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MICROCOPY RESOLUTION TEST CHART MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU Of STANDARDS-1963·.\ NATIONAL BUREAU Of STANDARDS·1963-A Structure and Toxicity of Insecticide Deposits lor Control 0/ Bark Beetles

By ROBERT L. LYON, Entomologist Pacific Southwest Forest and Range Experiment Station Forest Service, U.S. Department of Agriculture, Berkeley, Calif.

Technical Bulletin No. 1343 U.S. Department of Agriculture Forest Service October 1965 ACKNOWLEDGMENT

The author is grateful to the University of Ca~ifornia and the National Park Servicefor specin.1 materials and services i the late Charles B. Eaton, Forest Service, and Dr. 'V. M. Hoskin£, University of California, for their invaluable counsel; R. W. Bushing for 11is assistance during most of t.he study, especially in developing test. equipment; and J. R. Batchelder, ·W. D. Bedard, and B. D. Combs for their con­ tdbutions to the study. u CONTENTS Page Introduction______1 Litera.ture review ______.. ______1 Preventive controL ______2 Remedial controL ______. _ _ 4 Laboratory studies ______• 5 Deposit structure and toxicity______6 Structure and toxicity of deposits______19 Procedures_____ --______19 Effect of fu!l1igation______23 Effect of dose.______27 Effect of concentratiol1______36 Effect of solvonL______38 Effect of moisture content of substrate______39 Effect of wen,thering______40 Tests with caged bolts______46 Procedure______46 Results______49 Discussion______.______51 Summary and conclusions______52 Literature cited______54

NOTE.-Insccticides are poisonous and should be used only when it is necessary. Improper handling or application of insecticides, or careless disposal of excess material, may be injurious to humans, domes­ tic animals, desirable insects, and fish or other wildlife, and may contaminate water supplies. Directions and precautions on manu­ facturers' labels must be followed. Department of AgricuJlUre policy is to practice and encourage use of means of pest control which present the least potential hazard to man and animals. When residual or persistent pesticides must be used under this policy, the minimal effective amounts are to be applied precisely to infested areas, at minimal effective frequency. The studies summarized and reported in this bulletin are helpful in determining minimal insecticide application dosages, concentrations, and frequencies.

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m INTRODUCTION In a typical year in the United States, the volume of sawtimber·size trees killed by bark beetles amounts to more than 4 billion board feet (110).1 This In.r~e t1 loss can severely damage the eeonomy in locali­ ties that depend on forest resources. Prot6cting trees from these in­ sects is becoming increasingly vital to successful forestry (11). Insecticides f'roe indispensable in forest protection. Penetrating sprays of ethyl(me dibromide in diesel oil or in (1 water emulsion have been used in chemical control of bark beetles in the 'West in recent years (86,70,71,103,11'7). The sprays are applied liberally to the bark surface, penetmte to the inner bark tissne,and kill the bark beetle brood in place. Though expensive, the method is effective for remedial control. It cannot be used for preventive control because ethylene dibromicle is a fum igant (mel has a shott residual life. Residual sprays appliNl to the hark surface in low volume may efIecti\'ely prev'enl bttt'k beetle attack on green, uninfested trees 2 and also offer much promise for improving remedial control. Residual sprays are formulated and applied so that they act in one or both of t\\·o ways. They penetrate the bark and kill the tmderlying brood or ileposit the insedkide on and near the surface w11ere an emerging adult insect may pick up a lethal close. Thus, the form of the insecti­ cide deposit on the bark influences the effectiveness of the spray. The aim of this study was to leam how to develop bark deposits of residual sprays that will destroy b(;th emerging beetles and those attack-ing trees ancllogs. Because the strueture of the deposit is crucial in cletetwininp: contact toxicity, 'we must kno'w what governs deposit structure and how to eontrol it. ,Yith this information, ,ye can leave the kind of deposit that will suppress the bark beetle most efficiently. This study WaS Cllrried out near Oakhurst, Calif., at the Millmi Field Laboratory of the Forest Service's Pacific Southwest Forest and Range }jxperiment Station. LITERATURE REVIEW 1V11en ('hlorinated hydrocarbon insecticides became commercially available, they were tested against bark beetles. In reviewing these Rtlldies, Moore 3 conduded that. residual insecticides showed good promise for bark beetle contl·o1. Milch of this early work was done by applying the insecticide to rnnofl'--a procedure similar to that nor­ mn,lly followed in the use of pen etrn.tmg sprays. :Moore envisl'oned fl. different. manner of ('ontrol with residual spra.ys. The insecticide would be sprayec1lightly on the outer bark surface only, and the adult

t ltl1li(' numbers in parentheses refer 'to "T.Jiteratnre Oited," p. ;14. • MoorE', •.\.. D. StudlE';; on the toxicity of residual-type, organic insecticidE'S to bark beetles, with special emphasis on Ips COlltltSIIS (Lee.). 1956. (Un­ published Ph. D. thesis on file at the University of California, Berkeley, Calif.) • :\Ioore, op. oit. 1 2 TECHt'l'IC.-H. BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

bark beetle would contact it lethal dose as it emerged or attacked a new host. Published field studies of residualil1secticides for bark beetle con­ trol may be conveniently divided in two parts for review: (a) studies on preventive control and (b) studies on remedilll control. The control of ambrosia beetles also is re\'ie\\-ecl because they are closely related to hark beetles. PREVENTIVE CONTROL Oil solutions have received much attention as n, means for preventive control of bark beetles. For example, .Johnston (54) recommended :L O.5-percent gamma nne 4 solution in Xo. Z fuel oil to protect eastern hardwood and pille logs from nmbrosi,t and bark beetles. He got aileqllate protection for 3 to 4 months during the height of beetle activity when he applied the spray to runoff. Application of about 1 gttllOIl to 100 square feet of bark was sufficient for runoff to occur. Protection "-as improvcd when a. viscous oil, such as SAE 70 motor oi 1, was mixed with. the No.2 fuel oil at the rate of 1 part of viscous oil to 19 parts of fuel oil. In reviewing the iitnntul'e on bark beetles that. aired. southern pines, Thatcher (/08) indieated that O.~5- to O.5-percent gnmILtl BRC in fuel oil a.ppliecl to runoff was suc('esliful both as n. preventive and l'emec1 ial meaSUl'e. Smith {lOO) tested chloI'inatec1 hydrocarbons ill No.2 fuel oil for IH'otectiug fresh-cut illash pine stumps against the black turpentine beetle (Dendt'octonu8 tereb1'([n-~ (Oli\·.). Application was to runoff at the mte of 1 gallon oJ spmy to 40-50 square feet of bark. Sprays in order of decreasing effertiveness were: Concen­ tration Control (percent) (percent) Gamma Blle____ . ______Aldrin______• ______O. 5 96 Heptachlor ______2.0 95 Dieldrin______1.0 76 1.0 75 5. 0 60 DDT______2. 0 52 t~~id~~~~-:~====~======:======~======:======:== 5.0 48 No.2 (ud oiL______26

Oil alone provides proteetion against sonle bark beetles. Beckel' (/.1) found thtH saturating bark \\"lth No. 2 fuel oil or kerosene prevented elm bel'k beetles frol11 breeding. Rl1dinsky et a1. (96) applied a.j. to 6 mixture of refined kerosene and Velsicol AR50 5 to Douglas-fir' logs. Twenty weeks after spraying, there were 312 suc­ ressful attacks by Douglas-fir' beetle (Den(l1·octonu.~ 'PseudotS'lt,qae Hopk.) on the unspr'ayed check samples and only 11 on the sprayed ilampleil. Thel'ewere, respertively, 412 and 22 ambrosia beetle attacks. Rudinsky et nl. ascribed protection to repeI1ency.

• Tliseetipi(1('S ar(' IJlllll('d neC'Mcling to the npproved common nnmes published h)· th(' ('ollllllittet.' on Insl.'ctiC'i!l1' Terminology, Entomological Society of America. BUI. Ent. Roc. AllI('r. !)fill : lR!l-1!l7. l!)C,;t "Gamma BHC" is us('c1 when BBC ix fOl'lIlulatr!l on Uw gllnllnll ixolller C'ontellt. • Th(' idrntiticntion of eommereilll proc1uC'ts or citing of trade 1I11111('S in this /IulJlicntion Is soll.'ly for till.' purpose of information. Endorsement is not in­ trnded and must not lJe inferred. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 3

Johnston (54) found fuel oil solutions better than suspensions, and l:,,:>th these formulations superior to emulsions for preventing bark beetle and ambrosia beetle attacks on hardwood and pine logs. Smith (100) compared emulsions and No.2 fuel oil solutions of BHC for preventing black turpentin3 beetle attacks on slash pine stumps. The concentration of gamma, BHC was 0.5 percent in both :formulations. Applicatjon was Lo runoff, which deposited roughly 350 mg. of gamma RHO per square foot. After 29 weeks, prevention was D8 percent successful for the oil spray and 69 percent for the emulsion. Rlldinsky et al. (96) got better than 98-percent protecticn after 20 weeks with all formulations used on Douglas-fir logs against Douglas­ fir beetle and GnathotrichU8 8ulcatU8 Lec. Endrin, lindane, Thi­ odan, Sevin, and heptachlor were applied at 200, 200, 200, 400, and 400 mg. per square foot of bn,rk, respectively. Each insecticide was applied as a solution, suspension, and emulsion at 1 gallon per 100 square feet for solutions and 2 gallons per 100 square feet for emul­ sions and suspensions. En,dier studies by Allen and Rudinsky (3) on the Sflme host and bark beetle speciefl showed 19-week protection by three different wet­ table powders. Thiodan was most effective, lindane next, and Sevin least. A ppl ication was to runoff at two dosages, 95 and 285 mg. per square foot of bark. Logs treated with Thioclan had no attacks at ~ither rate of application; those treated with linclane had no attacks lI:t. the, 285-mg. rate, but had 0.2 per square foot at the 95-mg. rate. T;ogs tr'eated with Sevin hacl attacks at both rates of application: OA: per square foot at the higher rate and 0.1 at the lower rate. Check logs hacl2.3 attacks per square foot. Becker (1f2) found that DDT, methoxychlor, toxaphene, chlordane, and gamma EHe emulsions applied to elm logs at 1 percent concen­ tration all gave "excellent protection" all season against. the smaller European elm bark beetle (8colyt'tt.8 m1utistriatu8 CMarsham)) and the native elm bark beetle (Hylurgopimt.8 7'1.ljipes (Eich.)). Lindane rmulsiol1s applied to runoff were highly satisfactory for protecting reel, eastern white, and pitch pine logs from attack by Ip8, (Dend1'o('tonu,~ piceape1'da Hopk.) and Polygraphu8 ?vujilJennis Kby. t.ion was effertiye throughout the growing season, ranging from about 93 to D9 percent. The concentrations tested ranged from about 0.1 to 1.2 percent. Connola et al (134.) used lindane emulsions to completely protect windthrown red spruce for 2 months from the eastel11 spruce beetle (Dend7'octoml.~ piCfCLpC7'da Hopk.) and PoZygl'a.phu..~ '/'tLjiperllni8 I{jby. in the AdironchLck Mountains of New York. All concentrations­ 0.125,0.25,0.5, and 1.0 percent-proved effective. Hetrirk and Moses (45) tested a wide assortment of 0.5-percent emulsions for protecting loblolly, slash, and longleaf logs from 1718 species. Examinn,tions 4 weeks after application showed EHC as the most successful. Application was 0.4 gal. per 100 squue foot of bark. Kinghorn (5'/) rDmptlrec1 Thioclan and lindane emulsions as pro­ tective sprays against attu('k of l'rypodendron lineat1l/Tn (Oliv.) on Douglas-fir loge:. Tlu()dan sprays reduced attacks by 88 percent; lindane sprays by 63 percent. Thp. bark was saturated by 0.4-percent pmulsions on April 4, and results were assessed in mid-August. 4 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

In later tests on the same tree and beetle species, OA-percent sprays were applied to thorough wetness (200JLg. to 250JLg-. active ingredient P('1' sq. em.). "With Thiodan (emulsion) reductlOll in attacks was 82 percent; with lindane ('wettable pou'der) it was 74 percent (58). Phosphamidoll (aqueous solution) was also tried but. was ineffective. Sprays were applied in March and results measllred in August. In the interval, 6.5 inches of rain fell. Massey (69) found 2-percent DDT emulsions effective in protect­ i~g standing. ponderosa pine from infestation by the southwest~rn p1l1e beet Ie (Vendroctonu.<; bal'be/'i I-Iopk.). Two sprays were apphed, one at the end of May and one in mid-.ruly. "When the trees were examined in September !lite)· beetle flight ceased, inie.:;tations were found in 36 perc(,llt of 286 unsprayed trees and only 1 percent of 745 sprayed trees. REMEDIAL CONTROL Investigators have done less work on the remedial control of bark beetles by residual bark sprays than on preventive control. Smith (1(){)) found remedial control of the black turpentine beetle in slash pine stumps more difficult than prevention. One-haH-percent gfLmm!L BHC diesel oil sprays applied to runotf gave 75 to 80 percent kill of established broods compared with 98 to 100 percent prevention. Oil solutions excel emulsions in controHing the black turpentine beetle in the South (J()(}). A I-percent gamma BHe diesel oil spray iR recommended for- control of this insect in standing tr2es (101,102)" Postattack sprays applied to runoff on slash pine cut mortality of attacked trees by 70 percent. A. 0.5-percent gamma nHe oil spray is also highly efl"ective (60). The southern pine beetle (DencVrocton1l.s f'rontalis Zimm.) is effec­ tively controlled by 0.25- to 0.5-percent gamma BHe in fuel oil applied liberally to the bark. BHC was more effective than chlordane, DDT, ethylene dibromic1e, orthodichlorobenzene, 0)" trichlorobenzene (27, 108). Doane (28) applied emulsions to elm logs containing larvae of the Rmaller European elm bark beetle. Laboratory tests wl.th i~fested bark samples showed that lindane wns most effective, cheldrm was next, DDT followed, and malathion was .least effective. Kill was 99 p(')"cent with 6-percent lindane, 95 percent with 6-perrent dieldrin, and il\) percent with 12-pe)·cent DDT. On check samples, kill was 24 perc('nt. ~\. 110table result was that lindane killed most insects before t"hey could emerge, only :3 percent emer~ing. Dieldrin allowed .49 percent to emerge, and DDT 78 perrent. The sprays were applIed at the rate of 0.026 ml. per 100 sq. cm., so that the bark was "slightly moistened. ,~ Kinghorn (56) applied several chlorinated hydl'orarbon insecticides to lodg('pole pine bolts infested with "large larvae" of the mountain pine beetle (Dendl'oc/onus m.onti('olae Hople). The most effective WeI"e aldrin, heptachlor, lindane, and dieldrin; all four gave similar kills. Two others, chlordane and DDT that also gave similar kills, w('re much less effective. Except for lindane, one of the four most ('tYprtive, the sprays were formulated at 8.2 lb. per 5 gal.; the rate for lindane was muchkss at 0.6,1 lb. per 5 gal. All were emulsions except lindane and DDT, which ,vere formulated as solutions in ace­ STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 5 tone and Velsicol AR50-G, respectively. The sprays were probably applied to runoff, though this was not indicated. Kinghorn (56) applied aldrin, heptachlor, anti lindane as emul­ sions to standing infested lodgepole pine in a small-scale pilot test. He found all three compounds promising. Aldrin and lindane were tested on trees infested with the mountain pine beetle, and an three insecticid~s were tested on trees infested with the Douglas-fir beetle. LABORATORY STUDIES Only a few laboratory studies have been published on the contact toxieity of insecticides to bark beetles. Matthysse et al. (7B) tested thtj contact toxicity of elm twigs dipped in variollS insecticide emulsions (0.25 percent) to native elm bark beetle adults. The trenJed twigs had been weathered outdoors for various periods. Dieldrin, lindane, and parathion were the most toxic, DDT slightly less, and chlordane, toxaphene, and methoxychlor the least toxic. Doane (B8) carried out a similar type of test. ·with the smaner European elm bark beetle. Dormant Americn,n elm trees were sprayed to runoff with seveeal different emulsions (0.25 to 2.0 percent.). Twigs were removed 4, 8, and 13 weeks later, and adult beetles were exposed to them in glass jars for 24 hours. From most to least effec­ tive, the insecticides were dieldrin, DDT, lindane, heptaclllor, and malathion. Moore 6 (81) was the first to test the contact toxicity of insecticides to bark beetles by topical application. For each insect the acetone solutions-from most to least toxic-were: Califomia five-8pined ip8 (IZ)8 confus1.t.8 (Lec.) )-Lindane: endrin, dieldrin, EPN, isodrin, heptachlor, aldrin, DDT, toxaphene, dini­ trocresol, methoxychlor, DDD, DFDT. Wester'n pine beetle (Dendl'octOnlM bl'evicO'mis Lec.)-Isodrin, lin­ dane, DDT, toxaphene. Studies continued by Lyon (65) indicated: We8tem J>ine beetle-Endrin, isodrin, EPN, lindane, dieldrin, hep­ tachlor, dinitrocresol, DDT. i1fountain pine beetle (Dendroctonu".~ In!JnticoZae Hopk.)-Lindane, isodrin, EPN, enc1rin, dieldrin, heptaclllor, dinitrocresol, DDT. Fir en,ql'rtve1' (/:;colytll.~ 'vmtl'(diB Lec.)-Isodrin, endrin, dieldrin, lindane, EPN, DDT, dinitrocreso1. Other investigators later tested the contact toxicity of severa.l in­ secticides on the Douglas-fir beetle by applying sprays directly (95). The sprays were applied in solutions of one part acetone to one part Deohase. Decreasing order of toxicity at. LDso was lindane, Thiodan, isodrin, endrin, 8e\"in, heptachlor·, aldrin ancl dieldrin, DDT, chlor­ dane; and at LD90 was Thioc1an, elldrin, lin dane, isoc1rin, Sevin, aldrin, heptachlor, dieldrin, DDT, chlorclane.

