Insecticides, their design and evaIuation: Hummel 615 for use against certain nematode species, two directions. We can allocate our re- especially those species responsible for illa- sources equally or unequally, but the riasis in its various forms. There is an op- choice must still be made. portunity, if not a need, for new and im- The trouble with the empirical ap- proved drugs to complement or replace proach is that it is intellectually humiliat- those in current use. The greatest oppor- ing no matter how successful. The trouble tunity for improvement probably lies in with the rational approach is that it is the area of drug delivery systems. intellectually irresistable no matter how The discovery of new antiparasitic drugs un fruitful. is generally approached from either the point of view of empirical screening or of biochemical ("rational") design. Each ap- IATERATURE CITED proach has protagonists who have faith in I. Armour, J., K. Bairden, J. L. Duncan, R. M. its future success, with empiricists being Jones, and D. H. Bliss. 1981. Veterinary record. June able to boast of past success. Those who 20, 532-535. actually face the task of discovering new 2. Campbell, W. C. 1977. Control of parasites: drugs know that it serves no purpose to the role of drugs. Proc. Helm. Soc. Wash, 44:17-28. espouse the middle ground, important 3. Drummond, R. 1)., "F. M, Whetstone. and J. A. Miller. 1981. CoJltrol of ticks systemically with though semirational approaches might be. Merck MK-933, an avermectin. J. Econ. Entomol. Nor is it very daring or helpful to say that 74:432-436. we should have both approaches~for who 4. McCall, J. w., B. E. Lintlemann, and C. A. does not wish to hedge a bet? Who does Porter. 1981, Prophylactic activity of avermectins against Dirofilaria immitis. Pp. 126-130 in G. Otto, not applaud good basic research regardless ed. Proceedings of the Heartworm Symposium, Dal- of its short-term applicability to everyday las, Texas, January 1980. affairs? Who would deny the possibility 5. Rew, R. S. 1978. Mode of action of common of valuable but unpredictable spinoff from anthelmintics. J. Vet. PhalTnacol. Therap. 1:183- such basic studies? It is easy to say we need 198. 6. Sano, M., M. Terada, A. I. Ishii, and H. Kino. both empirical and biochemical approaches. 1981. Effects of avermectin Baa on the motility of But we cannot apply maximum effort in various helminths. Experientia 37:844-846.

Insecticides and Their Design'