"l\foore, A. D. StudiI's on thl' toxicity of residual-type, organic insecticides to bark beetles, with special emphnsis on Ips contI/BUS (Lec.). 11:)56. (Unpublh:;hL'O Ph. D. thesis on file at the University of Cal'ifornia, Berkeley, Calif.) i74-451 0-65-2 6 TECHNICAL B1;'LLETIN 1343, U.S. DEPT. OF AGRICULTURE

Also tested was the contact toxicity of deposits of these insecticides to the Doughs-fir beetle. Adult beetles were exposed to treated fiber­ board panels for 5 minuteS' under a petri dish after the deposits had aged for various periods. ,Yettable powders were applied at the rate of about 40 mg. actunl per square foot. Initial toxicity was highest for endrin, Thiodan, iso<1rin, and lindnne; and least for chlor­ dane. Sevin, DDT, dieldrin, aldrin, and heptachlor ·were intermedi­ ate. Lindane, llldrin, heptnchlor, nnd chlordanI' lost all toxicity in 6 "·ceks, and dieldrin and Sevin in about 10 to 12 ·weeks. The other rompounds lost most of their toxieity by the end of testing at 13 weeks. Both laboratory and field studies redewed nbon~ show consistently that th~ five insecticides most toxic to bark beetles are endrin, EPN, isodrin, 1indane, ancI ThiocIall. Next in line and also promising came aldl"in, dieldrin, and heptachlor. DDT has usually performed poorly despite its effecti,reness and widespread use. on forest defoliators. Howe,·er, elm bark beetles see.1l1 unusually susceptible to it. DEPOSIT STRUCTURE AND TOXICITY Few workers ha\'e studied the microscop:c structure of deposit'> on the various surfaces treated fol' residual control. Nevertheless, the physif!al condition of a residue governs its toxicity. Structure de­ termines how much of an insectieide is available to an insect moving o\'er it. The mo,·c a"ailable the insecticide, the more easily the insect. becomes contaminated with a lethal dose. Barlow and Hadaway (7), Potter and ,Vay (92:, and more recently Hoskins (51) have pub­ ]ishe(l on the general subjert of deposit structure and toxicity. Deposits are solid or liquid. Solid deposits consist of insecticide pn.rtides, either amorphons or crystalline, and usually result when clusts or suspensions are applied. Liquid deposits result when solutions or emulsions are applied, but may not remain liquid if crystallization occurs.. Inse(,ticides dissol"ed in oil in emulsions or solutions often crystallize after being applied to a surface. As th~ solvent evaporates, the solu­ tion becomes saturated, then supersatmated, sometimes becoming "is­ rose-almost gummy-and e"entually crystallizing (7~ 8/), 89. ,91). Pal (85) coiwlnsh·ely dcmonstrated with DDT-xylene solutions that sllpersatnmtion actually takes place. He weighed deposits periodi­ callr nntil cl'rstallization occurred. The amount of DDT reached a point well beyond the solvent capacity of xylene for DDT. . Supet·saturatiOll without subsequent crystallization or with greatly delayed crystallization may also occur with some insecticides and some solvents (88). Supersaturated deposits mas be induced to crystallize mechanically \"hen insects wn lk o"er them (7, 89). Factor:; controlling tlle rate of crysta11izatiml 11:1\"e received almost no attentlon. Pal (8.5) snggestl'd Hmt the rate (1epends on concentra­ tion, 5ubstrate, pl'esence of dust particles, and temperatures. But for DDT solutions applied to glas::l panels, he ouselTed no consistent differ­ ences in rate of crystallization in various types of solvents, such as aliphatic and aromatic hydrocarbons, halogenated hydrocarbons1 and ketones and esters. The consensus seems to be that contact toxicity of residmll insecti­ cides is greater in solution than in the solid state (7,10,31). But Pal STRUCTURE AJ.'l"D TOXICITY OF INSECTICIDE DEPOSITS 7

(85) found that supersaturated DDT deposits on glass panels in­ creased in toxicity when deposits crystallized. He also p )inted out that wetting power of spray deposits decrease as they age and become supersaturated. Amilabihty and I?ickup are reduced, thus toxicity may decrease while the deposit is stll1 a solution. On such absorbent surfaces as compressed fiberboard, the deposit may become more avail­ able and more toxic after it crystallizes i the solution when first applied is absorbed in the substrate and thus is not available to the insect (1). The induction of crystallization by the visitation of insects and other forms of mechanical stimulation has been observed. Most observ­ vations ha ve been made on DDT in such solvents as acetone and kero­ sene on nonabsorbent surfacer; like glass and absorbent surfaces like filter paper (10,21,592,88). The strlkinO' bloom of induced crystals of DDT is unique. DDD, dieldrin, and ftndane solutions in kerosene applied to fiberboard bloomed only slightly aiter brushing (1). Aldrin, methoxychlor, and toxaphene could not be induced to crystal­ lize at all. Maximum crystallization of DDT can be induced It few hours after application. Delay in stimulation results in a smaller and smaller crop of crystals (7). DDT crystal blooms removed by wiping are later replenished (18, 89). The type of mechanical stimulation can affed the deposit structure of DDT 111 kerosene on fiberhoard (8.9). Stimulation by flies caused numerous small crystals, whereas brushing with a paintbrush induced fewer but longer crystals. Induced DDT crystals may differ in form from spontaneous crystals. tVhen DDT-kerosene solutions were applied to fiberboard, brushing or the action of insects induced a dense, uniform mat of 501' to 1501' crystals that completely obscured the surface. In contrast, sponta­ neous crystals were stout, 501' w 3001' long, and arranged in isolated clusters (7). Pickup As used here, the term "pickup" refers to the process by which insects become contaminated with an insecticide during contact with a deposit. An insecticide can also be absorbed directly during such contact. Pickup from dry deposits is determined by the relative ad­ hesion of insectIcide particles to the substrate and to the insect cuticle. It is also determined by the extent of insect contact with the particles (;]7). Thus pickUp is greater when an insect is moving than when it remains stationary, as shown for bedbugs exposed to DDT deposits (~1). . The followinO' studies show the importance of relative adhesion. Oi1-coated lipopllilic dye paliieles (which act essential1y like lipophilic insecticide particles) were picked up at only one-tenth the rate of dry particles (693). DDT particles ,yet with Shell Risella 11 oil were picked up at only one-hundredth the extent that dry particles were picked up (-37). The oil seriously hindered pickup by increasing greatly the adhesion of particles to the substrate. And the more oil that is used to wet the particles, the less the pickup. The shape of DDT particles also affected pickup. Insect morphology also affects pickup. Insects with rough integu­ ments pick up more dry particles than insects with smooth ones (936). 8 TECHNICAL BULLET:U~ 1343, U.S. DEPT. OF AGRICULTURE

But a deposit is likely to have less effect on an insect with dense hair than one "with sparse hair. This is because dense hair often keeps insecticida particles awa,Y from the cuticle (38). David and Gardiner (126) found that increasmg resistance to dust deposits of DDT paral­ lels increasing hairiness among four different insect species. The structure of the tarsi affects pickup (61). The hind tarsi pick lip more than the mid 01' fore tarsi, probably because they exert greater thrust (38). The more hairy tarsal segments were found to pick up more insecticide particles than other segmellts or the pulvilli (61). The pickup of oil-soluble dye particles resembles the pickup of particles of DDT deposits (-18). Both were picked up in about equal quantity and showed the same straight-line relationslup between pIck­ up and exposure time. The bigger insects took up more total dye than the smaller ones, but less per unit of body weight. An insect may lose particles when walking on a clean surface (62). Also, retention is greatly influenced by particle size, Jarge particles being lost more readily than small ones (6). Cleaning movements ('an ('asily detach larger DDT crystals (40). Rut insects that "clean" themselves are likely to pick up more crystals by transferring- insec­ ticide from the appendages to the general body surface durmg the {"leaning (.38, 62). Large parti('les haying a· padie1e weight of the same order as the adhesive forces are easily dislodged, but particles with adhesive forces far in excess of this weight are readily retained (612). 'Crystal shape affects retention (78). Poorest retention was shown by platelike structures about 25p. in diameter. McIntosh (78) suggests that contact toxicities of insecticides can be ttl1ly compared only by considering the amount of insecticide retained on the insect. Particle Size and Form Partiele size and form are two of the more important properties of dry deposits of insecticides. Most attention has been focused on particle SIze as affecting toxicity. Toxicity to the pea aphid in­ ereased with exposure to a series of DDT-coated dust deposIts of in­ creasing particle size but having an equal number of particles per unit area (113). Flour beetle adults were dipped in suspensions of DDT that became increasingly toxic as length of needle crystals in­ creased (74,75). Needle lengths of 40p., 120p., and 400p. were tested. But DDT suspensions of uniform-sized crystals were less or equally as toxic as colloidal suspensions (78). Also, toxicity of rotenone sus­ pensions tested by clipping decreased as crystal size increased (76). Colloidal suspen8ions were most toxic. Most workers have reported higher toxicity as particle sizl>. decreased. Such was obtained by exposing houseflies to deposits, un glass, in three size ranges: 5p. to 15p., lOp. to 20p., and 20p. to 30p. (116). DDT particles p.veraging 5p. were more toxic to the large milkweed bug and potato leafhopper than paI"ticles averaging 15p. and 25p. (30). Van den Hende and Jacobs in 1950 (from Potter and 'Vay (9~» found that toxicity increased as size of DDT particles decreased. Hadaway and Barlow (40) exposed mosquitoes to deposits of [!round DDT suspensions on plaster of paris. They found that smaller particles were more toxic. Particles 60p. and Jess gave good kills; those (lOp. to lOOp., lOOp. to 130p., and over 130p. gave poor kills. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS . 9

Kill from particles lOp. to 20p. was 93 percent; that from particles 40p. to BOp. was only 5 percent-in spite of application of only 6 mg. per square foot for the smaner particles and 400 m~per square foot for the larger. Hadaway and Barlow (42) tested lJDT suspensions on wallboard, also with mosquitoes, and got similar results. Particles averaging 6p. were more toxic than particles averaging 24p. or 60p.. There may be. a particle size that confers maximum toxicity. Had­ away and Badow (40) exposed mosquitoes to suspensions of DDT, DDD, and methoxychlor sprayed on plaster blocks. For all three insecticides, lOp. to 20p. particles gave lar~er kills than 0 to lOp., 20p. to 40p., 40p. to 60p., or 60p. to 80p. particles. A turther test showed lOp. to 12p. crystals more toxic than 5p. to 6p. crystals. Many explanations may be given for the difference in toxicity be­ tween various p~trtic]e sizes. Thou~h more insecticide may be pICked up if particles are lar~e, rate of picl\:up may be faster if particles are small (1, 18, 6, 74-). bmaller insecticide particles have more surface area relative to their volume, which explains their more rapid. action (186). However,1Vheatley (113) could not measure the relatIOn be­ tween size of particle and its area of contact 'with the insect's relatively rough cuticle, Barlow and Hadaway (6) have sug;gested that the rate of penetration through the cuticle helps determme the effective­ ness of particles in difl"erent size ranges. As crystal size decreases, DDT dissolves in cuticle waxes more rapidly, and therefore penetrates to the site of action more rapidly. Lewis (61) found that. closely packed microtrichia on muscid and tachinid flies prevented inseetielde particles lOp. in diameter or larger hom contacting the cuticle surface. Hadaway and Barlow (41) mention that the persistence (toxic life) of an insecticide can be in­ creased by increasing the particle size, so that postponement of bio­ assay will favor the larger particle sizes. They indicate that particle ~ize influences potency less as intrinsic toxicity of the insecticide mcreases. Because influences on pickup differ, the particle size that is best for direct application to an insect may differ from the size that is best for exposure to a clepOfsit (9fa). Changing the holding temperature can reverse the relation between particle size and toxicity (76). The negative temperature coefficients of colloidal DDT versus 400p. crystals were sufficiently dift'erent that. relative toxicities could be reversed by increasing the holding temperature from 12° to 30° C. Little information on the efl"ect of particle form on toxicity has been published. DDT crystals of "straight line structure" (acicular) were more toxic than crystal aggregates (1118). Acicular DDT crystals were much more effective than colloidal DDT.1 To McIntosh (73), overall crystal length seemed most important (toxicity increased with length) und breadth less important, although increase in crystal breadth, from a needle to a plate in shape, reduced toxicity somewhat. Hadaway and Barlow (40) found60p. needles most effective, lOp. to 20p. ground crystals slightly less effective, Op. to lOp. ground crystals and op. rods much less en'ecti ve, and 60p. by l5p. plates least effective.

7 Malley, Nuri. The effevt of uniform size droplets on the toxicity of dieldrin to T"ibolillll~ Clll!tllllCum Rb. when !:lubjectC'1l to residual spray. 1953. (Un­ published Ph. D. thesis on file at the University of California, Berkeley, Calif,) 10 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE Factors Affecting Deposit Structure Three ffitLin fuctors responsible for deposit structure are (a) formu­ lation, (b) substrate, and (c) en vil·onment. Formulation Some investigators have shown that the major formulations-solu­ tions, suspensions, emulsions, and dusts- are equal in contact toxicity (;3)d, 35, .40) ; others point to very wide di tl'erences (7,55,96, 109, 11~). Oil solutions are generally higher in contnet toxicity than dry deposits, even when both types of deposits are equally available to the insect (7). Not only may the 1I1secticide itself profoundly afl"ect the toxicity of the deposit, but the purity of the insecticide compound mH,y 111so be im­ portant (89). Diil"erent brands of the same insecticide mlty also differ In toxicity (812). 8ol1'ent.-l\fuch information is available on the influence of the solvent on toxicity of insecticide deposits. Residual deposits of DDT in a "light oil" were Illore toxic to bedbugs than depOSIts of DDT in acetone· (10). DDT-kerosene deposits on wans nnd ceiling were more toxic 1"0 houseflies than DDT-alcohol deposits (87). DDT-oil solu­ tions on filter paper were more toxic to t11e granary weevil than DDT­ rtcctone solntions (104). The effect of the solvent is sometimes very great. DDT in n-hexadecrtne applied to filter paper was 50 times more toxic to mosquitoes than DDT in methyl abietate (.43). The auxiliary solvent also may be important (1112). The order of effectiveness of different solvents in influencing de­ posit toxicity was the same for DDT, Prolan, dieldrin, endrin, and I11lLla,thion (43). ·W'hen houseflies amI mosquitoes ·were exposed to filter paper deposits of these materials, the relative toxicities depenoed in part upon the solvent used. For example, dieldrin was two to three times more toxic to mosquitoes in n-hexadecane than in liquid paraffin, but DDT waS R times and Prolan was 13 times more effective ;n n-hexac1ecane. 1'he physical and chemical properties of solvents are important in determining the effectiveness of insecticides as deposits. On filter paper, DDT, Prolan, dieldrin, ench'in, and malathion were more effec­ tive against houseflies and mosquitoes in paraffinic hydrocarbons than in aliphatic, long-chain esters and related compounds U:J). They were least effective in aromatic e3ters. DDT in 38 different s01vents was examined as single eh'ops on fiberboard. The more important properties of the solvents in determ ining crystallization were vola.tilit.y, viscosity, and ability to dissolve DDT (7). The effect of the solvent is shown mainly through its influence on deposi t, st rnctu reo F or exam pIe, on fiherboard (an absorbent, surface) , solvents of lower boiling point cause more rapid and extensive spon­ taneons erysta11izatioll (7). Elmendorf et al. (31) observed this same phenomenon with DDT solntiolls applied to copper wire (a nonab­ sorbent sur·face). The more, volatile acetone and kerosene producecl crystalline deposits more rapidly than diesel oil, heavy mineral oil, or SAE oil No. 30 and No. 50. Aleohols and other low-boning solvents evaporate before soaking into absorbent. surfaces and mRy leave n solidinCl"ustation of the dissolved insecticide on the surface (7). STRUcrURE A:.t~D TOXICITY OF INSECTICIDE DEPOSITS 11

'When deposits do not crystallize, they are more toxic ill these less viscous solvents because of better pickUp. Also, light oils pass into the cuticle easier than heavy oils, thus increasing the rate of entry of illsecticides dissolved in them (115). Kruse (59) tested DDT in kerosene, xylene, and acetone on glass plates. He noted that fOl1nulations differed conspicuously in crystal form. 1Yith kerosene, crystals were much larger than the mmute crystals from acetone. 1Yith xylene, crystals were intermediate in size. Hoskins et al. (59) found that lindane, on glass surfaces, pro­ duced lumpy crystuls in petroleum ether and mostly flat plates, with a scattering of smaller }?latelets, in chloroform solutions. Pal et al. (86) in tests with DDT used a water surface as a substrate. DDT­ diesel oil sprays applied to water crystallized in 3 to 4 days, and the crystals averaged 170JL in length. DDT-acetone and DDT-cyclohex­ anone sprays crystallized in 1 to 4 hours, and the crystals averaged only 65p.. Because of the very uniformity of ,vater and glass surfaces, it is clear that observed differences in deposit structure were due to variations in the solvent. (loncentration and dosage.-Gahan et al. (35) showed that 1, 2, 2.5, and 4 percent DDT-kerosene solutions, 'applied to wood surfaces at 50 mg. per square foot, were equally toxic to mosquitoes. In contrast, Parkin and Green (88) found that with DDT in "pool burning-oil" applied at equal rates, the highest concentmtion was the most toxic. Later (89), they applied different concentrations of DDT in kerosene to wallboard at equal rates, recording greater kill as concentration was increased from 0.2 to 10 percent. The relation between exposure time and deposit toxicity was found to differ among the concentrations. For example, more concentrated solutions became more toxic with longer exposure whereas less concentrated solutions did not. This was probably related to differences in the amount of crystallization. Kettle (55) studied the effect of dosacre on toxicity of DDT deposits on aluminum panels to houseflies. Ife applied suspensions over a wide rtwge of dosages, from 1.25 to 320 mg. DDT per square foot. Toxicity was the same for all deposits. The situation was different for deposits of DDT-kerosene solutions on paper-covered aluminum panels. Toxicity increased for each increment in dosage from 2.5 to 40 mg. DDT per square foot. Parkin and Green (89) also got increasing toxicity to the housefly with increasing rates of application of DDT-kerosene solutions to wallboard. MlCl'oscopic examinations of exposed flies showed that the number of DDT crystnJs on the body increased as application in­ creased from 80 to 320 mg. f.er square foot. Kills were higher with four application!:! of 0.5 m. of 1.25 percent DDT than with one application of 2 ml. Hadaway and Btu'low (40) point out that there is a dosage threshold at which an insect will, with n, given exposure time, pick up a maxi­ mum amount of insecticide. An} increase in dosage above this threshold will have no effect on deposit toxicity. In tests on mosqui­ toes, they got equal kills from 3 mg. and 50 mg. of lOp. to 20JL DDT crystals per square foot applied as suspensions to plaster blocks. The th resholc1 was ;3 mg. or less. Lewis and Hughes (658) reported that pickup was very efficient at the beginning of IUl exposure. But the insectbec(l.me saturated quick­ ly, after wl11ch the insecticide lost from t.he body balanced uptake. 12 TECHNICAL B"L'LLETL."V 1343, U.S. DEPT. OF AGRICULTURE

The authors reported that uptake by blowflies declined 90 percent after a 15-second contact with dry dye deposits. Barres (10) showed that bedbugs exposed to DDT deposits on glass and wood picked up a maximum (1.ose in l1bout an hour. Hadaway and Barlow (43) reported a direct and constant relation between dosage and piekup. They exposed mosquitoes to DDT-liquid paraffin deposi ts on fi Iter paper. :So crystallization occurred before exposure. Grntwick (J7) exposed blowflies to 25p. (mass median di­ !uneter) DDT partie.les on both filter paper and carnauba wax surfaces. He IOlmd ;hat pickup inereased about threefold when exposure time was increased eightfold. Adjuuants.-Yarious compounds included in insecticide formula­ tions to impro\·e performance often have an important influence on deposits. 'Vetting agents commonly used to disperse suspensions have been found to reduce uptake and, in turn, to reduce toxicity (37). Their effeet on deposits from DDT, dieldrin, and lindane suspensions on 110npol"Ous surfaces was studied by analyzing {>ickup chemically (0,4-1). On these sul'iaces, the wetting:-agent solutIOn remained with the inspctieidal pattie-Ips. On drying It caused the palticles to cling ~tl"ongly to themrface so they could not easily be picked up. On porous surfaces such as dried mud or plaster, the wetting-agent solu­ tion penetrated and did not interfere with pickup. I?eposits <;>f DDT cryst.als f~~n~ suspensi~ns on glass increa~ed in tOXICIty dunng stomge (J9). Ihls was attnbuted to a. change III the wetting agent on aging that deel·eaRed itc1hesion of the crystals to the glass. Adding It wetting agent aetually iri\"reased toxicity of bean leaf de­ posits of malathion to mites (44). Kill was 11 percent without the wetting agent and 88 percent with it. Apparently the wetting agent increased the ava,ilability of the deposit. Inert diluents in 'wettable powders reduce the ava,iIability of an insecticide deposit. This was true for the several common diluents tested (diatomite, kaolin, and others), but they differed little in degree to which they reduced amilability (40). The reduction '.vas attributed to two effects of diluents: The masking effect of the dust covering the insecticide pltrticles, and the "competition)~ effect in which proba­ bility of picking uv the insecticide particles dec~"8ases as the number of inert p!trticles lI1creases. Stickers may reduce ttvailability by masking the insecticide particles and incre!1Sing their adhesion, especmlIy on nonporous surfaces. This is desirable to some extent, because it increases resistttl1ce to weather­ ing (7). Emulsifiers, too, Cttn mask a deposit and reduce its avail­ ability (33). A small amount of cottonseed oil added to DDT-kerosene solutions in(Teasec1 kill of tsetsp flips en. The crystals that fonned on glass pltnels sprayed with this mixture were coated with a tach.-y film of oil. Adding the oil appal·ently a,idecl both pickup of the insecticide and passage into the insect's lipoid eutiele, strikingly altering mortality. 1Yith one part of cottonseed oil for each part of kerospne, the deposits usually produced over 95 percent mortality; without the cottonseed oil, the deposits were nontoxic. Oil additives incretlsed toxicity of DDT deposits to mosquitoes (4») but fOl" a reason other than that described for tsetse flies. Par­ STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 13 affin oil added to DDT-kerosene solutions prevented crystallization that otherwise takes place. The resulting ullcrystallized, liquid de­ posits were more toxic to m09fuitoes than crystalline deposits. Conmamne resin added to kerosene solutions of DDT, lindane, dieldrin, and methoxychlor decreased crystal size and increased tox­ i.city B (111). The optimum ratio of resin to insecticide ranged from 1 part in 5 to 1 part in 20. Similar effects were obtained with other synthetic resins. These studies appear to have an important bea.ring On the l'(llayior of sprays on the bark of resin-producing conifers. Moore 9 noted that deposits of resin in the bark of sugar pine, pon­ dero~a pine, and Douglas-fir influenced the surface crystallization of the DDT-ueetone and DDT-alcohol solutions l1pplied. He concluded that the condition of these mriable resin deposits, whether crystal1ine Or noncrystalline and hi/!hly viscous, influenced deposit structure more than did sp€'cies of bark. Substrate The surfaee to which a residual spray is applied greatly affects C'ontaet toxicity of the resulting deposit. A detailed study of bark structure. of conifers by ('hang U2.J) abundantly illustrates the wide <1i trer'ences in bark strueture anel texture bet\Yc€'n spedes and between trees of it single speci€'s. This study sllg/!ests that deposits on bark could vary considerably. ~[oore 10 demonstrated this for deposits of DDT, from spot to spot, on It single small section of bark. S€'vet'ul studies have been conducted to test the effect of various substrates on toxicity. On glass, DDT deposits Were much more toxie in bedbugs than on eement and painted Rnd unpainted wood (jf}). Deposits of DDT-k€'rosene solution WHe more toxic to house­ flies on aluminulll than on pap€'r-c'o\'H€,d aluminum (55). Flour beetl€'s wen~ exposed to DDT and lindane emulsions on glass, filter pRpet·, and wax (93). The d€'posits were most effective on glass and least on wax .. DDT suspensions g:we higher mosquito kills on mud blocks and ",aIlboltrd than on glass or unpainted wood (4:2). Th€' substrate may atfeet relative potency. For example, Shawarby (.')7) found lindane :~ times more potent than DDT when testing both as suspensions 011 ntud blocks, but 20 times more potent \Yhen testing both asoil solutions on filter- paper. The substrate probably aJfects the toxicity of deposits mainly by its influence on iu-ailability and pickup. On glass, DDT-kerosene solutions erystallize in th€' form of aC'ieular crystals lying flat on the snrfa('e. On fiberboard, the crysta.1s stand upright and are more (w:tilable (7). On plaster- blocks, 0 to lOf! DDT crystals were much l€'ss toxic than 10f! to 20f! crystals, but on fiberboard and mud blocks both size ('lasses \\'('1'(' equaIl)' toxic (..if). Gr-atwiek (.J7) tested DDT suspensions on glazed paper, filter papPI', and It waxed sllI·fa('e. The d€'posits on glazed paper were most

• \'fln Tipl. X. Int\twnc'(' of rpsillS 011 crystal siz(' and toxiCity of .inseeticidal r('!,i!lllpg, H}:i:i. ({'u[lllh)i:;hl"<1 Ph. n. thpsis. Copy 011 fiI(' at the {'niversity of Ll'illen. Xetherlands.) • ,\fol)ft'. A, D. !:H1Hlips Oil th£' [oxic'ity of residufll-typ('. organic inseetiddes to bark bpC't1ps. with special ('mphasis on Ips ('ontu,yus (Lee.). Ifl56. (Cnpub­ IisllC'd Ph. ]). t!l('sis ou fil(' nt the University of California, Berkeley, Calif.) ,••'loon'. OJi. cit. jH~.j51 0--65-,- :1 14 TECHNICAL BULLETL.'{ 1343, "C.S. DEPT. OF AGRICULTURE toxic and those on filter paper the least. The smooth surface of glazed paper did not conceal the DDT particles. On filter paper, DDT par­ ti('les were concealed in the interstices between fibers and were out of reach of the insect crawling over the substrate. DDT particles were less effecth7 e on the 'waxed surface than on glazed paper because of the. affinity of DDT for lipoids, waxes, etc. The particles adhered more firmly to the wax than to the glazed paper and thns were less easily dislodged. The coarse, irregular sm'face of bark, Moore 11 noted, conceals the smaBcl" insecticide particles of suspensions, and makes them unavail­ able by contact action. For this reason, he felt that a study of solu­ tions 01.' emulsions that crystallized at the bark surface would lead to the deyelopment of toxic, bark-surface deposits for bark beetle control. On porous surfaces, snspensions have been more effective than solu­ tions or emulsions. DDT and lindane emulsions applied to mud blocks underwent much absorption, resulting in poor surface deposits (,5). Suspensions, on the other hand, provided good surface deposits. The- deposits were bioassayed with tsetse flies. Suspensions of several insecticides on fiberboard were twice as toxic to the Douglas-fir beetle as emulsions (96). The emulsions penetrated the porous fiberboard panels and carried the insecticide out of reach of the insect. The absorption of DDT-kerosene solutions into wallboard has also been demonstrnt~d (8.9). Thus, unlessinsecticic1e solutions and emul­ sions on absorbent substrates later Cl'ystallize at the surface, little or no rl'sidual contact toxicity results. Pretreatment of an absorbent surface (bl'iek) with poly,;inyl alcohol, starch paste, size, or water­ glass inereased contact toxicity by preventing or curtailing penetra­ tion of oil solutions (DO). The ability of oils to penetrate porous solids is a function of the 8lll·face tension and absolute viSCOSIty of the oil (50). On bark, for example, less \"iscolls oils penetrate better than more viscous oilso The outer layers are more easily penetrated than the inner layers. 'Vet bark intel"fetOes with penetration. Kerosene dissolves resins in hark and thel'eby becomes more viscolls. This greatly slows its penetration. DDT in kerosene solution was applied to mud blocks and absorbed hy the mud particles as the solution penetrated (39) ° This was con­ finned by a reduced DDT concentration in the spray solution recovered after the original spray solution seeped completely through the mud blocks. Lindane, dieldrin, and DDT suspensions applied to mud blocks at first resulted in a deposit on the surface (8). In time, however, the inseeticic1es were adsorbed and eventually became uniformly distrib­ Ilted throughout the blocks. Adsorption of lindane greatly reduced loss by volatilization. For example, deposits on glass were 50 percent vaporized in 1 week, but deposits on mud blocks were half gone in 16 weeks. The residual life of volatile insecticides on bark may perhaps be greatly extended if the insecticide seeps into the bark tissue instead of remaining on the sluoface. 80n1(' evidence for this has been given by

It Moore, op. cit. STRUCTURE A.J.~D TOXICITY OF INSECTICIDE DEPOSITS 15

Rudinsky et al. (96) for lindane, endrin, and heptachlor. Less insecti­ cide was recovered from weathered bark when applied as a suspension than as a solution or emulsion. However, Robinson and Mesmer (B..n analyzed surface and subsurface deposits of DDT on weathered bark and found equal loss. The toxicity of surface deposits to bark beetles would be enhanced if the vapors penetrated the bark. Doane (29) applied lindane, dieldrin, and DDT emulsions lightly enough to moisten slightly the surface of beetle-infested elm bark. Lindane achieved by far the greatest frac­ tion of total mortality within the bark, i.e., before the beetles could emerge, because it is much the most volatile. Of total mortality, thl percent that occurred in the pupal cell before the insects began to bore out was 69 for lindane, 9 for dieldrin, and 8 for DDT. Therercent of total mortality that occurred in the bark, both in the pupa cells and when beetles were boring out, was 98, 54, and 37, respectively. Environment Many tests have demonstrated the profound effect of environment on structure and toxicity of deposits. Siddiqi (99) concluded that an insecticide may control some insects well in one environment and be ineffective in another. Direct effects on the deposit should be distinguished from effects acting indirectly through the insect. As Barlow and Hadaway (9) point out, the weathe~' may affect an insect's behavior so as to alter the amount of insecticide picked up and thus the estimate of deposit toxicity. Various environmental factors that can indirectly influence estimates of toxicity of insecticides act on the insect before, during, and after treatment (20,98). The time element is important in evaluating the effect of environ­ ment on deposits. Environmental or weather effects are influenced by the duration of their action. Much of the discussion of various environmental influences on toxicity of deyosits, therefore, will refer to effects on persistence, and thus on residua or toxic life. The effect of temperature is well illustrated by the work of Tootia and Dahm (107). Deposits of aJdrin, chlordane, dieldrin, lindane, and parathion emulsions on glass panels were subjected to 52°,82°, and 108° F. in temperature cabinets. All tests showed a pronounced in­ crease in residual life as the aging temperature fell from 108° to 52° F. At 108° F., all deposits initially gave 100-percent kill, but declined in toxicity to below 20-percent kill after 16 days. At 52° F., parathion and dieldrin producedlOO-percent kill for the first 65 days compared with IO-percent kill or less from aldrin, chlordane, and lindane for this period and temperature. Though high temperature appears to be more influential, high humidity also shortens residual life of deposits (99). Humidity probably affects deposit toxicity by changing the availability of the insecticide to the insect during exposure (9). Sunlight strongly affects deposits, mainly by hastening evaporation and decomposition (25). In the laboratory, DDT-kerosene solutions on glass surfaces formed supersaturated droplets (with some crystals), and crystallization took plac~ readily upon stimulation (39). Similar deposits exposed to sunlight developed into hard, resinous masses. 16 TECHNICAL BULLETL.~ 13-13, U.S. DEPT. OF AGRICULTURE

SlmliIYht can have a greater effect on toxicity than ambient tempera­ ture. I~igh temperature aml weak natural lIght (800 ft.-c.) did not reduce toxicity of emulsion deposits of toxaphene, malathion, and methyl parathion Oll cotton leaves. HilYh temperature and intense natural light (13,000 ft.-c.), however, reduced their toxicity 50 to 70 percent (46). DDT emulsions and suspensions formed as dry deposits on glass and plywood and were not. harmed (especially on glass) as much by exposure to sunlight as were solutions (63). It. was concluded that sunlight decomposes DDT in solution more rapidly than as a dry deposit. DDT deposits on g1ass and wood were also exposed to ultraviolet light for 132 hours. The result. ,,-as a large drop in toxicity, especially wll('n high-boiling sol "ents ,vere used. A suspension of DDT exposed to ultraviolet light became less toxic. ~\lso tested was tb.", effect of "arious COS01Yellts on the behavior of DDT-hrosene deposits exposed to weathering. Loss of toxicity on both glas.,., and wood nuied enormously among di fferent cosolvents. Tho effect was related to the. boil ing range or Yolatility of the co­ soh-ent. Generally, deposits were not reduced in toxicity as much by cosolvents with a boiling range near kerosene. As thebOlling range inereased aboHI kerosene, sunlight exposure. had more harmful effects. The effect of rainfall on toxic'ity of def)osits ntries with the formu­ lation. Fot' example, dusts are more H.C versely afl'ected than sprays U)l)). ExceRsive rainfall reduces.the protective effect .of :SHC sprays agamst bark beetles and ambrosul beetles (54). A l/z-mch nunfan greatly reduced toxicity of toxaphene and Guthion emulsions on ('otton lelwes, but did not all'ect that of Sevin suspensions (46). Wind and rain coming shortly after dusting cause rapid loss of dust deposits from fol iage (926). Resichwl Zife.-Residual sprays for bark beetle control, especially protective sprays, must. remain lethal for a.n extended. period. Resid­ u.al life is main(y governed by environment, substrate, and formula­ tIOn (16,107). Little information is available on the effect of substrate on residual life. But as has been shown e..'1rlier, the substrate has a strong effect on the deposit and may influence residual life. Becauseroughsurfaces present a greater area than do smooth sUl'faces, Bertagna (16) sug­ gested that inseeticide on rough sllrfac.es is spread more thinly, thus lliwing a shorter residual life, than the same amount applied to smooth sudaces. The major formulations-whether solution, emulsion, suspension, or dust-may affect the persistence of an insect.icicle (16), although few Rtuclies have been designed explicitly to test this effect. Siddiqi (99) observed that DDT suspensions are more adversely affected by weather than are emulsions. Studies by Rudinsky et al. (96) tend to support this view. Lindane, e!ldrin, and heptachlor were extracted from Douglas-fir bark that had weathered for 10 weeks. Consistently more was recovered when applied as solutions and emulsions than when applied as suspensions. Solutions and emulsions were largely absorbed into bat'k tissues where their evaporation was retarded; suspensions l'emn.ined on the sllrface where they were exposed to higher evapora­ tive forces. It was not cleterm inecl whether the solutions and emulsions crystallized. at the bark surface. STRUCTURE L~D TOXICITY OF INSECTICIDE DEPOSITS 17