HANS E. HUMMEL ~ creasingly competed with man's best efforts Key words: chemical control, insecticides, tle- velopment, screening, mode of action. in food, fiber, and timber production; ur- banization accentuated the need for public Pesticides and drugs originated from the health efforts, and animal husbandry neces- need for controlling man's external and sitated advances in parasite control. Man internal environment. Since the advent of responded to these challenges with pesti- agriculture, pests in the widest sense in- cides and drugs. The chemical weapons used in this fight are part of the history Received for publication 24 January 1983. of insect toxicology, pharmacology, and 1Presented in a symposium at the 1982 Annual Meeting hematology. of the Society of Nematologists, Knoxville, Tennessee, July 1982. At first, nature was man's best teacher 2Department of Entomology, University of Illinois, Ur- bana, IL 61801. and exclusive supplier of pesticides. ~Vith Drs. R. L. Metcalf and T. A. Miller kindly called my the increasing independence from nature attention to relevant references in tile older and newest literature, respectively. Dr. Po-yung Lu, Toxicology Data came the push for synthetic substances, for Bank of the Oak Ridge National Laboratory, Oak Ridge, procedures to make them, and for theories Tennessee, and Dr. E. Davis and her staff of the Biology Library of the University of Illinois, Urbana, provided hard to understand their modes of action. copies of some lesser accessible references. Dr. G. Eger- The interaction of man with pests is not Hummel, Marburg, supported the preparation of this manuscript. static: it is highly dependent on natural 616 Journal of Nematology, Volume 15, No 4, October 1983 and man-made changes. Early in this cen- 1912 with the hydrocyanic acid resistance tury man had to cope with phenomena like in the California Red Scale (137), the oc- pest resistance to pesticides and drugs. This currence of documented cases of acquired forced him to search for new approaches resistance follows a predictable natural to control pests, and he soon left the scene growth curve. By the turn of this century, of plants and minerals. Today, all possible more than 1,000 insect species may show sources tor new compounds receive consid- some degree of resistance. eration (30,45,126,131). Since preceding papers of this sympo- sium dealt with nematicides and herbicides, tile scope of this paper will be restricted PRINCIPLES AND LIMITATIONS to the development, design, and evaluation OF APPROACH of insecticides. Today we are still far from tile ideal in pesticide development: tile denovo syn- HISTORICAL REMARKS thesis of agents based on rational design Realgar (arsenic sulfide) was used by (38,93). Limited knowledge of biochemical, the Chinese for garden insect control as pharmacological, and toxicological differ- early as 900 A.D. In late Roman times, ences between man, insects, nematodes, and and thorugh the Middle Ages, sulfur, pitch, plants restricts the rational development. mineral oil, brine, lye, and soap were used However, in a few selected examples, nota- as insect repellents. Records of these early bly in the field of organophosphorus and uses are excellently reviewed by Shepard carbamate insecticides, toxicologists are (137). Early insecticides utilized were the able to design pesticide analogs and, to a inorganic elements Hg, Pb, F, and As and limited extent, predict their properties. compounds of these elements. These are In most other areas, our search strategies broad spectrum biocides and can be equally are still largely stochastic. harmful to insects, man, domestic animals, In the past, almost exclusive emphasis and plants. In hindsight, one wonders what was given to the most effective agents with ecological side effects occurred when lead tile highest acute toxicities. Two decades arsenate was used in 1892 on a wide scale ago a subtle shift occurred toward environ- in an attempt to eradicate the gypsy moth mentally benign compounds. Killing pests, in Massachusetts. let alone their eradication, was no longer the prime objective. Instead, rendering the THE CONSIDERATION pests unfit for reproduction became tile OF SELECTIVITY goal. In the course of the last two millenia, The biochemical modes of action are methods for pest control have evolved, first known for a number of unselective poisons. slowly, then, since the late 19th century, Lesions occur at specific sites in the gly- with accelerated pace. We realized that as colysis pathway (GP), the tricarboxylic acid we act on the pests they react in response (TCA) cycle, and in the electron transport to our actions. This forced us to rethink chain (ETC) (86,91). These sites are of our strategies time and again. We probably ubiquitous importance for most animals. will never gain more than a one-step ad- Examples of poisons in the GP are iodo- vantage. The more progress we make, the acetamide, which interferes with the phos- greater the built-in pressure will become phorylation of fructose-6-phosphate to fruc- for pests to counteract. Resistant strains tose-I,6-diphosphate; arsenous acid (145), will occupy the niches vacated by tile which interacts with the ATP production strains susceptible to our interventions in the conversion step of 3-phospho- (118). glyceroylphosphate to 3-phospho-glycerate; By the end of 1975, 364 species of in- and fluoride ion, which blocks the conver- sects and mites were resistant, some of them sion of 3-phospho-glycerate to 2-phospho- multiply resistant, to all major insecticide glycerate. In the TCA cycle, fluoroaceta- groups (21,54). Starting in 1908, with the mide blocks aconitase; chloropicrin inter- first observation of lime sulfur resistance feres with the dehydrogenation of succinate of San Jose Scale in Washington, and in to fumarate; tricyclohexyl tin hydroxide Insecticides, their design and evaluation: Hummel 617 impedes the first oxidative phosphorylation and development alone. Collectively, they step in the ETC associated with flavopro- add about 16 new insecticides to the mar- tein and coenzyme Q; dinitro-o-cresol ket each year. This number falls short of (DNOC) inhibits the subsequent two phos- the anticipated need by the turn of the phorylation steps associated with cyto- century. As a result, a demand exists for chromes I) and c; and HCN poisons the a more rational, predictable, and less ex- last step of electron transfer from the biva- pensive approach to insecticide develop- lent Fe in cytochrome A~ (cytochrome ment. oxidase) to molecular oxygen (74). Since The "ideal" insecticide can be charac- the primary metabolism of all organisms terized by the following properties: is very similar, these metabolic poisons will 1) Specific for one target species only. affect most nontarget organisms including 2) Highly efficacious. man. "5) Biodegradable, with water soluble, Obviously, to achieve selectivity, we nontoxic, noncarcinogenic, and non- must turn our attention to other than mutagenic residues. metabolic poisons. Tile selectivity ratio, 4) Nonaccumulative in food chains. which is tile quotient between the toxicity 5) Easy to synthesize, formulate, and of a compound to the pest and to the host, apply. or between the target and the nontarget, 6) Cheap to synthesize from readily must be as high as possible. We can aim available starting materials. at differences in biochemistry, however 7) Free of selecting for resistant strains. small, provided we know them, and fine- Most of the presently known compounds tune the pesticide precisely to this differ- meet one or more of these criteria; no pres- ence; or we can identify differences in or- ently known compound is irresistible. In ganismal design, like tile presence (in all developing an insecticide, usually a com- nematodes, invertebrates, vertebrates) or promise is being sought between scientific- absence (in plants, fungi, bacteria) of a ally achievable and economically feasible nervous system; or we can aim at major considerations. differences in the presence (e.g., chitin in insects, nematode egg shells, fungi) or ab- MAJOR INSECTICIDE CLASSES sence of important skeletal building ma- Until the early 1940s, inorganic com- terials; or at differences in developmental pounds and botanical insecticides with a (mohing stages between larval and adult broad spectrum of activity provided the forms vs. nonmolting) or behavioral pat- backbone of all chemical control efforts. terns (use of species-specific sex attractants). Collectively, they are classified today as All of these approaches have been tried, "first-generation insecticides." Between 1940 and examples are given in the following and the late 1960s, the "second-generation pages. insecticides" consisting of the synthetic or- ganophosphorus esters, the carbamates, and THE PRESENT SITUATION the environmentally persistent chlorinated hydrocarbons were introduced. Broad spec- By the early part of the 21st century, trum pyrethroids joined the group about the world population will have doubled. 1970 when chlorinated hydrocarbons were Food for the growing numbers will mainly being phased out. Tile history of discovery come from increases in agricultural crop and the development of the second-genera- production. Undoubtedly, the demand for tion insecticides will be discussed in the pesticides of a wide variety will increase following sections. Table I provides a at the same rate (98). Unfortunately, pres- scheme and gives a short survey of the ent development costs for insecticides are modes of their discovery. Subsequent sec- excessively high due to the low rate of tions deal with "third-generation insecti- success in finding marketable compounds cides," a chemically diverse group acting (19,36). Costs are estimated at $20 million as modifiers of insect development; with per compound (55,91). Only large inter- "fourth-generation insectMdes," character- national companies can afford capital in- ized by their ability to modify insect be- vestments of this magnitude for research havior; and with a novel group of natural 618 Journal o[ Nematology, Volume 15, No 4, October 1983

Table 1: Some repre~ntative insecticides of the Ist to 5th generation and their modes of discovery.

Discovery classt Insecticide class 1 2 3 4 5 6 7

FIRST-GENERATION INSECTICIDES: Minerals containing the elements As, Hg, F, P, Se, Cu, Pb, Ca, Ba and their compounds ° * A Mineral oil and coal tar Raw plant products: tobacco, turpentine resin, powders of pyrethrum, A Quassia, Derris A SECOND-GENERATION INSECTICIDES: Organophosphorus esters • * B Organophosphates Phosphonates