The same authors tested enc1rin, heptachlor, lindane, Sevin) and Thioclan suspensions unit emulsions as deposits on fiberboard. The deposits were exposed to luboratory conditions and bioassayed with Douglas-fir beet Ie adults. Contact toxicity of both types of formula­ tions fen tlt the same rate with aging. The suspensions, however, were consistently more toxic at the three exposure intervals of 0, 4, and 7 weeks. Lindane and Thioclan were the most potent. Some deposits improve with age (17, 18). Paints and protective coatings containing DDT, lindane, and toxaphene were tested for surface toxicity to houseflies after various periods of aging. Coatings containing DDT continued to improve in toxicity to the end of the tests at 2 yC<'Lrs. This improvement was attributed to the slow crystaJ­ lizn;tioll of the DDT feo111 a snpersatur!llt:ecl condition. Lindane and toxn.plH'ne deposits aged ror 20 to 30 ,yeek~ also showed an increase in toxicity. ' ,Yithin any gi\'en YI1Rjor rOl'mulation, the amount of insecticide applied will influ('nce i'esidual life. ~\.s insects make contact with a deposit, tlwy remon' some of the inscc·ticide and deplete the. sUJ{ply UO). DDT 1'I',\'5ta1:-; "blooming" from a, treated surface, if WIped away, will be replenished thro\lgh furtlwr crystalliziltion (17), Wip­ ing could b€', repeated seYC'ral times and still be followed by renewed ('l'ystal growth. HE'ilvier applieations could provide longer residual protE'ctioll by making more insecticide twailable for crystallization at the sur face. Pn,rtir.le sizE' !L1so afl'ects residual life. Smaller particles, especially of volatile insecticides, lose their efi'ectiYeness more ra,piclly than larger partieles (Jo, 40), The solvent can influence residual life. The residual toxicity of DDT deposits on copper screening was prolonged by using heavy sol­ vents like SAE oil No, 30 and No, 50 (.'31). Diesel oil was not so effecti ve, and kel"Osene and ar.etone solutions showed poor residual toxicity. Respraying with solyent "revitalized" crystalline deposits on copper screen ing. Presumably, insecticide crystals were redis­ solved by the newly applied solvent, making the deposit more available to the insect. The con tad toxicity to houseflies of DDT deposits on pine and spruce foliage was q:reatel' when the solvent was methyl ethyl ketone tlmn when it was No.2 ine1 oil (106). After aging outdoors for 78 days, toxicity of deposits with methyl ethyl ketone dropped from 100 to 90 percent, while deposits from the fuel oil solvent became com­ pletely nontoxic. Insecticides ditl'er in residual life among themselves. In tests of oil solutions of YariOl1S chlorinated hydrocarbons on filter paper, rela­ tive toxicity after 6 months 'YaS almost. reversed (jI!jI!) , The order of potency was initiany lindane> dieldrin> aldrin> DDT> toxa­ phene j at 6 months it was DDT = dieldrin = toxaphene> lindane = aldrin. Insecticides in order of decI'E'asing persistence of deposits on painted and unpainted wood \\,('1'(' dieldrin, aldrin, parathion, chlordane, and lindane (107). The dE'posits were exposed to the environment in Kansas from November to ~[al'ch. Average monthly precipitation was 2.17 inches. 18 TECHNICAL BULLETIN 1343, U.S. DEPT. OF/AGRICULTURE

Toxaphene, dieldrin, anel EPN deposits retained toxicity better un­ der a wiele range of climatic conditions than did aldrin, endrin,linclane­ DDT mixtures, isodrin, and malathion (99). 'Vettable powdel' deposits on fiberboard were bioassayed with Douglas-fir beetle adults after 2, 4" 6, D, and 1::3 \\P.eks of laboratory aging (9/i). Lin(lane, aldrin, heptachlor, and chlordane were greatly reduced in toxic'ity after 2 weeks :md became nontOXIC after 6 weeks. ~evin was intern1l'diate in persistence. Thiodnn, DDT, endrin, and i~odrin lo~t to~i('ity most slowly-not greatly affeded after 6 weeks, a Imost nontoxJ(~ after 13 weeks. The toxie life of a, deposit appears to be closely related to the vapor pressure of the inseetieicIl'. The half life (time for loss of half the deposit by \'olatilizntion) of lindane on impervious surfaces ,,,as one t",enl'y-fifth that of dieldrin and one-se\'entieth that of DDT (16). Lindane and parathion are volatile enough to be lost in "si~YJ1ificant amoullts" even before the spray droplets reach the target (35). Higher jemperatlll'e may greatly aceelerate evaporation. McIntosh (77) cal­ eulnted thai an inel'ease :11 temperature from 11 0 to 30 0 C. will raise Ynpor prl:'ssure of lin<1al1l' 21 times. Among the fe\\' pnhlished analyses of the persistence of insecticides Oil bark IS the stu(ly by Hobinson and ~resml:'r (94). They sprayed rofl'ee tree bark with 2-per{'('nl DDT emulsions. Artel' 40 weeks they analyzed the amount of insedicide remaining on the surface and inside the bark. Little elilferen('e was found between the respectiye Tates of loss or DDT. Heavy rain fell during the aging period. Ruclinsky et al. (lJC) analyzed the lindane, endrin, and heptachlor I'pmaining in Douglas-fir h'ark nftl:'r 10 weeks or exposure to the weRther. They recovered the following percents of original dosage: Lindane Endrin Heptachlor 50 30 70 20 19 50 22 39 55

The sud!H'e toxic'ity of this same. bark after 10 weeks was zero as gnged by bioassay with Douglas-fir' beetle adults. Exposure time was ,~O minutes. Bill'l\: treated with Thiodan and Sevin was included in the bioassay. All treatments gave about 10 percent or less kill, except Thiodan whirh !!il\'e Hi- to 20-pereent kill. Apparently much of the inserti('ide, seeped beneath the bark surface, becoming unavailab1e to tlw beetle upon superficial contact. E;1.'len(/('Iw.-The use of extenders to pl'Olong the residual life of \'olatiJe inse('tieides (espe('ially lindane) has been studied extensively (47, 4.9. 105, 106). ,Vorkers IU1\'e de\'oted special attentioll to the chlorinated terphenyl extendl:'r ArO('lor' 5-160. On aluminum, deposits of lindane in Aroclor-l to 1 and 1 to 2 mixtures-were less toxic to h

10 days for both insecticides. Results with DDT, however, were quite different; deposits were less toxic after aging when Aroclor was used (106) . The more volati1e the insecticide, the more effective is Aroclor in extending toxic life. The residual life of relatively volatile chlori­ nated hydrocarbon insecticides like aldrin, heptachlor, and lindane and tha.t of organic phosphate insecticides like Diazinon, malathion, and parathion is effectively incl-eased by adding chlorinated terphenyl extenders (48). Aroclor is thought to prolong toxic life by reducing the loss of the insecticide by evaporation. ",Vhen the carrier solvent evaporates after application to a surface, n, tacky mixture'of insecticide and extender remaills. Evaporation of the insecticide at the surface of the remain­ ing film causes more insecticide to diffuse to the surface and replenish the deposit (4-7,49). The insecn~ide-Aroclor mixtures do not crystal­ lize as does the insecti<;ide alone (105). . The lowel' the vap)r pressure of the insecticide, the more slowly it is lost at the surface of the jnsecticide-Aroclor film by evaporation. In­ secticides with low va.por IH"essures such as DDT decompose. at the film surface instead of evaporating and being l-eplaced by fresh in­ secticide. The insecticide becomes unavailable to the insect and the deposit is nontoxic. (48). This explains the observation by Sullivan et al. (ZOO) that DDT deposits were less toxic with Aroclor than without. STRUCTURE AND TOXICITY OF DEPOSITS For several reasons these studies of insecticide deposits were begun with the use of solutions. Tests had shown that the upright. crystal produced in solutions is the one most available to an insect contacting lL deposit. 'When insecticide solutions are applied to an absorbent substrate, the result. clln be a crystal 'bloom on the surface. But when snspensions are appl ied, the result is a deposit of small particles wholly on t he SUI" face ilnd the rapid loss of the volatile insecticides. Also, on rough porous stll'faces like bark, the finer particles of suspensions may be lost. to direct contact in the myriad, minute, bark-surface "pores." Emulsions require large amounts of water for a carrier; a controlled deposit of a eOllcentrated insecticide at the bark surface would seem diffie-lilt to achieve. But formulations othel' than solutions are not nec('ssaI"ily inferior, and should be studied. PROCEDURES Test Insects The test insects, California fi ve-spined ips (Ip8 confu.~WJ (Lee.)), were reared from ponderosa pine bolts, usually 1.5 to 3 feet long, in an outdoor, wire-scre~n insectary (fig. 1). ",Vhen adults were needed for bioassay, they were colleded, each singly, in No. 000 gelatin capsules, The insects were isolated in capsules to prevent them from damaging eaeh 01 her. The rea.ring cages were cleaned perioclicnJly so that adults more than 6 hours old from emergence were not used in the tests. 20 TECIDrICAL BULLETIN 1343. U.S. DEPT. OF AGRICULTURE

1!'-505871 FIOURE I.-Insectary for rearing test beetles.

Insecticides DDT, dieldrin, dinitrocresol, endrin, heptachlor, and lindane were chosen for study. These. insecticides were the most. promising resid­ uals in topical application on bark beetles in California 12 (65, 81). Te('}lll ieal DDT and heptachlor were purified by recrystallization from solution in ethyl alcohol, arriving compounds with melting points of 108° to 1000 and !l~0 to 06° '., respectively. The melting point of tIll' pure PI/ isomer of DDT is 109° to 110° C., and that of pure lwptachlor IS D5" to D6° C. (79). Dieldrin, clinitrocresol, and endrin were tlsC'd in the te('hniel.! gmc1C', but formulations were a11 based on the adh'e prilwiple. The lindane. was labeled better than 99-percent galllma isomer and was formulated as though pure. The insecticides were tested mostly in !t('()tone solutions. Acetone's high \"olatility enluUlces thl\ deye]opl1lent of crystalline deposits and its high clissoh-ing power eliminat.es need for adjuvants whose effect. on deposits is not. well understood. Substrate Compressed fiberboard, one-eighth-inch thick, cut in 3- by 3-inch seetio))s was used as the substrate. The dark brown color of the fiber­ board contrasted with the light color of the opaque to translucent. insecticide crystals. Both tiberbo(lrd and bark are cellular and ab­ sorbentso that the forces gOYel'l1ing deposit structure on each substrate

1! :\!oore, op. cit. STRUCTCRE A..'m TOXICITY OF rxSECTICIDE DEPOSITS 21 are probably basically similar. Moore 13 was the first to use fiberboard as a substrate for testing the contact toxicity of residual insecticides to bark beetles, and Rudinsh.-y and Terriere (95) later adopted it. Preparation of Deposits Deposits were made by applying insecticides to the fiberboard panels with a pipette (fig. 2). The quantity of insecticide applied could be held easily within nn error of '2 perrent, including errors in fornlU­ lation. The pipette was mOYed back and forth about an inch above earh panel as the liquid flowed drop by drop oYer the entire surface. 1Yhen application was completed, an apparently uniform and COll­ tinuous film of solution remained momentarily 011 the surface. The treated panel::; were mo\'ed to shelves and stored at room temper­ ature until ready for bioassay. No panels were reused. The micro­ scopic structure of deposits was described through a dissecting scope set usually at 45 or 90 power.

,. Moore, op. cit.

11'-506873 ~'IGl:&E 2.-Pipette used to apply insecticides to fiberboard panels. 774-4510-65--1 22 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE Bioassay Deposit.s were bioassayed by exposin~ Ips conju8u.s adults to them for increasing lengths of time, mostly from 5 seconds to 30 minutes. Time-mortality curves were usually plotted as a basis for comparing the toxicity of different types of deposits. Data were tmnsformed by the standard probit. method (.14-) or by a modified version (64-). The latter was used for some of the data on the eti"ect of weathering on the behavior of deposits. Each time-mortality cun"e depicts the activ­ ity of a. given deposit. It was obtained from bioassay usually with 100 to 200 adult beetles at 5 to 20 different exposure limes. A separate fiberboard panel was used for each sample of 10 insects. Moore 14 bioassayed insecticide deposits ,,"ith adult bark beetles by confining the insects to treated surfaces under a watehglass. Rudin­ sky and Terriere (95) later used a petri dish for the same purpose. In a modification of Moore"s \)\'ocedure, 10 adult beetles were re­ moved from their gelatin eapsu es and placed together in the con­ cavity of a 65-lI1m. wnJchglass (fig. 3). A fiberboard panel, treated surface down, was plueed o\"er the concavity and the watehglass and panel inveded to bring the beetles in contact with the deposit. Ex­ posure was timed with a stopwatch, and was ended by reverting the

" Moore, op. cit.

't,... 1'-506872 FIGURE 3.-In bioassay, beetles were exposed to a deposit under a watchglass. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 23 watchglass and panel. Most of the beetles fell back into the watch­ glass and were then replaced in gelatin capsules. A light tap on the substrate removed beetles that sometimes would cling to the panel. Control insects were exposed to untreated fiberboard panels. After treatment, the samples of 10 beetles each, enclosed individually in capsules, were held separately in open petri dishes in a basement room having a relatively constant temperature and humidity. Tem­ perature ranged from 61° to 74° F. during four summers of research; humidity ranged from 54 to 79 percent. In any 7-day period, tem­ perature fluctuations were from 2° to 4° F. at most. A holding time of 72 hours was used because Ips confwus begins to show measurable natural mortality, though usually less than 10 per­ cent, in about that time under conditions at the Miami Field Labora­ tory (table 1). After 72 hours, the insects were removed from their capsules and prodded with a dissecting needle. A beetle was classed alive if it moved, dead if it did not.

TABCB l.-Typical nat'~tral mortality of Ips confusus adults at the Forest Service Miami Field Laboratory

Holding time Cumulative mortality

1______Days _ Number Percent 2______o o 3 ______1 0.4 4 ______10 4.5 5______26 11. 7 6 ______73 32.9 7______121 54.5 8______173 77.9 9 ______201 90.5 10______210 94.6 11 ______219 98.6 12 ______221 99.5 222 100. 0

EFFECT OF FUMIGATION Some mortality may occur from the accumulation of toxic vapors under the watchglass during bioassay if exposures are long enous:h. Mortality from this cause would exaggerate the toxic effect of depOSIts, and would weaken estimates of the comparative contact toxicities of insecticides having widely differing vapor pressures. Tests were con­ ducted to find out if deposits did have a fumigating action under the conditions of bioassay employed. Materials and Methods A Coplin jar cover was substituted for a watchglass. Bark beetles were exposed to the environment under the jar cover by suspending them about 3 mm. above the deposit on a 40-mesh wire screen (fig. 4). The screen was cut with a slightly larger diameter than the inside of the jar cover and turned up on the edge to .fit snugly into the cover. The standard bioassay procedure was followed. 24 TECHNICAL 13ULLETIN 1343, U.S. DEPT. OF AGRICULTURE

Fiberboard Coplin jar cover Exposure chamber

FIGURE 4.-Diagram of a Coplin jar cover on fiberboard panel as used totes!: for toxic vapors during bioassay (natural size).

DDT

6 ­

20 500 1000 2000 20 SO 100 200 500 1000

fT) DINITROCRESOL ENDRIN !:' 7 :0 / ~ Q. 0/0 .!:; 5 ;;0 .~ o /00 ~ 3 o ~ 3_ ~ ~ 20 50 100 10 20 50 100 200 500

f HEPTACHLOR 7

0.5 10 20 50 0.5 10 20 Ex po sure time (minutes) (log scale)

FIGURE 5.-Mortality of Ips confusus adults exposed to vapors of six insecticide depoSits. STRUCTURE L"N"D TOXICITY OF mSECTICIDE DEPOSITS 25 Results Time-mortality curves (fig. 5) were plotted from the data of fumi­ gation tests. Table 2 shows exposure times needed for 10- and 50-per­ cent kills. The exposure time required for 10-percent kill was used to approxi­ mate the minimum exposure time for fumigation to begin under the watchglass. More than 30 minutes of exposure to the vapors of DDT, dieldrin, and endrin were needed for a 10-percent kill. The bioassay of insecticide deposits was rarely done with exposures of that length, so that mortality expressions for these three insecticides were pre­ sumed not to be complicated by a fumigating effect. Tests of the contact toxicity of lindane deposits also were presumed to be unaffected by a fumigating action. Lindane vapors rose to toxic levels in about 1¥2 minutes. But lindane deposits are ordinarily so toxic that exposures of 1 minute or more were rarely needed for estimates of contact toxicity. Dinitrocresol showed a 10-percent mortality from fumigation in 13 minutes. This insecticide was used only in studies on the effect of dose on toxicity, and some of the exposures exceeded 13 minutes. But these studies do not seem to be voided by this result. Toxic vapors in effect dampened, though not greatly, the mortality-dose expression. The mortality trend (fig. 6) would have been further exaggerated if fumigation had been prevented. The less toxic deposits, requiring long exposure periods, would have shown even less mortality. The most toxic deposits, requiring exposures so short that toxic vapors did not accumulate, would have been essentially unchanged. As with dinitrocresol, the bioassay of heptachlor was probably biased by a fumigating action. Confinement above heptachlor deposits for as little as 30 seconds produced a 10-percent kill. But, like dini­ trocresol, the data on the mortality-dose relation were dampened rather than nullified by the fumigation effect. Heptachlor was used only in tests of the effect of dose on deposit toxicity.

TABLE 2.-J.l:lortality of Ips confusus adJuZts ercposed to insecticide 1Japors

Vapor Insects LTIO and LT/iO and Insecticide pressure 1 treated standard standard error error

Mm. Hg at 25° O. Number Minute8 Minute8 DDT 2______1. 5X 10-7 200 59. 87±6. 88 101. 14± 6. 98 Dieldrin______1. 8X 10-7 180 67. 05±7. 71 130. 62± 12. 02 DinitrocresoL ______5 Endrin______5. 2X 10- 400 13. 06±3. 90 55. 47± 11. 48 2. OX 10-7 220 36. 96±4. 25 74. 94±5. 17 lfeptachlor______3. OX 10-4 150 0.57±0.19 2. 61±0. 42 Lindane 2______3.0X10-2 220 1. 40±0.16 2.45±0.17

I From N egherbon (83). 2 Vapor pressure determined at 20° C. 26 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

w :il z 0 on

0 ;: ~ . ~ ~ W '" I 0 ~ • N ~ ~ ~ ~ ~ ~ .. ~ N N 0 ..0 0 '"0 ci 0- '" <> ~ <> ;; '"0 <> 0 ci ci 0 0 0 ~ 0 U VI CO ..Q -1 o ~;:: II) ~ ~ 0 w o 0 a: a: u o -; ­ o z ~ w o 0 a: .. :J ~ z "' CTVI o ~ o N QJ ~ 0­ -~ E E CO g == l oJ o o '" o 'u; o 0 0­ 0'" 0 "'~ ~ I­ a: o o ~ o -1 W o ~ ~

0 '"

~

:;'

FIGURE 6.-Contact time for 5O-percent kill of Ips ccmtu8u8 adults e:s:posed to six insecticide deposits.

Discussion The relative toxicity of the insecticide vapor is prdbably determined by both the inherent toxicity of the compound and its vapor pressure. From them, the extent of a compound's fumigating effect is roughly predictable. Heptachlor and lindane are many hundreds of tlmes as volatile as DDT, hence their very high potency by fumigation. Dinitrocresol is about 350 times more volatHe than DDT and appears more toxic by fumigation, even though inferior by topical application. Dieldrin, ell(h'in, and DDT have simihu' va.por prmlsures and similar toxicities by fumigation. STRUITURE ~"D TOXICITY OF INSECTICIDE DEPOSITS 27 EFFECT OF DOSE Litt1e has been published about the effect of dose on deposit toxicity and, so far as known, nothing on the effect of dose on deposit structure. The few observations made by various authors indicate that deposit toxicity either is unaffected by dose or is directly re1ated. It would normally be expected that the more insecticide applied, the more toxic would be the resulting deposit. Materials and Methods Acetone solutions of aU six inse.cticides tested-DDT, dieldrin, di­ nitrocresol, endrin, heptachlor, and lindane-were studied at appli­ rations ranging from 40 to 5,120 mg. per square foot. We tried to begin the series at the lowest concentration at which some mortality could be expected with reasonable exposure periods (30 minutes or less). In nature the adult bark beetle remains on the bark surface for a matter of minutes at most. Exposures greatly exceecling this contact time were considered unrealistic. The age of the deposits differed somewhat at the time of testing because bioassay was carried out when crystal formation had reached near-maximum development in each insecticide. With some insecti­ cides such as lindane, testing was done when deposits were but 1 or 2 clays old because the deposits began to deteriorate (by sublimation) noticeably after 3 or 4, days. Other insecticides, like DDT, dieldrin, and l'ndrin, were much more durable, allowing more freedom in the timing of bioassay. Results The insecticides fell into two groups, depending on how the deposits responded to changes in dose. Toxicity of om~ group, endrin and heptachlor, continued to increase as dose incrl'ased; that of the other group-DDT, dieldrin, dinitrocresol, and lindane-did not increase consistently with progressively larger doses, but tended to be highest at some intermediate point in the range. These latter insecticides fell off in toxicity as the amount of insecticide fluctuated from the optimum level. This behavior will heI"ek'tfter be called "dose effect." If the amount of insecticide applied was raised high enough, the toxicity of the deposit generally improved again, though not grell.tly. The effect of changes in dose on toxicity C.!1Il be shown by plotting the LTso values withstanclat"d ('rrors (fig. 6, p. 26; table 3). The in­ tensity of the "dose effect" varied. It was particularly strong with dieldrin and dinitrocresol. and weaker with DDT and lindane. The (,hanges in toxicity that followed increasing dose resulted from changes in deposit structure. Some terms used to describe deposits are defined as follo;vs: Border crystals.-Crystals arising at the borders of crystal patches that arc usually much longer than the general body of crystals. OO'l'el'age.-The fraction of a giyen area clothed by insecticide particles. 28 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

Density.-The number of insecticide particles per unit area, or the closeness of spacing of these particles. IncrlJ,8udion.-An indistinct granular deposit often undedyin~ areas of crystal growth, composed of very small (less than 51l) amurphous particles. PartZ:r:le.-The discrete microfragments of a deposit, whether crystalline or noncrystalline and amorphous in nature.

TABLE 3.-LT50, slope, and 1'egl'eS8ion equation.s j1'om exp081u'e of Ips confusus adults to in.sectic-ide dep08it-8 of inerea8ing dose

Insecticide and dose Insects LT50 and Regression (mg. per sq. ft.) treated standard error equation: y=-

DDT: 80 ______Number l\[inutes 160______80 (1) ------­ 320______80 9. 12± 1. 47 2. 862+0. 676X 640______\ 160 2. 06±0. 33 O. 691 + 1. 862X 1,280______200 1. 52±0. 31 2. 701 + 1. 053X 2,560______180 5. 37± 1. 36 1. 298+ 1. 356X 180 1. 42±0. 26 1. 722+ 1. 523X Dieldrin:40______80______150 5. 04± 1. 29 0.945+ 1. 481X 160______220 O. 21±0. 03 2. 211+2. 097X 200 (2) 320______----2~ -166-+2~ g-i7X 640______150 O. 1O±0. 02 1,280______150 O. 52±0. 14- 2. 268+ 1. 593X 2,560______150 O. 82±0. 21 1.213+1. 978X 180 O. 47±0. 06 1. 423 + 2. 133X Dinitrocresol:80______160______50 28. 25± 18. 84 -7.437+3. 129X 320______150 1. 28±0. 18 -0. 051+2. 396X 640______300 O. 97±0. 11 O. 227+2. 404X 1,280______200 4. 73±0. 54 -0. 395+2. 017X 2,560______150 11, 75± 1. 35 - 4. 328+ 3. f)38X 220 8. 4l±0. 97 3.161+2. ',90X Endrin:80 ______1 160______150 2. 05±0. 38 O. 396+1. 991X 320______150 O. 73±0. 10 O. 864 + 2. 223X 640______. 150 O. 21±0. 06 2. 484+ 1. 898X 150 O. 06±0. 02 3. 513 + 1. 903X 1,280_ -- --. -- _------___ I 250 O. 02±0. 01 4. 235+ 1. 963X 200 O. 02±0. 01 4. 455+ 1. 573X Hep/d~~~~; - .------1 16080------1______150 4. 87±0. 15 2. 475+0. 939X 150 3. 91±O. 72 0.459+1. 752X 150 1. 24±0. 23 1. 715+ 1. 570X ! 1,2801:~::::~::::::::::::1______150 O. 41±0. 06 1. 422+2. 214X 2,560 ______210 O. 07±0. 02 3. 993 + 1. 176x 5,120______330 O. 03±0. 02 4.331+1. 340X 150 O. 1O±0. 02 2.562+2.473X Lindane:80______160 ______180 O. 35±0. 04 O. 790+2. 718X 320.______170 O. 02±0. 01 4.456+ 1. 855X 640______160 O. 05±0. 01 3. 038+2. 924X 180 O. 07±0. 02 2.812+2.594X .1,280 __..______3.081+2.702X 2,560______190 O. 05±0. 02 ! 180 O. 12±0. 02 1. 373+3. 370X

1 Deposit nontoxic. 2 LT.IO less than 0.05 minute. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 29

DDT.-DDT deposits exhibited a weak dose effect (fig. 6, p. 26). Toxicity was high at 6-10 mg. and again at 2,560 mg. Coverage at the MO-mg. level was 100 percent; crystals were most dense, some were 100!' long (table 4:). The reappearance of border crystals at the 2,560-mg. dose probably accounted for the high toxicity.

TABLE 4:.-0lmracteristics of DDT deposits

Dose Cover­ (mg. per age Particle shape Particle size Remarks sq. ft.)

80______Percent 0 ------­ No deposit visible. 160______(1) -G;a-;ti"l;'~;---- Less than 51'______Very sparse amorphous. deposit. 320______50 Acicular and 101' to 201' long, Highly variable granular. usually less than coverage. 51' wide; border crystals 501' to _____ do ______1001' long. 640______100 101' to 201' long, Greatest density, some 50!, to 1001' light incrusta­ in scattered tion. clusters. 1,280 _____ 100 _ ____ do______101' to 401' long______Less dense than at 640 mg., light _ ____ do______incrustation. 2,560_____ 95 101' to 201' long, Uniform coverage, border crystals deposit less dense 501' to 100!, long. than at 1,280 mg., light in­ crustation.

1 Negligible.

Dieldr·in.-The dose effect with dieldrin was pronounced (fig. 6, p. ~6). Toxicity WilS maximum at 160 mg.l~ A tabulation of deposit dutl"llcteristics (table 5) helps to explain the trend in toxicity. The 160-mg. cleposit consisted of a dense mat of long acicular crystals (fig. 7), which appear to be the most toxic form of crystal. An incrustation was not as toxic as the crysbds, as evidenced by the 40- and 1,280-mg. deposits in which only an incrustation developed. A deposit of unusual structure develo£8d at the heaviest dose of 2,560 mg. Dinitroc·/'esol.-Deposlts of dinitrocresol, Eke dieldrin, showed a prollounc,ed dose efl'(>('t (fig. 6, p. 26). The 3:20-mg. deposits displayed the maximum toxicity. The drop in toxicity at 640 and 1,280 mg. was striking, as was the diminishing density of insecticide particles. No deposit whatever could be detected at the 1,280-mg. level. Again, a conspicuolls but uncommon deposit structure developed when the amount of insecticide applied was very high (table 6). Deposits of 80 anc11,280 mg. showed some toxicity even though the deposits themselves were not yisible. Some undetectable depOSIt may have been present, but probably fumigation caused some of the mor­ tality observed at these two dose levels.

,. An LT", value could not be estimated because of tbe extreme toxicity of the lBO-mg. dePQSit. 30 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

TABLE 5.-0ha'racteristics of dieldrin deposits

Dose (mg. per Cover- Particle shape Particle size Remarks sq. ft.) age

40______Percent (1) Granular, Less than 51-1 ______Extremely light 80______amorphous. incrustation. (1) Acicular ______201-1 to 5001-1 long, Very sparse mostly 1001-1 to crystals. 2001-1; less than 160______do ______51-1 dowide. ______95 Greatest density, uniform cover­ 320______do ______do ______age. 640______50 Variable coverage. (1) Acicular and 201-1 to 1001-1 long_____ Crystals rare, granular. light incrusta­ 1,280_____ tion. (I) Granular, Less than 51-1 ______Only a very light amorphow,. 2,560 _____ incrustation. (1) Amorphous___ Indeterminate______Milky white mounds, 501' to 1001' in diameter.

1 Deposit boundaries indefinite because of low density or small size of insecticide particles.

TABl~E 6.-0ha1'acte1"istics of dinitToC1'esol deposits

Dose Cover­ (mg. per age Particle shape Particle size Remarks sq. ft.)

80______Percent 160 ______0 -.. _------No deposit visible. 90 Acicular to 201' to 1501' long; Uniform coverage; platelike. less than 51' to much variation in 251-1 wide. size, shape, and density of par­ ticles; light 320______incrustation. 100 _____ do_- _____ Same, but some crystals 3001-1 640 ______do______long. (I) ------Deposit virtually lacking; crystals rare. 1,280_____ 0 ------No deposit visible. 2,560_____ 90 - Gr~·;;~;~: -- -- Less than 51-1 ______Uniform coverage; amorphous. amorphous mounds, mostly 201-1 to 601-1 in I diameter, with multiple peaks. --- I I 1 Negligible. Fr'f1 :~:. 7' I~l ... ('\.·!~··;'tl~· ~h·pu.... it ...... hu\\iD.~ tYl'if'nl f·ry~tal f"rIll. :1. 1~11H\ ueirulnr ':', " I:~ ,.1' l.lt'!' • :XI" N, '.,rd.,!" 'Tystals of IllYI'I >< ,I;;); (', tiIll'. neir-ula!" "I'> ~·.1,' .f '!It·.dr:l. , • !~J I • /1, g"[";l!lular ,l"ll{),':t uf I'utiriu ('()Ul~('

Hll.! .1!U ,r;-h )[1:""< i)ar~4~·~t"S t '>I ~Hll 32 TECHNICAL BULLETh,\," 1343, U.S. DEPT. OF AGRICULTURE

En(l1in.-Endrin deposits increased in toxicity c.onsistently with each increment of insecticide (fig. 6, p. 26). DeposIt structure was con­ sistent with the trend in toxiCIty (table 7). Coverage and density improved with each increase in application; otherwise each succeeding deposit resembled the previous one. The particles that composed the deposits were typically cuboid or ilTegula.r (fig. 7, p. 31) and did not vary not,iceably in size and shape from one application to the next.

TABLE 7.-0lwracte-l'iBtic8 of end1'1'n dep08it8

Dose Cover­ (mg. per age Particle shape Particle size Remarks sq. ft.)

Percent 80______L60 ______------.. ------No deposit visible. ° ------Do.1 320 __ ... _ 50° -c"'u-b;i~r o~ i~--- 10", to 40", in Variable coverage i regular. diameter. particles very sparse, closely appressed to 640 ______do______substrate. 80 _ ____ clo ______Variable coverage i particle density _____ do______do. ______increasing. 1,280____ . 100 Particle density _____ do______do______increasing. 1,920. ____ 100 Greatest particle I density. I There may have been a very sparse scattering of particles 5", or smaller.

Endrin deposits were unique in being remarkably uniform in density and particle {onn from panel to panel at any given dose level. But they did vary considerably in coverage within tests, as did deposits of all insecticides. FJeptach107'.-As with endrin, the toxicity of heptachlor deposits (fig. 8) was enhanced by each increment in dose except that from ~,560 to 5,120 mg. The 5,1:20-mg. deposit was less toric (fig. 6, p. 26), which is not consistent with t!le observed deposit structure. This de­ cline may have been due to experimental error rather than to a real difference in potency. Heptachlor deposits never exceeded 10 percent in coverage (table 8), but were nevertheless highly toxic, Applications of 80 and 160 mg. were toxic notwithstanding the ab­ sence of a visible deposit (fig. 6, p. 26). The LT60 for heptachlor vapors was about 0.6 minute, so that mortality from exposure to 80­ and 160-mg. deposits was probably due largely, if not entirely, to a fumigation effect. Part of the toxicity of the 320 mg. deposits may a.lso have been caused by toxic vapors. The curve of LTso values for heptlU'hlor deposits (fig. G, p. :W) \\'ouId be steeper if there had boon no fumigation. Lincirme.-Lindane PI'o\'ecl extremely toxic. The dose effect was clampcnecl by the uncommon toxicity of the insecticide, and pronounced changes in deposit structure did not greatly afi'ect toxicity. 33

F<' <':1" lr>.,.<'i-".\t· ,krK"it"s 'h"\\ill,~ typh'nl ('ry~tfil form. ,I. Thick. 14tllhhy ~,,', , ,1" ',Pi":! 1,_,:- • !Hl,; 11. phllplik(' I'ry-tub or lindnJl(' • x: 1(0); ('. ~:. !:... ~, lr ~.'r" ,f ~.daljt' r11iU fli (''Irs at hi~h Ip,·pI~ (If dosp I X 4;)); [J. < , ' " «<.' h.<,j HI' ..I, it·;.: "f a·llll! har!. 1...,<11", ,/'" l'OIt/II,

TABLE 8.-0haractelutic8 0/ heptachlor deposits

Dose Cover­ (mg. per age Particle shape Particle size Remarks sq. ft.)

80______Percent o No deposit visible. 160 ______o Do. 320 ___ . __ Deposit virtually lackingj particles rare. 640. __ • (I) Thick colum­ 1O}l to 40}l in Sparse deposit. nar, cuboid, breadth and or amor­ width, 20}l to -I phous. 100}l long. 1,280•• _ (I) _____ do______. ____ do._. ______Particle density "I increasing. 2,560__ 10 r... --dO- ..._.L. ___ do_ ..• ____ _ Uniform coveragej .\ maximum particle ,t density. 5,120__ . . • ! 10 l-----do•. _---- _____ do_ - ______1 Do. .\

1 Negligible.

The greatest toxiC'ity was at 160 mg. and coincided with maximum development of plnJe crystals (table 9). These crystals appear to be rhe most toxic. form of lindane. (fig. 6, p. 26). As dose was increased, a loss of toxicity followed the drop in number and final loss of plate (Tysbds. Even [tn incrustation of lindane (fig. S, p. 33) was highly toxic, though jts partirJes were tightly appressed to the substrate and probably less !wailnble to the insect. The SO-mg. dose showed no yis­ ible deposit. Its toxicity may have been due to a fumigating effect during bioassay.

TABI.B 9.-0h(LracteriBtiC8 0/ lindane deposits

Dose Cover­ (mg. per age Particle shape Particle size Remarks sq. ft.)

Percent o No deposit visible. 40 ------~---~~Broad, thin, ~~------~-1O}l to 200}l - Variable coveragej wide platelike. wide, 501l to variation in cryatal _____ do______300}ldo ______long. _ size and shape. 320 ____ •• Very sparse deposit. 640. ____ _ Granular, Less than 51l ___ _ Only light incrustation. amorphous. 1,280... ______do.______do______do_____ . _____do______Do. 2,560•• _'_ Incrustation appears heavier than on pre­ vious 2 deposits.

I Deposit boundaries indefinite because of low density or small size of insecti­ cide particles. STRUcruRE A~D TOAICITY OF INSECTICIDE DEPOSITS 35 Discussion An important concept in the study of deposit behavior is "crystalli­ zation threshold," or the level of dose at which crystallization charac­ teristically begins. If an insedicide never crystallized at tho bark surface after having first soaked in, the deposit would have little or no surface contact toxicity. The lower the crystallization threshold, the more efficient the insecticide may be considered as a surface deposit when applied to bark as a solution in oil. Dieldrin had an especially bw threshold, whereas endrin and heptachlor had high thresholds. Co\>erage approached or reached 100 percent at some level of dose with most of the insecticides studied. But heptachlor and lindane were conspicuous in showing poor coverage at all levels of dose. Hep­ tachlor dl.'posits developed a maximum coverage of 10 percent, and lindane 40 percent. ('overage above 10 percent had little or no effect on toxicity; when below about 10 percent, mortality fell sharply. Theshape or the particles composing the deposit was vital to toxicity. This effect was shown especially well by dieldrin, dinitrocresol, and lindane deposits in which discrete, upright crystals gave way to a granular form as dose was increased. Toxicity dropped markedly whenever this change occurred. For this reason too much of some insecticides, as well as too little. can be applied when seeking the opti­ mum contact toxicity. The insecticide deposits rrom a given formulati.on usually varied widely in size and shape of particle, density, and coverage. Wide differences were found not only from panel to panel, but also from one area to the next on a given panel. Endrin deposits were the least variable, dinitrocresol deposits the most. The tendency to vary may indicate an insecticide's pI iability. It may indicate the extent to which the deposit can be intentionally manipulated to conform to desired standards. Dieldrin and endrin arc similar in chemical structure, but differ radically in deposit structure. Dieldrin crystals were acicular, endrin crystals ::,rranular. Deposits of dieldrin and DDT were similar though the two insecticides differ greatly in chemical structure. Both dieldrin and DDT deposits contained acicular crystals of similar structure, though the dieldrin crystals were generally longer and broader. Diel­ drin deposits did not contain border crystals nor the heavy incrustation typical of DDT deposits. In tests described in a later section, insecticides were used against f p8 ('onfu,~us adults in caged ponderosa pine bolts. Toxicity was far greater than could be explained solely by contact with a surface residue. In practice, toxicity of a deposit may result from more than one mode of exposure (e.g., contact, fumigation, and stomach action). Thus the toxicity of a given insecticide in the laboratory as determined by contact may be complicated by other forms of toxic action. 36 TECHNICAL B1JLLETIN 1343, U.S. DEPT. OF AGRICULTURE EFFECT OF CONCENTRATION Materials and Methods The effect of concentration (weight of insecticide per volume of sohrent) on toxicity of deposits was studied with DDT and dieldrin. The deposits were bionssHyed with a constant I-minute exposure. Tests were replicated 10 times (100 insects) with DDT deposits and fi times (50 insects) with dieldrin deposits. A sel'ies of increasing dose levels "'as chosen for e:tch insecticide that would bracket the close shown pl'e,-iously to haw optimum potency. The close SHips wpre, obtained by applying' n fixed spray ,'olume and incl'C'asing the concentration, and by inerea:;ing' the yolume and keeping' the concentration constant. Dieldrin deposits were 1 month old when tNlted, and DDT deposits 2 months old. Results DDT.-The eft'ect of concentration on DDT deposits was slight (table 10). No 8i::-,n ifirant differences were found in mean mortalities whethel' conrentmtion was fixed or increased in g'eometric progression except at the 640-mg, dose le,'els. At this close the difference between mean modalities was si!-,TJ1ificant at better than the 2-percent level of confidence (t=2.84, d.f.=18) , The deposits were virtually identical tlt a given close whether con­ centration was fixed or not. EYen at the 640-mg, dose in which mor­ talities 'were significantly different, no differences in the deposits could be detected.