Carbamates • * B Sulfenylated carbamates as "pro" insecticides • D Chlorinated hydrocarbons Lindane Toxaphene DDT, biodegradable DDT analogs • * * C Cyclodienes Mirex, chlordecone Pyrethrin group Pyrethrins, natural Synthetic pyrethroids Hybrids between pyrethroids and DDT Minor classes of synthetic insecticides Formamidine group, chlordimeform • C Nitromethylene heterocycles Botanical insecticides Alkaloids and nicotine Nonalkaloids and rotenone Insecticides from microbial origin Bacillus thuringiensis E Avermectins E Minor fungal metaoblites with insecticidal activity THIRD-GENERATION INSECTICIDES: (DEVELOPMENT MODIFIERS) Mol ti og hormones Molting hormones from insects o Molting hormones from plants Antimolting hormones Juvenile hormones (growth regulators) JH from insects Juvenoids from syntheses Chitin synthetase inhibitors of the benzoylphenylurea type C Precocenes FOURTH-GENERATION INSECTICIDES: (BEHAVIOR MODIFIERS) Antifeedants Pheromones FIFTH-GENERATION INSECTICIDES: (PROMISING LEADS FOR THE FIlTURE) Novel natural products • • Brain hormone antagonists Insecticides, their design and evaluation: Hummel 619 products that is still in the early stage of many accidental and deliberate cases of exploration and may tentatively be labeled poisoning occurred. as "fifth-generation insecticides." Between 1945 and today thousands of This classification scheme is by necessity OP compounds were described. Their total crude because it descriptively combin~s sales volume in the United States in 1971 chemical, historical, and toxicological char- was 50,000 tons (91). Many of the OP com- acteristics. Yet it will demonstrate the pounds have been extensively reviewed progress made in sophistication of approach (133,144,15B). and insecticide use within the last halt" All OP compounds, including the very century. sMe insecticides fenitrothion and mala- thion, work as cholinesterase inhibitors through a common, relatively well under- SECOND-GENERATION stood mechanism at cholinergic synapses INSECTICIDES (l10,136,) (Fig. 1). The work of Murphy Organophosphorus (OP) esters: These (110) illustrates the similarities between insecticides emerged from secret research acetylcholine, the natural cholinergic neu- on nerve gases before and during the sec- rotransmitter, and the OP compounds. Car- ond world war. The first OP insecticides bamates, with subtle differences, follow the were tetraethylpyrophosphate (TEPP) and same mechanism of action (Fig. 1). The the systemically active octamethylpyrophos- more electropositive the phosphorus atom phamide (schradan R) (133). Developed in in tile OP compound becomes, the higher Germany as a substitute for nicotine, TEPP is its phosphorylation potential, and, hence, was targeted against the nervous system. the larger its reactivity with the serine Because of its high and indiscriminate tox- hydroxyl group occupying tile active site icity to insects and mammals and because of acetylcholinesterase (ACHE). A direct o1 its hydrolytic instability, it was replaced correlation exists between the lethal dose by parathion and methylparathion. In spite for 50°,o of the insects tested (LD.~0) and of their mammalian toxicity, the OP esters substituent effects on aromatic rings at- gained a 13~:~ share of the U.S. insecticide tached to the OP group. Hammett's sigma market as inexpensive plant protectants. constants are a measure of this substituent As a group, they approximately complied effect (48,49,72) (Fig. 2). Using graphs with requirements 2 through 6 of the depicting this linear structure-toxicity cor- "ideal" insecticide, but disappointingly relation, the designer of new compounds failed with respect to numbers 1 and 7. can make quantitative comparisons and Unfortunately, the compounds were not reasonable predictions. replaced when slightly more expensive but Efforts to make OP compounds envi- much safer OP analogs such as chlorothion ronmentally more stable lead to the intro- and fenitrothion were discovered (99). In duction of methylated and halogenated many European countries, China, Indo- substituents. It also led to the replacement nesia, and in Japan, the use of parathion of the labile OP triesters by diesters with and methylparathion is restricted because one P-G bond (phosphonates). Unfortun-

J I/Ill \\\\\ ~-Key for discovery class: I = original observation followed by chemical identification. 2 = screening of pZatHs or organisms. 3 = pure accident. 4 = inspired guessing. .5 = synthesis, screening, structure optimization. 6 = enlightened research. 7 = other modes of discovery. A: Ob~rvations from antiquity originating either in the Mediterranean, Indian, American Indian, or Chinese culture. B: Quantitative structure-toxicity-relationships established by Hammett's constant. C: Semiquantitative structure toxicity-relationship data established by Hansch's ~--constants. D: Enlightened research supported by in-depth metabolic studies. E: Discovered by observations of insect pathologists. 620 Journal o/Nematology, Volume 15, No 4, October 1983 AC ETYLCHOLINESTE~(ACI'E) /~ esteratic ....~ - wv//'s/s//site H/////HH .... anionic gl~eplQ site Fig. 1. Comparison of the bio- chemical interaction with the ac- tive site of AChE of the physio- logical ncurotransminer ACh, the + acet~choline (ACh) ~rbaryl . paraoxon carbamate insecticide carbaryl, and the OP compound paraoxon (rood- c+l O flied from Murphy [110]). The (CH3)3N-(CH2)20~CH3 aminoacid sequence -glu-ser-ala- de- Ct{-+ notes the active center of ACHE; OH stands for the reactive hydroxy choline group of ser. The box emphasizes the similarity between the interme- diately formed acylated ACHE. De- pending on the acyl-moiety, it has acetytated AChE carban~ytated ~h~lale6 a half-life from msec to hours (for AChE further explanations see the text). INtO IH20 acetate,AChE N-methyt-carba_m- diethyLl:~. - (very fast) - ate * AChE phafe ,ACHE (slow] {very slow)

ately, some members of the OP, phospho- nated nerve sheaths resulting in progressive hate, and phosphonothioate series, snch as ataxia (100). However, the phosphonate leptophos, also act as delayed neurotoxins series also includes trichlorfon, which is for mammals and birds (1,2,100,145). In ad- safe and has tile excellent environmental dition to their moderate acnte toxicity, rating of 4.3 (101). Tile biological conver- tltey cause irreversible damage to myeli- sion of the pbosphonate trichlor[on to the OP compound dichlorvos is a prime exant- ple of selective metabolic activation and - lOG I50 (MOL.CONC.) toxication in insects and of inactivation J FLY BRAIN ChE ÷ • ' _m.NCH~}.~ in warm-blooded animals. In addition to modification of the aro- ~-~/ .~-CH0 matic or aliphatic substituents, another • • avenue exists for gaining greater selectivity j _-,-Nell3)2 ..... :H3// .E-CN in OP compounds. Substitntion of oxygen by sulfur leads to different metabolic ac- 6 _m-t_-be• / p-CO tivation patterns in insects and warm- blooded animals. As summarized by Mur- phy (110) (Fig. 3), phosphorodithioates and phosphothionates can be "toxicated" in insects to compounds binding to the critical AChE (toxic action). In mammals t-g0 . ct these compounds are metabolized to less '1 "+"/' toxic compounds that bind to noncritical enzymes or to sites where they can be eliminated before they reach dangerous 3. levels (sparing action; see pathways II, III, ~H3 HAMMETI'S d-CONSTANT IV, V, and VI in Fig. 3). Carbamate insecticides: Unlike OP com- -0.~ pounds, a few carbamates occur as natural Fig. 2. Linear relationship between anticholine- products. The toxicological properties of sterase activity and Hammett's sigma constants for the calabar bean Physostigma venenosum substituted aromatic diethylphenylphosphates (48). ]}ave been known since antiquity. The ac- Insecticides, their design and evaluation: Hummel 621