TABLE 10,-Effect of conrent7'ation on torJ:icity of DDT dep08it8 to Ips conTusus amults

Mortality Concentration (insecticide Dose Volume Cover- Insects and to solvent in mg. per Ill!.) applied age treated standard error

lo,/g. 7Jer IncrellSing: sq. ft. MI. Percent Number Percent No treatmenL ______10 ______------90 4 20______160 I (I) 100 52±5 40______320 1 25 100 79±2 80 ______640 I 95 100 62±4 1,280 1 95 99 67±5 Constant: No treatment. ______20______------­ 90 4 20______160 0.5 5 100 59±3 20______320 '1 25 100 75±3 20. ______._ 640 2 95 100 44±5 I, 280 4 95 100 65±6

1 Coverage less than 5 percent. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 37

The crystals were acicular, ranging in length from 20p. to 500p.. Most were fine and about 5p. wide, a few reaching 20p. The great body of crystals was less than lOOp. long. Border crysta:ls were gen­ erally lOOp. to 500p. long. An incrustatlOn existed, but could not be resolved clearly. Dieldrin.-Concentration had a marked effect on dieldrin deposits {table 11). At a fixed concentration, all dose levels were equally toxic, krlling about 85 percent of the beetles. Doubling the concentration with each increment of dose led to optimum mortality at the l60-mg. level. Crystal structure was indistinguishable from that of DDT deposits. :Most crystal growth was in the form of needlelike crystals, with slight incrustation. The crystals were similar in structure whether concen­ tration was fixed or increased: 20p. to 300p.long, but mostly 50p to l50/L, and from less than 5/L to 20/L wide. Concentration definitely affected coverage. Covera~e on the 80­ and 64g-mg. deposits was greatly reduced in the series In which con­ centratIon was not fixed. Discussion The important influence o.f concentration on dieldrin deposits seems to be exerted on crystal coverage and not on crystal form or density. But covemge ('.nn vary widely before differences showup in mortality.

TABLE 11.-Effect of concentration on torcicity of dield".in deposits to Ips confnsus adults

Mortality Conc('ntration (insecticide Dose Volume Cover- Insects and to solvent in mg. per mt.) applied age treated standard error

Mg. Iter Bq. t. Ml. Percent Number Percent Increasing: No trcntmcnL ______5_____ . ______------137 7 10______80 1 ll) 50 28±5 20______160 1 85 50 98±2 40______320 1 80 50 72±10 640 1 (1) 50 50±5 Constant: No treatment______10 ______------137 7 10______80 0.5 45 50 82±8 10______160 1 85 50 84±4 10______320 2 75 50 88±5 640 4 25 50 84±4

I Coverage less than 5 percent. By implication, dieldrin deposits of different dose may show ex­ aggerated differences in toxiCIty if the dose series is broad enough and made by adj!1sting the concentration of the spray. On. the other hand, the close etl'ect may be less pronounced if the series is made by changing the vohmlc at a fixed concentration. 38 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE EFFECT OF SOLVENT Many workers have shown that the solvent is important in deter­ mining toxicity of deposits. But few have ~xamined its effect on deposit structure on absorbent substrates of plant fiber. Materials and Methods Deposits of DDT were tested in benzene, xylene, kerosene, and diesel oil at three dose levels-320, 640, and 1,280 mg. per square foot-and in acetone at two levels-320 and 640 mg. per square foot. The stand­ ard bioassay procedure was foHowed. Results The kerosene and fuel oil deposits of DDT did not crystallize even after aging for 2 months, and therefor!' had no contact toxicity. Xylene deposits at 320 and 640 mg. also did not crystallize and were nontoxic. All other formulations showed crystal growth. Table 12 includes a description of the deposits and theLT50 values.

T.\B1,Jo] 12.-St)·uct11're and toa:icity of DDT depo8it8 7lBing different 8olvent8

Solvent and Cov- Particle Particle Insects dose (mg. erage shape size treated LT50 Remarks per sq. ft.)

Per- ilJicrons Num- Min­ cent ber utes Acetone: 320______30 Acicular 20-100 290 1.3 Variable coverage and (5 to 90 percent), granular. light incrusta­ 640______do______tion. 40 20-60 320 2. 1 Variable coverage (10 to 90 per­ cent), crystals muoh less dense then above, light inorustation. Benzene: 320______do_____ 35 50-100 240 1.1 Crystals fine (less than 51' wide) and dense, light incrusta­ 640______do_____ tion. 100 50-100 239 .9 Do. 1,280_____ 100 Gran ular___ (I) 240 1.9 Incrustation only. Xylene:320______No deposit visible. 640______------_ .. ------Do. 1,280 _____ 40 Aoicular 20-100 280 1.2 Crystals fine and (less than 51' granular. wide) and dense, light incrusta­ tion. Kerosene 2 __ ------No deposit visible. Diesel oil 3 __ ---- .. _------1------Do.

1 Less than 101'. 2 Doses 320, 640, and 1,280 mg. per square foot. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 39 Discussion Solvent influenced deposit structure far more than toxicity esti­ mates indicated. Deposits that crystallized gave 50-percent kill in about 1 to 2 minutes of exposure. As Barlow and Hadaway suggested (7), the influence of the different solvents on deposit structure seemed to be related to their boiling point. The high-boiling kerosene and diesel oil solvents (b.p. 170 0 + C.) caused no crystallization at any dose level. Xylene (b.p. 140 0 C.) deposits did not crystallize at the two lowest dose levels. Acetone (b.p. 56 0 0.) and benzene (ib.p. 80 0 C.) produced by far the most abundant crystal blooms. The dose threshold at which crystallization began also was related directly to the solvent's boiling point. Other, less conspicuous effects produced by the different solvents included differences in crystal form and density. Crystals were usually much finer and more dense when benzene and xylene were used as solvents. All crystals were acicular and exhibited some clumping typical of DDT. Coverage was greatly affected by the solvent. Benzene deposits gen­ erally showed superior coverage. Coverage was zero on all panels treated 'with the two heaviest solvents, kerosene and diesel oil, and on some of the treatments with xylene. The benzene deposits that developed at the intermediate dose level were the most toxic. Needlelike crystals were completely absent at the heaviest dose. EFFECT OF MOISTURE CONTENT OF SUBSTRATE No references were found on the effect of moisture content of sub­ strates of plant ori~n on contact toxicity of insecticide sprays applied to them. The vanable nature of deposits on fiberboard is possibly caused in part by differences in moisture content of the panels at the time insecticides are applied. Materials and Methods Tests were conducted with DDT-acetone and DDT-xylene solutions. Acetone is soluble in water in all proportions and xylene is insoluble. These extremes of solubility were chosen in the event that water sol­ ubility would affect deposit behavior. Duplicate deposits of the two types of solutions were prepared on both dry and moist fiberboard panels. Some panels were dried in an oven until weight loss averaged 3.9 percent. Others were moistened in a desiccator over a, free wat.er surface until weight gain averaged 3.3 percent. This artificial control of moisture content produced a range of 7.2 percent between maximum and minimum weights. In contrast, the daily range in weight of fiberboard exposed to labo­ l'rutory conditions was only 0.24 percent. Ponderosa pine bark showed a range of 0.61 percent. Panels dried in the oven were allowed to return to room tempera­ ture before they were treated. During this time, they reabsorbed about 11 percent of the weight of water removed in the drying process. The deposits were bioassayed by exposing groups of 10 insects, replicated five times, to the deposits for 1 minute. 40 TECHNICAL BULLETIN 1343, U.S. DEl'T. OF AGRICULTURE Results No important differences were found in structure or toxicity of the deposits of DDT that could be ascribed to the wetness of the sub­ strate (table 13). Standard errors overlapped in all tests but that of the DDT-acetone, 640-mg. deposits. However, a t-test of these 640-mg. deposits showed no significant difference at the 0.05 level of confidence (t=1.52, d.f. = 8) . A dose effect developed in both formulations and was more pro­ nounced with acetone. The xylene deposits were unaccotUltably more toxic than the acetone deposits at the two upper dose levels in spite of more favorable coverage on the latter.

TABLE 13.-Towicity of DDT deposits to Ips confusus adults on d'I'Y and nwist jibe?'boa1'd panels

Coverage I Corrected mortalitv I and standard error Solvent and dose (mg. per sq. ft.)

Dry Moist Dry Moist panels panels panels panels

Acetone:320 ______Percent Percent Percent Percent 640______20 20 72±7 82±6 1,280 ______85 90 62±4 44±11 95 (1) 34±4 28±5 Xylene:320______I 640______(2) (2) 74±5 66±1O 1,280 ______10 15 96±2 98±2 85 90 86±5 80±13

1 Coverage greater than 95 percent, 2 Coverage less than 5 percent.

EFFECT OF WEATHERING Surface deposits on bark normally can be expected to weather and erode. Information on the effect of weathering on toxicity would help in estimating the probable residual life of an insecticide and in timing and spacing of spray operations. Although weather can have a pro­ found effect, little information is available on its influence on deposit structure. Materials and Methods

DDT.-Earlier tests with DDT used procedures slightly different from those of later studies with dieldrin and endrin. DDT deposits were prepared at three dose levels: 320, 640, and 1,280 mg. per square foot. All deposits were made up on the same day so they would be exposed to identical weather. The deposits were bioassayed after 1 month of weathering and again after 2112 months. The sample size was 10 insects replicated 10 times in tests of the I-month-old deposits and 10 insects replicated 5 times in tests of the 2Vz-month-old deposits. All exposures were 1 minute. STRUCTURE A..'TD TOXICITY OF INSECTICIDE DEPOSITS 41

The treated panels WE1:e placed so as to provide four intensities of exposure to the environment : L.-Exposed on shelves, horizontally, in an open building where no direct and little indirect sun1ight reached the deposits. These are termed r.'L" deposits, and their disposition simulated deep shade. They are used as a standard by which the weathered deposits below could be compared. J.f, J.f1"V.-Exposed on tables in an open, sunny area, standing on edge with the deposit facing east, l\f, or south, MM (fig. 9). Sunlight impinged at a wide angle of incidence on these deposits. This ex­ posure was intended to simulate partial shade, or the exposure re­ ceived by the sides of fallen logs or standing trees. H.-Exposed on tables in an open, sunny area, lying fiat, for maxi­ mum exposur·e to the sun, H, simulating the effects of no shade. Letters above wi11 be used to refer to both the deposit and the con­ ditions of wenthering. For example, L deposits are those that re­ cei ved L exposure. Dieldrin and end7'in.-Deposits were prepared at dose levels of 80, :320, and 1,280 mg. per square foot. The deposits were bioassayed after 2 and6 weeks of weathering, usually with 10 to 15 increasing exposure times (100 to 150 beetles per regression). The data 'were transformed by the approximate method of Litchfield and 'Wilcoxon (72). The treated panels were weathered like the DDT deposits, except that M deposits faced north and there were no illr deposits.

11"-506877 Frnl'RE 9.-Fiberboard panels treated with insecticides were weathered to test the effect on deposit toxicity. 42 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE Results DDT.-Table 14 shows the effectiveness of the deposits. 'Weathering did not always weaken deposit toxicity, but strength­ ened it under some conditions. The magnitude of the effect on toxicity depended on both the degree of weathering and the dose. M and MM deposits showed similar results and so are de,\lt with jointly and refen·ed to henceforth by the letter M. In general, M exposure caused an increase in deposit potency at all dose levels. H exposure generally increased toxicity at the two heavier dose levels, and reduced toxicity at the lowest dose level. The dose effect was altered by weathering-. L deposits behaved as would be expected from laboratory tests: deposit toxicity decreased as dose was mcreased. M deposits tended to be similar, but showed a less pronounced mortality trend. H deposits, in contrast, showed a rtwerse trend of increasing toxicity with increasing dose.

TABLE 14.-Effect of wecttherinq on toxicity of DDT de7J08its to Ips confusus adults [Percent kill and standard error]

Toxicity 2 at dose (mg. per sq. ft.)- Exposure symbol l 320 640 1,280

AFTER 1 MONTH

L ______354±5 t 33±3 328±4 ~I ______79±4 82±4 50±6 M~f ______H______82±5 366±6 t 59±4 525±6 55±6 74±4

AFTER 27~ MONTHS L______84±5 48±5 o 56± 10 ~I ______90±3 96±2 94±4 wI~I ______H______96±3 96±4 74±6 544±7 354±9 380±5

1 Symbols follow order in which exposures are described in text. L exposure was deep shade, M and MM exposures were partial shade (M deposits faced east, MM deposits faced south), and H exposure was maximum sun. 2 Differences, as footnoted, are statistically significant (0.05 level). J Less than M. t Less than H. 5 Less than L. o Less than MM. Weathering had a fronounced effect on deposit structure. L de­ posits were typical 0 those studied in the laboratory. They were composed of granular particles, acicular crystals lOp. to 150p. long.; and border crystals up to 600p. long. Land M deposits were matted and so fine as to appear cottony. This change was probably due to the action of wind and visiting insects. Coverage appeared to be the largest variable, decreasing in the order of L to M to H. STRUCTURE A...~D TOXlCrry OF mSECI'ICIDE DEPOSITS 43

DieZdrin.-Deposit toxicity was usually but not always reduced by weathering (fig. 10). The lightest, SO-mg. deposits were severely impaired after both 2 and 6 weeks of weathering. The intermediate deposits ot 320 mg. were unaffected after 2 weeks of weathering, but moderately impaired after 6 weeks. The heaviest deposits, 1,2S0 mg., were unaffected by M exposure, but showed moderately to greatly increased toxicity under H exposure. The typical dose effect ,vas not usually expressed in the weathered deposits. The L deposits showed a trend of toxicity decreasing with dose, a trend consistent with earlier laboratory findings. The dose effe('t was obscured in the deposits weat-hered fOI" :2 weeks. In deposits weathered for 6 weeks, the mortality trend was reversed, mortality in­ c['easing with close. MierosC'opic struetlll"e of deposits seemed consistent. wit.h toxicity. Particle dimensions were similar to those of DDT. In general, weath­ ering tended to reduce or eliminate 11 IH'i!!ht, acicular crystals and to increase fhe gml1l1lar type of deposit. Though alll,2S0-mg. deposits consisted entirely of granular particles, the H deposits were much heavier and had better coYemge. This difference may explain the higher toxicity of these H deposits. Some weathered panels were not used, but left in their respective positions outdoors over winter. "Then these were reexamined a year later, some were still blanketed with what appeared to be highly toxic residues. A bioassay (table 15) showed that these l-year-old deposits were more toxic than the 2- and 6-week-old deposits (cf. fig. 10, p. 44). The structure of the l-year-old dieldrin deposits differed from any lleret{)fore described. Particles were usually lOp. to 20p. and either (1) granular, with plane surfaces or amorphous, resembling endrin, or (2) platelike but smaller than the platelike lindane crystals. Both types were extremely toxic. The development of endrinlike deposits of dieldrin might be ex­ pected because the two inserti('ides are similar in molecular stTIlcture. They are cyclodiene. compounds with identical molecular formulas characterized by an endomethylene-bridge structure; dielddn is the endo-exo-isomer and endrin the endo-endo-isomer. Some of the ~f panels of 1,280 mg. per square foot dose fell, deposit fac!;' down, on the tables where they were exposed to the environment. Their deposits were. shiel(led from sunlight and the direct impact of min and Snow. These panels had no visible insecticide residue, where­ as their upright counterpart sllOwed a heavy deposit. This observa­ tion supports the hypothesis that exposure to certa.in environmental factors can greatly enhance toxicity of deposits of dieldrin, even over long periods. Endrin.-On the whole, weathering impaired the potency of endrin deposits (fig. 10, p. 44). Six weeks of weathering diminished toxicity at all dose levels. But 2 weeks of weathering was not entirely adverse. The lightest deposits, SO mg. per square foot, were greatly enhanced in potency. Results at 6 weeks showed the typical endrin close-mOl·taLty trend of increasing toxicity with increasing dose. This trend was con­ Jused or actually reversed after 2 weeks. 44 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

50 01 ELDRIN DIELDRIN 1\ 2 weeks 6 weeks 20 \

10 5 k\\\ \ \ 2 \\ \\

0.5

0.2 ,, \ -qj \ 0.1 \ (5 \ (j \ VI 0.05 en \ Q L deposits \ ;::: \ Vi M deposits 0.02 ~ "5 H deposits ~~------~------~--~ ·S 50 -S ENDRIN ENDRIN 20 2 weeks 6 weeks ~ t=:: 10 5 k\ \ \ 2 \ \ \

0.5

0.2

0.1

0.05

0.02 I I 320 1280 80 320 1280 Deposit (milligrams per square fool) (log scale)

FIOUR~; lO.-Contact tillle fOr flO-percput kill of Ips ContU81t8 adults e.'\:posed to weathered deposits of dipldrin and endrin. The yertieal line at each deposit level shows thE' Oil !1E'reent {'onlidpllee limits. L deposits were exposed to deep shade, M deposits (facing north) to partial shade, Ilud H deposits to lIIllXilllUlll sun. STRUCTURE AND TOXICfrY OF INSECTICIDE DEPOSITS 45

TABLE 15.-MortaZity of Ips confusus adults emposeit to itielit11Jn deposits 'weathered /01' 1 yea?'