HO\p.~S ~ II ] RO\/px//S II1= RO\ ~S Fig. 3. Toxication (toxic action, RO/ \OR I RO ORI Ro/P~oH pathways I plus VII) in insect tis- sues and detoxication (sparing ac- PARENT tion, pathways I plus VI) ill mam- malian tissues of dialkyl (R)-mono- INSECTIClDE aryl-(R1) phosphorothionate insec- ticides. Pathways II and IV (oxida- tive desalkylation) and III and V HO\p//O <,v RO\'p//O v RO\ #0 (hydrolytic cleavage) represent mi- nor metaho'Ac routes whose prod- RO/\OR1 w RO/'\OR1 ~ Ro/P~oH ucts do not inhihit AChE (modified after Murphy [110]). binding to noncritical binding to enzymes or sites AChE tive principle, eserine, was chemically iden- carbamates by analog synthesis in all think- tiffed as a carbantate by Stedman and Bar- able variations (13,58) and for many uses. ger (149) who also recognized the carhamic Carl)amates, like OP compounds, are acid ester moiety -OCONHCH.~ as the tox- inlaihitors of ACHE. Quantitative structure- ophoric principle. toxicity correlations are known from the The introduction of a naphthyl group work of Kolbezen et al (83), Metcalf (97), to carbamic acid ester produced carbaryl, and Kukuto (95,96). Toxicities, expressed a very effective insecticide with low mam- as -log K~ values and plotted against Ham- malian toxicity (83). In 1971, 25,000 tons metrs sigma values, follow straight lines of carbaryl were produced in the United for hoth the meta- and para-suhstituents States (91). Between 1950 and 1970, indus- (Fig. 4). Meta-substituents produce higher trial laboratories synthesized thousands of toxicities than do para-substituents (83). -LOG K i 7. °m-t-Bu m-Me3N 6. "~-..,.~_m-Et .p_+Bo 5'~ ~_NO 2 4 Ph

3' "P"NO2

2. HAMMETT's O'- CONSTANTS -02' 0' 0.2' 04' 0.6' C{8 1.0' 1.2' 1.4 Fig. 4. Linear relationship of -log K i for fly head cholinesterase inhibition to Hammett's sigma values for mela- and para-substituted phenyl N-methylcarhamates (modified after Kolhezen et al. [83]). 622 Journal o~ Nematology, Volume 15, No 4, October 1983 Unlike the situation in OP insecticides, thus avoiding poisonous levels in these addition of electron accepting substituents nontarget species (11). For example, N- (e.g., the NOz group) to aromatic rings re- (4-t-butylphenyl) sulfenyl-2-isopropoxyphe- suits in a reduction of inhibitory potency nyl methylcarbamate has a selectivity ratio and thus insecticidal effectiveness, while of 30 for the white mouse. Similarly, amino- electron donating substituents (e.g., OCH3 sulfenyl derivatives of the hazardous aldi- and SCH~) to the aromatic rings enhance carb are available (51) which offer selectiv- the inhibitory power and hence the insec- ity ratios for mammals of 10 and are 7- ticidal potency. The negative slope of the 8 times more toxic to mosquito larvae than straight lines is evident from the Fig. 4. the respective methylcarbamate. As a sys- Quantitative correlations of this kind assist temic insecticide, bisaldicarb sulfide is sub- in the design of analogs with desirable stantially more effective against cotton toxicological and environmental properties. aphids, perforators, and mites than the Predictions are possible to a certain degree. parent compound (34,51). Higuchi and Another approach to selective toxicity Stella (62) reviewed the progress already is the design of "pro" insecticides in the made in the related field of "pro" drugs. carbamate family. Their active principles, Fukuto (51) foresees major applications of the parent carbamates, are set free by dif- proinsecticides and a future proliferation ferent organisms at different rates. Both of the underlying toxicological principles. N-arylsulfenylated and N-alkylsulfenylated Chlorinated hydrocarbons: This group, derivatives which operate by this principle once believed to be the ultimate solution are effective insecticides for mosquitos and for insect control, rivals the proceding OP houseflies (10). esters and carbamates both in production N- (2-toluene sulfenyl) carbofuran (Fig. volume and price. 5) is desulfenylated in the housefly to the • Lindane (Fig. 6A). Hexachlorocyclohex- more toxic parent compound carbofuran; in ane was first synthesized in 1825 by M. white mice and other mammals, competing Faraday as a crude mixture of isomers (98). reactions to nontoxic conjugates are faster, It took 117 )'ears until research groups in I. AROMATICS ALIPHATICS ,x x A. ( RO)2P-O'~O2 (RO)2 P-S -Y

R=C2Hs R =CH 3 Fig. 5. Second-generation insec- X=O PARAOXON X=S MALATHION ticides with built-in partial selec- Y =QH-CO2Et tivity. R=C2H-x=s~ PARATHION CH2CO2Et I. OP compounds. A = phos- pilate esters, B = phosphonate R=CH^ METHYL" ,,-..,~"~n300IMETHOATE esters. The aliphatic members with Y = C2H 4 S Et (demeton). Y = X=S ~ PARATHION Y=CH2C-NHCH3 CH 2 SEt (phorate) share with dimethoate strong systemic action B, S Ct CH.O0 OH in plants. CH sO"J~-n~r cH~)P-CH-CCI3 lI. Proinsecticides composed of C6H(..- v B~ - TRICHLORFON the carbamates carbofuran and al- LEPTOPHOS dicarb with protective OP-, o-tolyl- sulfenyl., and carbofuranoyl-moie- ties. Upon cleavage by insect tissue II. .'C~N/CH3 n-C-N,CH3 the parent carbamates are set free resulting in specific insect toxicity and a high mammalian selectivity phosphorylated carbofuran biscarbofuran sulfide ratio. o.-~/C H3 o "3 Hs -y_ u c. --CH 3 ~-"('('('('('~ surfenytated carbofuran sulfenylated aldi:#~ Insecticides, their design and evaluation: Hummel 695 CHLORINATED HYDROCARBON GROUP A ctL~c' CI.2~, C[ C[ C,,,,~ ,7C' R~.~R" c,C~<.2-ct c I Cl cll el" ¢'Cl~"Cl cr METHOXYCHLOR CE;ON! CYCLODIENE GROUP ctC~ct Cl, Ct ctc~ct c, 0 C[ C[ CI C[ ct (&CHLORDANE HEPTACHLOR ALDRIN ISODRIN ISOBENZANE CiCI~sT._~0 cl c[ CL.-CL..,, n ctt~V~'s~o rCl C(l"~Cl " Cl Cl Cl ct ENNE/~HLOR HEPT,tkCHLOR- DIELDRIN ENDRIN a-ENDOSULFAN EPOXlDE