Exposure l\fortality Exposure and dose (mg. per sq. ft.) time Insects after 72 hours

.M exposure: 1 .MiTluUa Number PerceTll 320 _____ ~ ______0.00 125 11 .05 50 50 .10 50 64 .20 50 58 .40 50 82 ~280------.00 50 14 .05 40 97 .10 40 100 .20 40 100 .30 40 100 H exposure:2 1,280______.00 120 18 .05 30 100 .10 30 100 .15 30 100 .20 30 100 .25 30 100 .30 30 100 .40 30 100 .50 30 100 .70 30 100 1. 00 30 100

I Partial shade, fucing north. 2 ·Maximum sun.

The principal weather effect was a reduced size and density of endrin particles as though vaporization was accelerated. The heavier deposits were still highly toxic, even after 6 weeks of exposure. ('overage was high, almost always 80 to 100 percent. Particle climen­ sions were usually 10ft to 30p.. Unique for emh·jn, smal1 platelike crystals appeared llluong the granular particles in the 320-mg. weath­ erecl deposits, but disappeared after 6 weeks. Discussion ·Weather·ing can alter the relation between toxicity and dose by enhancing toxicity under: some circumstances and impairing it under others. Consequently laboratory results are difficult to interpret. They may not accurately foretell performance in the field. Not only may weathe6ng change the relative potency of different dose levels of llll insecticide deposit, but it may also great1y alter the relative potency of different insecticides. Dieldrin find enclrin de­ posits of 80 In!!. weathered for 2 ·weeks Se,l'Ye 'as It good illustration. The ratio of LT50 estimates shows that the enc1rin deposits were about 77 times more potent than dieldrin deposits giyen the most Revere (H) exposure. But relative toxicity was reversed under con­ ditions of minimum (L) exposure: dieldrin deposits were about 47 tinJ(>s more potent than enc1rill deposits. The reRic1ual1ife of Rnch insecticides as dieldrin) endrin, and DDT nppears to be long enongh to provide remedinl control throughout the 46 TEC.HNICAL BULLETIN 1343, U.S. DEPT, OF AGRICULTURE

typically extended emergence period for bark beetles, The outlook for long-term preventive control seems equally promisin~, That some dieldrin deposits remained extremely toxic for more than a year supports this belief. The structure of dieldrin deposits in these studies proved to be uncommonly pliable, Depending on conditions, particles were gran­ ular (either amorphous or cuboid), platelike, or acicular. The po­ tential for dh'ersity in form should perhaps be regarded as an important asset because form may affect toxicity, Greater pliability thus would proyic1emore room for controlled adjustment 1n the deposit. The shape of ic~ crystals developing in air is control1ecl by tem­ perature a lone, and the effect is very precise and reprodncible (68). A given erystnl 'while stil1 developing in one ~1Hlpe may be induced to continue its growth in another pl'edictable and entirely different shape by shifting to a different temperature, lYe know of no research on the. e·fi'ects of various environmental factors on the development of insecticide crystals. TES1'S WITH CAGED BOLTS Thus far we have described laboratory st,udies of deposit structure and toxicity, lYe also conclucted small-scale tests uncler simulated field condit'iolU; on short pine bolts infested with bark beetles (a) to being out relationships that may bear on the research problem but are not discoverable in the laboratory, (b) to gll ide future laboratory studies along fruitful lines, and (c) to test the applicabi11ty of lab­ oratory findings, PROCEDURE Ten separate tests were conclucted over a period of three seasons as Tollows:

Number of bolts in Test: Insecticide .~ample A~ ______~_. ____ • DDT______B______.. __ . ______do______24 C.______._. ___ _ lindnnc______20

l)___ ~ ___ ~ ______do______]5 'E ____ • ___ - ______do ______12 F______do______16 G______dieldrin ______16 H ______do ______16 1______16 J______lindancDDT, dieldrin, ______cnelrin, and lindnnc______20 16

Tests A and B 'were conducted in 195i, C and D in 1958, and the I'estin ]959, Ponderosa pine (sugal' pine in test E) infested with the desired bark beetle species was rollected in the wild, usually ,,-hen the insects were cll.llow adults n.nd nearly ready to emerge, The in­ festeel pine was cut into short bolts, 16 inches long tlnd usnally 8 to ]2 inches ill diameter. The bolts were apportioned in a given test to give It comparable hark area and bolt dill1l1l'ter to ea.('h formulation, Formulations wen' replicated three to six times, This usually pro­ vided It sample. bark area oilO to 15 square feet for each formulation, ST.RUCTU.RE AND TOXICITY OF L.,,"SECTICIDE DEPOSITS 47 Preparation and Application of Insecticides :Most of the formula,tions were ill xylene because of its good dis­ solving action. Concentration was based on the active ingredient and cosolvents were not needed. Test J consisted of formulations in xylene, kerosene, and diesel oil. The insecticides were applied at various dose levels from 5 to 2,560 mg. per squRre foot. All bolts were tl'eRted with 24 ml. of finished spray for eRell square foot of bark surface. Since only about 50 percent of the spray impinged on the bark, the actual deposit was estimated at half the amount expelled, or 12 1111. of finished spray pel' square foot. The bark was visibly wet by this volume of spray, but no runoff resultecl. Spmying to wetness but short of runoff makes a good criterion for judging application of it spmy in the field, where the amount ftppliNl {'an be only coarsely estimatecl and must be judged visually. The bolts were treated {'omparably by adjusting spray volume either for bark surfaec are:t Ol', as in tests A and B, for average bolt size. Holts in tpsls .\ and B wel'p !';u!';peIHll'(l with twine and rotated manu­ Idly whill\spl'ilYs wen' appliNl. .\11 othel' bolts were treated sepam,tely by l'evoh'ing each on a :-;p('cial tllrnt!Lhle while applying the full rneu..qlll'l'd eontpllts of a Rrnall hand spl'fI,yer (fig. 11). ('he('k bolts were unsprayed ('xcept in test D, in which both un­ sprnye

In tests A and B, all fiYe 01' six replicates of a given fOlmulation were pla{'(>d together in ()- by 6-foot compartments of the wire-screen inSe('t~ll'Y (ng.l,p.:W). In thl'otherteststhe.boltswerec.ngedsepa.­ rately in -to-mesh, \\'i[·('.-5('1'l'en cfLg('~, IX inchps hi/!h and Hj inehes in diameter. The holt was plac'pel on a pl,)'bo[\nl base and the ('age, open It!" tl1(' bOltom, was pllH'ed oyer the bolt. A poly foam strip at the base of the c'age made :1, tight seal between the ('agp and plybonrd so that em('rging bret les could not (,!-WH Pl'. Thr ('aged bolts \\'rre arranged in a mnc10mlzNl block d('si!!ll (H!!. l~). To ('oiled the ('mug'ed b('(~tlps, a ('ag-p was first lifted from a bolt and placed on a ('l('an plyboltl'(l base to trap the bpetlps clinging to t]le inside of tl1(> C'ltg-P. The "ot h(,l'fl werr uswtlly walking about the upper cut flt(,e of till' holt and \\,prp gathel'pd first. The cag(' was th('n ll1\'edecl, till' bpet les ('ollr('tecl front thp inside sUl'face, and the cage replaced oYer tIl(> holt. EaC'h I)('('tle collected was enclosed in a separate No. 000 ~plntin ('apsulC'. Collpcfion was at abont -:l: p.m. daily, and twice daily 111 tests A and B. Esenpps w(,re mre, generally well below 1 percent of pll1prgen('e. Holding time was 7~ hOllrOl, hut was rec1u('ed to 48 hours in tests I and .r h('('au:{(' of high n:lIurnl mortality. Beetles were considered (1('11(1 if thpy did not re~pond to probing. They were ('ollsic1ered mor­ ibund when ]O(,OlllOrioll WitS not pO:'1~ible and movement. of appendages was f('phll' and UIH'ool'dina('(l. :.'!IOl'ibund be('tles were included in IlHll'!al!ty pstill1atl'~ h('c'uusp many were fonnd in the treated samples. .\. IllOI·Jhund. h(,p(l(' prolmlJly would not \)t' able to Olueeessfully attack a. nl'\\, host and t!tu:{,ill ('f1'p('l", would 1)(' i1 dead insect. All expressions of mortality wen' C'oI'l'edNl with Abbo(fs formula. 48 TECIDHCAL BULLETIX 1343, C.S. DEPT. OF AGRICULTURE

Before a cage was !'eused, it was decontaminated by washing with a high-\'('!o('ity water stream, and renewing the poly foam cushion (at the rim of the cage) and plyboard base. The thoroughness of the clean­ ing IH'O('ess is shown by the 7~-hollr mortality of beetles exposed in the decontaminated cages for 4: hours:

Insects Treatment for which cage was used (mg. lindane per sq. exposed lVorlality ft.)0;______(number) (percent) 40 ______26 4 luO ______28 11 640. ______28 11 2,560______28 4 28 4

F-!)06874 FrGL'1IE H.-Applying spray to an infetited pine bolt. ~ :t.' ,': 1 " \\'[1 I', \.' ;'1 Y of 1'."'" '1 1, '!lH. IH.Pp:-'IT:-; .}\)

REsrLTS

,. 1,'.f .,1\!! .tl,', t'·,.· 'I:';'l:!!" tlo' (·,tliflll'liia lin' "I'illl'd il''' I." I I!' .,--1 ,\ ,111.1 ":!Illl" rill' \\\"[('1'11 l'ilH' Ill,plli' . L,·, :\. 1",1 B 'Itl.!p 11;), T!lxi"il \' \\;t" ;I" I', : /. '.j. I \\1' lllll' "rIll)'!' :ll'l.Jil'd 1!ll'i,·:dIy'IS1\. J.' ,! I' ." I ,,\',','1.,1'1\ 1•. \ 1.. /," "".f"'i,- "\I'lI :11 d:t'I(J\' do,.;p of .' ~' ,." 'i" i",' :, .... : to,,1- I ",,'I I), II \',:t- :dlll"" a- 111:\\" III tlll' !t;P"[' t: jl'.~ I">t .' __ • /1 ..,J u _/.1," t.!r'l .,: 'I Ilo}dc) ,tp~t It:)" Illlt

:~ i' ~ !IJ\ °fl /J, ,,,1,.,,. ", +,11 .... JJr~ H Ut,I!" j tt· ... t I'" J. ~rhi .... i~ tIll' ... allIP I •• h'",. i"""" .',p\\!, t., 1.,.I'!',,· \\i"'j; ;tI'l'hl'd Ifll'll';dl,\' (Ij". ,\'jl, \ ",!"I' ,r '•.• "f '.',1- '" lll.! (i ,111l\\_ d1t,!.t!'ill 10 Ill' Illfl!'l' l'll'l'din' 1,;,' f j'._ d' !J "Jl!, ~jd;ll." I.j'; ,," 'If.", ''', I.irud:lHP i'"'l 11I()1~p toxi(' .' '.,;, • ':t' 1'-'" t i"-Il''' r"I',"ally appl:t·.( 'I;,: I ..\ pal'liall'xplan:t· •. '11, ! .. (" ,_ I'p', "l'-t! .. f r ..\:,'! \' ;n":I:!t'd [",It l .."t'" ilia\, I){· that dil'ltlrill ...• ,' ,: •.,[ " .. -'Irf ,. " '1:.:1 I lI,htH' ,hi ):,,' 'I'll;' ~l1l'f';[,'p tippo...;!t 1': t·, i,,, :"':':11'.\"'1 1._ I -pr"lr'th' ""nt" ,.,,1' ",.1 t'llll'!lal loll (hat ,'ollt I'ihllt('~

7 ,( I • ~ ~ r I ~ ! j <: • ; ~ I J I) .,',1'1"' ','. I, ,'II',' ,_ 1..\,,' "- l:li'htil' II) II" "III.tI/"I" ill tt,,,t.; C, 1>, :" II,,' ,. ." ,11' ;,,1' d 1 "dtlll' \I.,.!,,: :tll 'filiI!' ":lltil:tl' ill po' '1':.', ',.! r. lI.d :q'I.I"I',,:,d.ly 1l11lt"'1():\II'lhan 1)1)'1' inll'.;l 1. I:. t" ••1, ':"t' "\ " \' "I' ;,!ldlld' It) / J" 0'';''''/'' \\;t-' !llll !.!Tl'~ltly l:f.. : • .j, I,' I. ,;.\ flll:!l!"]'! 0:. !l (!!l'l't.'.oht'llt- that dit!\·n·.t'wi,h·['· :1. \" '" ' •. \.1:"';' i'l" Ill't'.,lll.t. i.,'wi",t!·lr'I"'IU'I', • 50 TECHNlCAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

TABLE 16.-Results Of caged-bolt tests of residual in.'Jecticiiles

Insecticide and Total Corrected Insecticide and Total Corrected dose (mg. per insects mortal- dose (mg. per insects mortal­ sq. ft.) ity I sq. ft.) ity I

TEST A TEST F I p8 con/usus Dendroctonus brevicomis DDT:0 ______Number Percent 320______533 ------Lindane: Number Percent 732 79 0 ______640 ______20______273 ------­ 568 86 40 ______205 38 1,280______597 94 80______217 56 164 64 TEST B De1ldroclonu3 TEST G brevicomis Dendroctonus brevicomis DDT:0 ______Dieldrin: 320 ______738 ... ------0______640______677 48 316 ------­ 20___ ,. ______322 1,280______634 71 40 ______45 485 70 80______171 62 231 94 TEST C Ip~ confusus TEST H Ips con/usus Lindane:0______346 Dieldrin: 40______------0____ - ______579 160______374 95 20 ______------­ 332 100 80______287 96 '40______180 100 2,560______184 100 320______149 100 288 100

TEST D2 TEST 13 Ips confusU8 Ips con/usus Lindane:0______Lindane______20______80 ------­ 99 ------147 97 Xylell(~:0______Dieldrin: 20____ 149 89 5______64 ------Endrin: 20 _____ 133 94 10______147 93 DDT: 20______132 73 20 ______117 92 52 100 TEST J3 TEST E Ips con/usus Dendroctonu8 Lindane in monticolae 0 ______xylene: 20 ______277 ------Lindane:0______191 98 40 ______118 ----,------Lindane in 160 ______143 89 kerosene: 20 __ 135 97 640______131 96 Lindane in diesel 38 100 oil: 20 ______201 93

I Holding time 72 hours unless noted. 2 Corrected morta1ity is based on the combined data from unsprayed and xylene-treated controls. 3 Holding time 48 hours. STRUCTURE Ai'W TOA""ICITY OF INSECTICIDE DEPOSITS 51

In general, kill increased with dose and no dose effect was evident. All deposits 'were quite variable even within small areas of the bark surface. Crystal structure and the threshold of crystallization were generally similar to those on fiberboard panels in the laboratory. J.Jinclane crystals became badly eroded within a week after they formed. DISCUSSION Kill by all four insecticides-DDT, dieldrin, endrin, and lindane­ was greater than would be expected from contact with deposits on the bark surface alone. The insecticides were clearly acting while bark beetles were still in the bark, as shown by the dead beetles under the bark. Tending to confirm that poisoning under the bark was an important ('ause of kill are the (a) often high morta1ity when no suda!'e deposit ('olll(l be found; (b) undiminished mortality through­ out the emergent!' period, e\'en in test. C in which the surface deposit of lindane was eroded to virt1tal extinction at the end of an emergence period of oYer -~ weeks; (c) frequent presence of dead insects on the treated bolts with head and sometimes thorax protruding from the bark, as though poisoning ended .in death before the insect could eseape i and (d) death of beetles in place as shown by the consistent drop in el1lel'gence in many of the tests as dose was increased. It is not cleal' whether poisoning under the. bark resulted from con­ ta(·t, fumigation, stomach action, or a combination of the three. Stoma('h adionis possible bec'ause bark beetles ingest bark tissue as they ('hE'w their way out (8()). A highly volatile insecticide like lindane may well fill the exit gallery with a lethal vapor concentration as the emerging insE'ct reaches the outer bark tissue containing the insE'ctic'icle. POBsibly lindane vapors also enter the egg and larval galleries where tIlE' bark is very thin (as near fissures) and the spray i'oaks to the cambium. Stomac·h action here is also possible. E\'en though somE'. insecticides develop a surface crystal bloom, somE.' toxicant SE'E.'pS into the bark when applied and remains there. The merit of surfaec deposits reJa,tive to tIssue deposits is not clear. . \ voln.til(> insec·tieiclo. like lindallE' may be more effective if :it does not crystallizE.' at tilE' bark surface, but instead remains within the bark tis;;ue. Pel'llllps with lin(lane, crystallization should be avoided because ('rystals in slIrfa('£' (1£'p05its qui('kly vaporize. This loss would seem less important with insertieides less volatile than Hndane. Their more stablE' slIt'flu'e deposits would perhaps be advantageous in pre­ '1E'nl'i,'o. rontrol wherE'. toxic action is wanted as soon as the attack begins. ThE'l'e were no instanc(>s of an intermediate level of dose being more potent than 10WE'I' and higher levels such as was observed in the laboratory. Bnt surface cleposits tended to show fuller development at. an intE'rmNliate le\'elin tests A. B, and C. So, in practice, the dose effect may 110t bE'important. The close effect in the 1aboratory was expressed at a relatively high-dose range and the caged-bolt tests were conducted mostly at low-dose levels. The additional toxic action by way of the stomach and perhaps also by fumigation in caged-bolt tcsts evidE'ntly great 1y lowers the effective dose level. ThE' dose t11r"ec:1101cl of crystall ization parallelecllaboratory findings. Dieldrin crystallized at a much lower l10se than lindane. This result seems to have given dieldrin superiority over lindane in tests on Dendrocto-nu.s brevicomis (tests F and G). 52 TECHNICAL BULLETIN 1343) U.S. DE.PT. OF AGRICULTURE