B CH3,,x,CH3 CH3...CH3 CH~_/--%/\ CH3. CHT coO'~o H ALLETHRIN DECAMETHRIN CH3x.CH3 CH3x,CH3 Fig. 6. Second-generation insecti- CH3~--~L H cides, A) Chlorinated hydrocarbons c., coo-climaXJ CH3 COO-CH2~ a.d cyclodienes. B) Synthetic py- rethroids. C) Holan's DDT-py- TEI'RAMETHRI~ BIORESMETHRIN rethroid-hybrids (63,64). C 1 = DDT -analog part. C 2 = py- C.H3 CH3 CN O ~ rethroid-analog part. C 3 = syner- c,./~,~.0-c-4h-5 gistic part. ~, i~ x=/ FENVALERATE C .sc2o..r~ ~,co2c.~.~o' ' t"'cq " I i I 624 Journal o] Nematology, Volume 15, No 4, October 1983 France and England by chance and inde- ered until 1939 (20,98,108). Research into pendently discovered its insecticidal prop- public health applications of DDT went erties. Of the seven isomers, only the gam- on in secrecy during World War II, but in ma isomer, lindane, is appreciably active. 1944 DDT was already praised as the mira- Recently, synthesis conditions were ad- cle insecticide that would solve all future justed to produce mainly the gamma iso- insect problems. DDT, indeed, soon could mer. In 1976, the cumulative world pro- show a spectacular success record in con- duction of lindane had reached a total of trolling insect vectors of malaria, typhus, I million tons. In spite of this impressive and other debilitating diseases. Agricul- volume, the mode of action is insufficiently tural applications followed immediately. In known. Several theories, ranging from meta- 1961, a peak production year, 80,000 tons bolic interference with meso-inositol to were synthesized. However, by 1971, pro- CNS effects have been proposed (91). Ac- duction had dropped to 20,000 tons, a level cording to a brief review by Brooks (20), that represented 30/% of all chlorinated efforts in structure optimization had mar- hydrocarbon insecticides (91,98). Environ- ginal success. Only some mixed halogen mental concerns (27), combined with the analogs with gamma configuration sur- buildup of resistance (21,26) by all major passed, and a gamma methoxychloro-analog insect pests, resulted in its discontinued use came close to, the activity of lindane itself. in the industrialized nations by 1973. More • Toxaphene (Fig. 6A). The success biodegradable, but more expensive, DDT stories of lindane, DDT, and the cyclo- analogs like methoxychlor (Fig. 6A) re- dienes paved the way for the preparation placed it (33,98,102). DDT still is registered and testing of other chlorinated hydrocar- for emergency use in the United States in bons. In 1948, camphene from pine tar, an case of public health threats. In third-world unorthodox starting material, was selected countries, its low price still makes DDT for chlorination, and the polychlorinated the most attractive chlorinated hydrocar- terpene toxaphene, a mixture of 175 indi- bon insecticide. vidual components, was produced (129). • Cyclodiene insecticides (Fig. 6A). Soon Compounds such as 2,2,5-endo-6-exo-8,9,10- after World War II, great opportunities ex- heptachlorobornane were identified as in- isted for use of newly discovered, cheap secticidal components (29); the most toxic synthetic intermediates derived from petro- component to mice, goldfish, and house- chemicals. Around 1930, Diels and Alder flies was the octachlorobornane homolog had discovered the general 1,4-addition re- (Fig. 6A) (129). action named in their honor (66). This re- More than 100 components, totalling action facilitated simple, inexpensive, and 75% by weight in the crude toxaphene previously unavailable routes of syntheses mixture, remain to be characterized. Since for chlorinated compounds. The group of its discovery in 1948, 450,000 tons of toxa- cyclodiene insecticides (aldrin, dieldrin, phene have been produced worldwide, isodrin, endrin, isobenzane, heptachlor and mainly for use against cotton and livestock its epoxide, chlordane, endosulfan, mirex, insects. But concerns with accunmlation and chlordecone) (Fig. 6A) are all derived in food chains, its possible carcinogenicity from hexachlorocyclopentadiene by Diels- in man, and its effects on immune responses Akter reaction with a variety of dienophiles in mice (4) prompted the EPA to restrict or by a dimerization reaction of the start- its use as of 1982 in the United States. ing material. In 1944, Hyman (20) took • DDT (Fig. 6A). Dichlorodiphenyltrichlo- credit for recognizing the potential of this roethane (DDT) can be considered as the simple synthetic route when he made chlor- forerunner of the chlorinated hydrocarbons dane, aldrin, and dieldrin, while Kearns insecticides. Its production in 1971 in the et al. (79) first reported the insecticidal United States reached a total of 65,000 activity of chlordane. The precise biochem- tons. DDT was first synthesised by Zeidler ical lesions caused by cyclodienes never in 1874 via the then newly discovered Aivon were clarified. They probably interfere with Baeyer condensation reaction between chlo- nerve axons and act as CNS stimulants. In robenzene and trichloroacetaldehyde (158); spite of this lack of knowledge, many other its insecticidal properties were not discov- cyclodienes were made by the manufac- Insecticides, their design and evaluation: Hummel 625 turers. As a group; cyclodienes reached esters. Subsequently, decamethrine was record sale volumes comparable to DDT-- synthesized by systematic structure-toxicity 20,000 tons in 1971 (91). optimization (40). With an LDs0 of 0.01 With the exception of endosulfan, the mg/kg and effective application rates of cyclodienes are all environmentally quite 10-20 g/ha, it represented the then most stable. Because of their lipophilicity they effective insecticide. Developments since tend to accumulate in food chains and in 1974 have heen broadly reviewed (42). nerve tissues. Reports of their detrimental Several systematic improvements of other effects on mammalian fertility, chronic in- synthetic routes have been reviewed by terference with the electrical activity in Naumann (113). They led Japanese groups the brain, and suspected carcinogen;city to tetramethrine and fenvalerate (Fig. 6B). (27) lead to their gradual phase-out during Simultaneous use of synergists (sesamin, the 1970s. Because of its reduced persist- piperonyl butoxide) can increase effective- ence, endosulfan can be considered the ness of the natural pyrethrins by blocking most benign of the group, although on metabolic degradation systems. One note- Metcalf's scale (101) it ranks in category worthy approach is the chemical incorpora- 9.7, comparable with mevinphos, azinphos- tion of tile methylenedioxy moiety (the methyl, and lindane. group confering synergism) into the insec- • Mirex and chlordecone (Fig. 6A). These ticidal pyrethroid molecule (Fig. 6B), highly chlorinated compounds are prime rather than physically mixing it into the examples of "hard pesticides." Both com- formulated product. This principle is dis- pounds, synthesized by dimerization of cussed further under "hybrid insecticides." hexachloropentadiene, were used for fire The primary targets of pyrethrins are ant control. They are highly persistent, tile ganglia of the insect CNS. With the bioaccumulate in food chains, induce multi- exception of criteria 1, 6, and 7, pyrethrins flmction oxidase enzymes (150), and have are close to the "ideal" insecticide. Gradual poor environmental ratings (101). Chlor- buildup of resistance by heavy selection decone (kepone ~) has been linked to cases pressure will, however, be unavoidable. of reduced spermatogenesis and infertility Hybrids between pyrethroids and DDT: in man (150). A different avenue for the design of Synthetic pyrethroids: The use of py- new insecticides has been suggested by rethrum powder as an insecticide probably Holan et al. (63,64). They identified "tox- dates back to antiquity, but the natural aphores" for pyrethroid analogs and for product, contained in the flowers of Chry- isosters of DDT, combined them in one santhemum sp. is still being extracted for chemical structure, and found 1) hybrid commercial use (28,92,114). Its high price structures with considerable insecticidal ac- and limited environmental persistence tivity and biodegradability, with the R- stimulated efforts in several laboratories in enantiomers being more toxic than the Japan and Switzerland to extract and char- S-enantiomers (as commonly found in py- acterize the active principles. Given the rethrins); 2) shifts of the site of action from crude analytical methods of the time, the the peripheral level (characteristic of DDT chemical identification of pyrethrins by type compounds) to more central (but still Staudinger and Ruzicka (147, 148) is con- unspecified) CNS sites; and 3) a strong po- sidered a milestone in natural product tential for synergism by addition of the chemistry. The full characterization of the methylenedioxy moiety, a structural feature numerous structural, geometric, and optical found in many mixed function oxidase in- isomers took another 50 years. Schechter et hibitors. The hybrids are nontoxic to mice. al. (130) initiated the search for synthetic One example is the 3,4-methylenedioxy- pyrethroids with improved stability and benzylester of 1-p-ethoxyphenyl-2,2-dichlo- ease of preparation. They developed alle- ropropane-l-carboxylate (Fig. 6C), thrin (Fig. 6B); less expensive, photosta- Although the hybrids between pyre- ble, synthetic analogs were prepared in throids and DDT show cross-resistance with Japan, England, and France. Elliott et al. both parent compounds and will therefore (39,41) synthesized bioresmethrine, the first not solve the crucial problem of irresisti- compound more toxic than the natural bility, they nevertheless point in the direc- 626 Journal of Nematology, Volume 15, No 4, October 1983 tion new developments may take. In sum- activity with a chance for recovery. Other mary, the pyrethrins, their synthetic ana- mechanisms, such as stimulation of mito- logs, and their hybrids provide one of the chondrial ATP'ase activity and inhibition best examples to date for enlightencd re- of oxidative phosphorylation (different search leading to structural optimization. from that caused by the uncoupler dinitro- The theme of hybrid insecticides will phenol), are discussed by Matsumura (91). appear again in the section on third-genera- Problems may arise from the carcino- tion insecticides, with Bowers' JH analogs genic potential for bladder epithelial cells (15,16) composed of features of JH and caused by chlordimeform degradation prod- the synergistic 3,4-methylenedioxyphenyl ucts like p-chloro-o-toluidine which is found moiety Fig. 8A. in the urine of treated mammals. Chlordimeform is an example of a MINOR COMPOUNDS promising class of compounds with various The search for new classes of compounds different activities. Its ability, at sublethal is an ongoing endeavor in many industrial doses, to modify behavior, is somewhat laboratories. Of equal importance is the reminiscent of behavior modifiers isolated refinement of bioassay procedures. Orig- from natural sources. Its discovery under- inally, bioassays were designed to test for scores our ability for developing new leads toxicity against fungi, weeds, mites, or that may provide keys for entirely new nematodes, and only for short-term insect modes of action. toxicity. Because of tiffs blind screening Nitromethylene heterocycles (NMHs): strategy, promising leads were missed. Re- NMHs originated in the Shell laboratories fined tests introduced within the last dec- from structure optimization of unspecified ade and long-term screening corrected this synthetic compounds (142,143). In their shortfall and rewarded the new initiative toxicities for a number of insects, NMHs with the discovery of several new insecti- with the pyridyl moiety are comparable to cides such as chlordimeform, the nitro- parathion while being relatively safe to methylene heterocycles, and the growth reg- mammals. They are cholinergic agonists at ulators. the CNS level. Although they are fast acting Formamidines: This class of compounds "knockdown" agents like the pyrethroids was originally developed as acaricides. and show a favorable weight-to-toxicity ra- Later bioassays revealed excellent ovicidal tio, agricultural applications have been activity. Also, the compounds could in- hampered by their environmental insta- hibit some Lepidoptera and Hemiptera bility. Problems also arise from the fact larvae from consuming plant tissue treated that they have a zwitterionic charge distri- with the compounds. bution and are not readily taken up by the Chlordimeform (N-4-chloro-o-tolyl)-N, lipophilic surface of arthropod appendages. N-dirnethyl formamidine) and a series of Field resuhs during the last decade have analogs have been reviewed (65,82,89). To- been erratic, and the compounds, in spite day, the formamidines are extensively used of their promise, have not yet gained reg- for control of cotton and garden insects. istration. In some situations, chlordimeform is pre- Minor botanical insecticides containing ferred to broad spectrum pyrethroids which nitrogen: Like pyrethrins, the minor bo- tend to kill insect predators and parasitoid tanical insecticides have been used for cen- populations and thus generate secondary turies (37). Nicotine, nornicotine (Fig. 7) outbreaks of new pests that are originally and anabasine occur in tobacco, Nicotiana kept in ecological equilibrium. sp,, and in the small perennial shrub .4na- The hyperexcitation and the "crawling basis sp. In 1951, 750 tons of nicotine, the off" reaction observed in treated larvae is predominant alkaloid of this group, was explained by noncholinergic, central in- sold in the United States and 2,800 tons hibitory effects of chlordimeform and some worldwide. Although it was first isolated of its metabolites on the monoamino- 150 years ago and synthesized only 75 years oxidase system. Accumulation of biogenic ago, its use as an aphicide can be docu- amines like serotonin, norepinephrine, and mented in France in 1763 (137). Nicotine octopamine (44) results in temporary hyper- was the most toxic of all the synthetic Insecticides, their design and evaluation: Hummel 627