The effectiveness of highly potent insecticides may not be greatly influenced by changes in structure of the surface deposit. Barlow and Hadaway (6) melltioned this for the relation of crystal size and contact toxicity. Wide latitude possibly may be permitted in deposit structure in practice; great care to reproduce a uniform deposit of a certain structure may not be necessary. Formulation in three differ­ ent solvents (test J) made no difference in the potency of lindane. Caged-bolt tests wHh lindane early indicated that an application of 20 mg. of lindane in 12 ml. of solvent per square foot may be effective on Ips confu81lS. A O.2-perc~nt lindane spray applied to bark wetness has 'been recommended for field trial as a remedial con­ trol against this insect (66). Lindane has since been field tested as a remedial control against Ips confu8u8, DC1UZl'OCt01W.9 b1'evi('()1nis, and Dend1'octonll,,~ monticolae and shows good promise (53, 67, 114). Osburn (8.4) has given an example of costs. SUMMARY AND CONCLUSIONS

Selected residl1al insecticides were studied for their potential in controlling bark bpetles. Lo,y-volume sprays were applied to the bark of the host tree. Deposit structure strong-ly influences contact toxicity and how much insecticide the insect picks up. Shortly after applica­ tion, insecticides often bloom as tiny crystals from the surface. De­ posits from insecticicle solutions were studied to determine the causes of variation in crystal structure and the methods of controlling struc­ ture to achievemaximnm control benefits. Much of the work was done in the laboratory with acptone solutions applied to fiberboard. The insecticides used were DDT, dieldrin, dinitrocresol, endrin, heptachlor, and lindane. Deposits were bio­ assayed for contact toxicity with the California five-spined ips (Ips con/usus (Lee.)). I..Iaboratory findings were examined under simu­ lated field conditions by using pine bolts cut from infested trees. The bolts were treated with the experimental insecticide solutions and then raged separatelY for rearing. Maior results of the laboratory and caged-bolt studies were: 1. The crystalline structure of deposits that developed from app1i­ cation to fiberboard panels was unique for each insecticide. DD1'.-Deposits were typically composed of a patchwork of acicu­ lar erystals, usually 20/L to 150/L long anel less than 5p. to 10/L wide. The crystals 'were often clustered, several arising from a common focl1s. Usually the deposits had a granular component. of isometric particles less than 10/L ill diameter and seemingly amorphous. Acicu­ lar or border crystals that developed at the edges of the crystal patches were generally longer (100/L to 500/L) than the usual crystal. Die7dr'in.-Cr.vstals were also acicular and clustered like DDT, usually 20/L to 500/L long, and from less than 5/L to 10/L wide. A light granular deposit was sometimes present. Dinitrom·esol.-Crystals varied from needlelike to broad and plate­ like, 20/L to 150/L long and from less than 5p. to 25p. wiele. Occasional1y a light granular deposit containing particles less than lOp. in diameter developed. Endrin..-Deposits were composed of granular partir.les only, usually lOp. to 40/L in diameter. Some particles were definitely crystal­ line, being cuboid. Others were irregular or amorphous. STRUCTURE AND TOXICITY OF INSEerrCIDE DEPOSITS 53

Heptachlor.-Crystals were thick and stubby, like massive columns, 20", to 100", long, and 10", to 40", in breadth and width. Some particles were granular and less than 10", in diameter. Lindane.-Crystals were platelike, often massive, usually 50", to 300", long, 10", to 200", wide, and less than 10", thick. Granular particles less than 10", in diameter also occurred. 2. Lindane was uncommonly toxic. Bioassay with Ips ConfU81.l8 usually showed a 50-percent kill in less than 5 seconds of contact. Dieldrin, endrin, and heptachlor were intermediate in contact tox­ icity. DDT and dinitrocresol were the least potent. 3. Much unexplained and unpredictable variation in deposit struc­ ture occurred among and within single treated panels. Variation in crystal form, density, and coverage was noticeable. The amount of variation ranged widely between insecticides. Dinitrocresol varied most, endrin least. 4. The amount of insecticide applied for each unit area of substrate (dose) profoundly affected depOSit structure and thus toxicity. En­ eh-in and heptachlor generally became more toxic with each increment in dose. DDT, dieldrin, dinitrocresol, and lindane '5howed markedly greater toxicity at an intermediate dose, falling off in toxicity above and below this optimum level. This characteristic-termed the "dose effect"-was more pronounced in dieldrin and dinitrocresol than in DDT or lindane. Typical crystal forms of endrin and heptachlor increased in density and coverage consistently as dose was increased. Above the optimum dose the other insecticicles showed less of the typical upright, more available, and more toxic cr.vstal forms, developing a granular habit and becoming less toxic to the insect. 5. The effect of weathering on DDT, dieldrin, and endrin deposits, though generally adverse, showed some benefits. Some weathering generally improved the toxicity of heavier DDT and dieldrin deposits. The eft'ect was so pronounced that the usual dose offect noted in labo­ ratorv studies--in which intermediate dose levels were most toxic­ was absent, and toxicity varied directly ,,,ith dose. The high potency of these heaviPr deposits was maintained even after several weeks of weathering. This indicates that toxic life may be long enough to provide effective bark beetle control. 6. The solvent had a marked effect on deposit structure, but not on toxicity. Studies with DDT showed that benzene solutions gave ex­ cellent coverage-100 percent for many deJ?Osits-with fine, dense, acicular crystals. Xylene and acetone solutIOns resulted in deposits averaging but 30- to 40-percent coverage with less dense and more coarse crystals. Kerosene and diesel oil solutions did not crystallize. 7. Concentration affected dieldrin deposits but not DDT deposits. The dose effect in dieldrin appeared when concentration was increased and spray volume kept constant. But, when concentration was fixed and spray volume was changed, the dose effect did not appear. 8. Large changes in deposit structure were needed to affect contact toxicity significantly. It was concluded that, in practice, wide lati­ tude may be permitted in the control of deposit structure without sacrificing mOl'tnl ity benefits. Die1drin, cnclrin, and linchtne each showed good promise for effec­ tive bark beetle control in tests on infested pine bolts. DDT, also 54 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE tested, was least toxic. All the insecticides, especially lindane, were more toxic than could have been p'redicted from laboratory tests of contact toxicity. In addition to contact action, stomach poisoning and perhaps fumigation contribute to mortality. Dose levels below the threshold of surface crystallization were often llighly effective in the caged-bolt tests. The dose effect noted in the laboratory at rela­ tively high-dose levels, therefore, may not be important in an actual control operation. LITERA TURE CITED

(1) ALEXANDER, P., KITOTIE.....ER. J. A., and BRISCOE, H. V. A. 1944. Inert dust insecticides. 1. ~Iechanism of action. Ann. Appl. BioI. 31(2) : 143-149. (2) ---, KrrclIENER, J. A .• and BRISCOE. H. V. A. 1944. Inert dust insecticides. II. The nature of effective dusts. Ann. Appl. Bioi. 31(2) : 150-156. (3) ALI.EN. D. G.• and RUDIN SKY, J. A. 1950. Effectiveness of Thiodan. Sevin, and lindane on insects attacking freshly cut Douglas-fir logs. Jour. Ecoll. Ent. 52 (3) : 482-484, illus. e4) ASCHER, K. R. S., and REUTER, S. 195.1. The physical state and insecticid'al properties of D.D.T. in spraying residues: laboratory experiments on glass plates. Rh'. di Parassitol. 14(2) : 115--122. (5) BARLOW, F .. and HADAWAY, A. B. 1047. Preliminary notes on the loss of DDT and gammexane by absorption. Bul. Ent. Res. 38(2) : 335--346. (6) ---, and HADAWAY, A. B. 1952. Studies on aqueous suspensions of insecticides. II. Quantitative de­ >terminations of weights of DDT picked up and retained. Bul. Ent. Res. 42(4) : 769-777. (7) ---, and HAPAWAY, A. B. 1952. Some factors affecting the availability of contad insecticides. Bul. Ent. Res. 43 (1) : 91-100. (8) ---, and HAPAWAY, A. B. 1958. Studies on aqueous suspensions of insecticides. VI. Further notes on the sorption of insecticides by soils. Bul. Ent. Res. 49(2) : 315--331. illus. (9) ---, and HADAWAY, A. B. 1058. Studies on aqueous suspensions of insecticides. YII. The influence of relative humidity upon the sorption of insecticides by soils. Bul. Ent. Res. 49(2) : 33~{-354, iltus. (10) BARNES. S. 1945. The residual tOxicity of DDT to bed-bugs (Cimea; lcctltlariu8, L.). Bul. Ent. Res. 36(3) : 273-282. (11) BEAL. JAMES A. 1000. Biological and chemical control of plant 'and animal pests. Oontrol of forest insects. AAAS Symposium 1957. Pub. 61. pp. 23-32. (12) BECKER, W. B. 1950. Sprays to preyent scolytid infestation of elm logs. 1I1ass. Agr. Expt. Sta. Bul. 459: 47. (13)-­ 195.'5. Tests with BHC emulsion sprays to keep boring insect.'! out of pine logs in Massachusett.'1. Jour. Econ. Ent. 48(2) : 163-167. (14)-­ 1959. Further tests with BHG emulsion sprays to keep boring insects out of pine logs in Massachusetts. Jour. Econ. Ent. 52(1) : 173-174. (15) ---, ABBOTT, H. G., andRICH, J. H. 1956. Effect of lindane emulsion sprays on the insect invasion of white pine sawlogs and the grade yield of the resulting lumber. Jour. Econ. Ent. 49 (5) : 664-666. (16) BERTAGNA, P. 1050. Residual insecticides and the problem of sorption. WHO Bul. 20(5) : 861-889. STRUCTURE AND TOXICITY OF INSECTICIDE DEPOSITS 55

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(39) HADAWAY. A. B., and BARLOW, F. 1949. Further studies on the loss of insecticides by absorption into mud and vegetation. Bul. Ent. Res. 40(3) : 323-343. (oW) ---, and BARLOW, F. 1\)51. Studies on aqueous suspensions of insecticides. Bul. Ent.. Res. 41(3) : 003-622. (41) ---, and BARLOW, F. 1952. Studies on aqueous suspensions of inseeticides. III. Factors affecting the persistence of some synthetic insecticides. Bul. Eut. Res. 43 (2) : 281-311. (42) ---, and BARLOW, F. 19"'.)3. Studies on aqueous suspensions of insecticides. IY. The beha\ior of mosquitoes in contact with insecticidal deposits. Bu!. Ent. Res. 44(2) : 255-271. (43) ---, and BARLOW, F. 1958. Some aspects of the effect of the soin~nt on !fhe toxicity of soiutions of Insecticides. Ann. Appl. Biol. 46(2) : 133-148. (44) HARTZELL, ALBERT, and WILCOXON, FRANK. 1960. The ImI)()rtance of wetting agents as affecting the toxicity of certain Insecticides. Boyce Thompson lnst. Contrib. 20(7) : 421-424. (45) HETRICK, L. A., "md ~IosEs, P. J. 1953. Value of insecticides for protection of pine pulpwood. Jour. Econ. Ent. 46(1) : 160--161. {40} HrofIrowER, B. G. 1959. Bioassays of weathered residues of several organophosphorus insec· ticldes. Jour. Econ. Ent. 52 (5) : 840-842. (47) HOJI:I"STEIN, Y.. and SULLl\'AN, '''.X. 19;:;3. The role of chlorinated polyphenyls in improving lindane residues. Jour. Econ. Ent.4t:(6) : 937-940. (48) ---, Sl'LLIYAN, \VILUAlI N., and TSAO, CHINo·Hsr. 1955. Residual effectiveness of mixtures of organic phosphorus insecticides with chlorinated terphenyls. ,Tour. E('on. Ent. 48(4) : 482-48.'3. (49) ---, SULLl\'.-I.N, W. N., TSAo, CHT:I"o-HsI, and YEO~!ANS, A. H. lO5-l. The perSistence of lindane-chlorinated terphenyl residues on outdoor foliage. Jour. Econ. Ent. 47 (2) : 332-335. (50) HOSKINS, "T. ~I. 1fl33. The penetration of insecticidal oils into porous solids. Hilgardia 8(2) : 4~82, illus. (51) --­ 1962. Some important properties of pestieide deposits on vaonous surfaces. 1/1 Residue reylews 2: 66-fll, Francis A. Gunther, ed., New York: Academic Press Inc. (52) ---, WITT, J. ~r., and ERWIX. W. R. 1952. Bioassay of l,2,3,4,5,S-hexachlorocyclohexane (lindane). Anal Chem. 24(3) : 5[>5-560. (;'3) JACKSOX, 'VILLARD L. 1!)6(). A trial of direct control of pine engraver beetles on a small logging unit. r.s. Forest Sen'. Pacific Southwest Forest and Range Expt. Sta. Misc. Paper 44, 7 pp., lIlus. (54) ,TOIl:l"STO:'i, H. R. 1952. Insect control-practical methods for the control of insects attacking green logs and lumber. South. Lumberman 184(2307) ; 37-39. (;5.'5) KETTLE, D. S. 1949. The speed of action of insecticidal sprays and deposits and its use in assessing the biological efficiency of BHC, DDT, and pyrethrum. Bul. Ent. Res. 40(3) : 403-429. (56) KIXOHORN, J. ~I. 1955. Chemical. control of the mountain pine beetle and Douglas·fir beetle. Jour. Econ. Ent. 48(5) : 501.,-504, illus. (57) --­ 1000. Chemicals for preventing ambrosia beetle attacks. Canada Dept. Agr. Forest Biol. DIv. Bimo. Prog. Rpt.16(5) : 3. (58) -­ 1961. Ambrosia beetle preventives. Canada Dept. Agr. Forest Ent. and Path. Branch Bimo. Prog. Rpt.17(6) : 3-4. (59) KRUSE, C. W. 1948. Roae-h ('ontrol. Soap and Suuit. Chem. 24(11) : 131, 133, 135, 137, 139, 169. STRUCTURE} AJ.'l"D TOXICITY OF INSECTICIDE DEPOSITS 57

(60) LEE, R. E., and S!.UTH, R. H. 1955. The black turpentine beetle, its habits and control. U.S. Forest Sen-. South. Forest Expt. Stu. Occas. Paper 138, 14 pp., illus. r61) LEWIS, C. T. 1954. Studies concerning the uptake of contact insecticides. I. The anatomy of the tarsi of certain Diptera of medical importance. Bul. Ent. Res. 45 (,1) : 711-722, illlls. (62) ---, and HUGHES, J. C. 1957. Studies coneerning the uptake of contact insecticides. II. The con­ tamination of flies exposed to particulate deposits. Bu!. Ent. Res. 48(4) : 755-768, illus. (63) LINDQUIST, ARTHUR W., JONES, HOWARD H_, and )IADDEN, A. H. 1946. DDT residual-type sprays as affected by light. Jour. Beon. Ent 39{1) : 55-59. (64) LITCHFIELD, J. T., and WILCOXON, F. 1949. A simplified method of evaluating dose-effect experiments. Jour. Pharmaeo!. and Expt. Ther. 00(2) : 9'J-113, illus. (65) LYON, ROBERT L. 1059. Toxicity of several residual-type insecticides to selected western bark beetles. Jour. Econ. Ent. 52(2) : 323-327, illus. (66) -­ 1000. Directions for using lindane sprays to control ips beetles in Califor­ nia. U.S. Forest Servo Pacific Southwest Forest and Range Expt. Sta. Misc. Paper 33 (rev.), 8 pp. (67) --- and \VICKMAN, BOYD E. 1960. Mortality of the western pine beetle and California five-spined ips in a field trial of lindane. U.S. Forest Serr. Pacific Southwest Forest and Range Expt. Sta. Res. Note 166, 7 pp., illus. (68) MASON, B. J. 1961. The growth of snow crystals. Sci. Amer. 2M (1) : 120-123, 125-131, illus. (69) ;\IASSEY, CALVIN L. 1960. DDT-a preventive control for the southwestern pine beetle. U.S. Forest Servo Rocky Mountain Forest and Range Expt. Sta. Res. Note 40,1 p. (70) ---, CHISHOD£, R. D., and WYGANT, N. D. 1953. Ethylene dibromide for control of the Black Hills beetle. Jour. Econ. Ent. 46(4) : 601-604. (71) ---, CHISHOLM, R. D., and WYGANT, N. D. 1953. Chemical control of the Engelmann spruce beetle in C{)lorado. Jour. Eeon. Ent. 46(6) : 951-955. (72) ::\I.ATTHYSSE, JOHN G., )IILLER, HOWARD C., and THO!.[PSON, HUGH E. 1954. Insecticide deposits for control of elm bark beetles. Jour. Econ. Ent. 47(5) : 739--746. (73) McINTOSH, A. H. 1946. Relation of crystal size and shape to contact toxicity of D.D.T. sus­ pensions. Nature 158(4012) : 417. (74) -­ 1947. Relation between particle size and shape of insecticidal suspensions and their contact toxicity. I. D.D.T. suspensions against Tri,boliunt castaneum Hb. Ann. App!. BioI. 3±(4) : 586-610. (75) -­ 1949. Relation between particle size 'and shape of insecticidal suspensions and their contact toxicity. II. D.D.T. and rotenone suspensions against Oryzaephilu8 8urinamensis L., with some time-mortality studies. Ann. Appl. BioI. 36(4) : 535-550. (76) 1951. Particle size of insecticidal snspensions and their contact toxicity. III. Temperature coefficients and tests by injection. Ann. App!. BioI. 38(3) : 567-576. (77) -­ 1954. Temperature coefficients of insect kC~ by volatile solid insecticides. Bu!. Eut. Res. 45(1) : 137-139. (78) -­ 1955. Particle size of Insecticidal suspensions and their CO'Jtact toxicity. V. Effect of physical properties on toxicity of compounds in the DDT group. Ann. App!. Biol. 43 (2) : 161-181. 58 TECHNICAL BULLETIN 1343, U.S. DEPT. OF AGRICULTURE

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