I'1" ~)O"!H~ CH3"i%' ~ PYRETHRIN I PYRETHRIN CH3

R R = CH3 NICOTINE R = H NOR NICOTINE RO~]ON~

-...,/ CH3 NEREISTOXIN 6 CARTAP

HLI..i'(~ V H ° n u.~t"'~U~..,,....IK~cCH3

",...[LRW~H CH3 AVERMECTIN Bla b,,..~ .~CH" H i .'~OH-"•J Fig. 7. In~'cticides of botanical, marine, and microbial origin, analogs investigated (157). Its high toxicity enocaulon sp.) gained commercial impor- to warm-blooded animals is explainable tance as insecticides against lice, plant by its easy skin and lung penetration and feeding Hemiptera, and thrips. But their by its interaction with postsynaptic acetyl- high cost, chemical lability, and toxicity choline receptors. The broad range toxicity to mammals limit their usefulness (94). of these alkaloids, even when used as sul- Similarly, the root and stem of Ryania sp., fate salts, calls for great camion in handling which contain ryanodine (30,94), have been and limits their undisputed value as insec- applied for the control of European corn ticides. borers and other crop insects. The higher Veratrine alkaloids from hellebore roots air and light stability and the improved (Veratrum sp.) and sabadilla seeds (Scho- residual activity of these plant extracts gave 628 Journal of Nematology, Volume 15, No 4, October 1983 them a temporary advantage over py- light; difficulties with standardization of the rethrum and rotenone. virulence obtained in different batches have Nonalkaloid botanicals: Water extracts also been reported. from wood chips of the tropical tree Quas- zlvermectins: These novel microbial in- sia amara contain quassin and neoquassin, secticides are produced by cultured mycelia tetracyclic diterpenes with considerable of the soil microorganism Streptomyces toxicity against sawflies and aphids (37). avermitilis (123). Originally, avermectins Foday, they have only historical value. were discovered as broad spectrum anthel- Rotenoids have been described in 68 mintic agents for domestic animals with a plant species of the family Leguminosae, very high activity in the range of 10-300 including 12 species of Derris (137). Origi- ppb (78). Later tests on confused flour nally used as fish poisons, the insecticidal beetles, sheep blowflies, and ectoparasites properties of the water soluble rotenoids including mites and ticks encouraged fur- were known as early as 1848. Rotenone, ther testing. Activity against all major or- a pentacyclic flavonoid (30,47) (Fig. 7), was ders of insects, as well as against plant- first isolated in 1902 and chemically char- parasitic nematodes, was obtained at LD.~,, acterized in 1932 by four independently levels as low as 10 ~tg/kg. One of the dis- working groups. The synthetic substance tinctive features of avermectins is their is too expensive for agricultural use, but novel chemical structure identified as a even today the natural rotenoids are valued 16-membered pentacyclic lactone disaccha- for their insecticidal properties on home ride (3). Eight different compounds were and garden crops and for their low toxici- isolated, each with slightly different struc- ties to plants and mammals. After the intro- ture and activity spectrum (Fig. 7). duction of second-generation insecticides, The symptoms of poisoning appear U.S. imports Of Derris roots dropped from slowly and would have been missed by the 6,600 tons in 1947 to 2,300 tons in 1948 classical short-term bioassay for toxicity. (137). Paralysis of the caterpillar pseudopodia re- Insecticides from microbial origin: Mi- stilts in inhibition of movement on the crobial insecticides are accessible by large- plant and feeding. Egg production in firc scale fermentation and are relatively insect ant queens is permanently inhibited by specific. Today they claim a minor but sig- doses as low as 0.12 g/ha. nificant share of the market. Among the Avermectins have a unique mechanism more than 1,500 known insect pathogens of action. Unlike OP compounds and car- are several types of virus, microsporidia, bamates, avermectin has no cholinergic and the spore-forming bacteria. The bacte- effects. Rather, it blocks transmission of rium Bacillus thuringiensis (BT), discov- electrical potentials between interneurons ered in 1915 by Berliner, has been exten- anti excitatory motoneurons in the ventral sively studied (90,112). Preparations of BT nerve cord; it inhibits transmission between contain a crystalline toxic protein (&endo- inhibitory motoneurons and muscle by toxin), a thermostabile water soluble fl opening the chloride channels on the mem- exotoxin (30), and spores infective to in- branes; but it has little effect on excitatory sects (5). Commercial production of BT neuromuscular junctions. The block of began in 1958. interneuron to excitatory motoneuron In contrast to the fast acting chemical transmission can be reversed by picrotoxin, insecticides, microbials cause slow paraly- a known gamma-aminobutyric acid sis and eventual death by septicemia. The (GABA) antagonist. Avermectins cause re- serotype BT israelensis specifically kills lease of GABA from brain synaptosomes in mosquitos and blackfly larvae and has con- rats (120). Interference with the chloride siderable value in public health programs. gate has been observed by Fritz et al. (46). BT preparations may be applied to crops It is too early to assess the potential of shortly before harvest without human avermectins. They certainly will stimulate health or environmental hazards. In many more research and investments. respects, BT comes close to the ideal in- FungaI secondary metabolites with in- secticide, even though problems have been secticidal activity: A recent example was encountered with its lability against sun- described by Claydon and Grove (32). In Insecticides, their design and evaluation: Hummel 629 surface cultures, one strain of Vertici!lium which regulate molts at the pupal-to-adult lecanii produced pyridine-2,6-dicarboxylic stage. Insects treated with JHs do not reach acid. Its insecticidal activity against the ~dulthood; they are arrested in their de- blowfly Calliphora erythrocephala was de- velopment and eventually die without re- termined as 50 yg/fly. producing. Industrial scientists like to call the JHs "insect growth regulators." This THIRD-GENERATION INSECTICIDES alternative term is justified because JHs This term was coined by Williams (156). have no hormonal activity in warm-blooded It characterizes a chemically diverse group animals. consisting of the ecdysteroids, the juvenile After a painstaking purification proce- hormones and juvenoids, the chitin synthe- dure monitored by quantitative bioassays, tase inhibitors, and the precocenes. Several R611er et al. (128) isolated and chemically feeding deterrent natural products have identified the hormone as a terpenoid re- striking morphological effects, in addition lated to farnesol. Today, three more nat- to their ability to modify insect feeding be- urally occurring hormones are known (Fig. havior. They interfere with tile molting 8A), and hundreds of synthetic analogs process and are therefore included in the have been prepared. group of third-generation insecticides. Since the discovery of the "paper factor" Their example shows that the borderline juvabione in balsam fir (Abies balsamea) to tile fourth-generation insecticides is less (140) and the registration of methoprene clearcut than the systematic mind might (139) as a synthetic JH analog with im- wish it to be. proved environmental stability, progress in Ecdysteroids: Arthropods have relatively this field has been vigorous. Many reviews rigid exoskeleta. Growth and development highlight our relatively high degree of from the larval to the pupal and adult knowledge (17,56,59,135,146) and sophisti- stages requires periodic molting. This proc- cation. JHs are convincing examples for tile ess is under tight control of the mohing success of enlightened research. hormone ecdysone. It was originally iso- If JHs ultimately did not revolutionize lated by Butenandt and Karlson (23) and tile insecticide field, it is probably because structurally identified as a C2T steroid by of the formidable crux of resistance (21) Karlson et al. (76). The term "ecdysteroids" and the impatience of the user who wants is now used for the entire family of closely to see "dead insects" instantly after appli- related hormones which have been isolated cation. This is, however, a societal rather from insects, crustacea, and plants (67). than a toxicological problem. By design, A number of synthetic analogs (67) have JHs come close to the ideal insecticide. toxic effects, prevent egg development, or Their specificity is limited, but it can be interfere with cuticle hardening after molt- improved by appropriate formulations (84, ing. However, because the compounds were 85). environmentally unstable, or would not The chitin synthetase inhibitor diflu- penetrate the insect cuticle, or were too benzuron: Chitin (poly-N-acetyl-glucosa- expensive, they did not gain acceptance. mine) is a structural component of the cell Moreover, insects do have the capacity to wall of pathogenic fungi and also adds inactivate structurally identical, or similar, strength to the inner proteinaceous cuticu- phyto-ecdysteroids contained in some of lar layer of insect larvae. Inhibition of their food plants (141). Development of chitin synthesis will therefore interfere with the juvenile hormones and juvenoids has growth and development of these organ- been more successful. isms; insect larvae will be unable to molt Juvenile hormones and juvenoids (JHs): properly. The discovery of a factor controlling insect The discovery of the fungicidal proper- differentiation and development occurred ties of polyoxin D (43,105) prepared the bio- in 1937 (154). Twenty years later, an extract chemical background necessary for the from male cecropia silkmoths was found search for other compounds with similar to have potent juvenile hormone activity activity. Diflubenzuron, an inactive analog (155). JHs regulate molts at the juvenile obtained during screens in the herbicide or larval stage, in contrast to ecdysteroids synthesis program, turned out to be a po- A B R R' R" CH3-'~ CO2CH3 CH3~0~0H H0~0H R "~"'~H'O HO~ ~OH R R' R" CH 3 CH 3 CH3 JH-II/ DI MBOA GOSSY POL C2H 5 CH3 CH3 JH- I_] C2H5 C2H5 CH3 JR- i CH3 C1"13 _ ,.,,, .0Hj~,, (~ O 0 ~ C2H5 C2H5 C2H5 JH-O ~.~'-.,-. o2c..q.,~.-.~U/ ~ ~' CO2CH3 ~ < CHACO2~0fio 0H" JUVABIONE METHOPRENE CH302C" LO - C'b Ro 13 - 5223 AZADIRACHT!N TRICHILIN ,~ o~

....'0~ PRECOCENE .v I and ]I CH3~__~,H H H BOWERS' HYBRIDS BETWEEN JH-1 Ho~¢ ~ R : CH3,C2H5 AND A SYNERGIS- TIC GROUP CLERODENDROSIDE R': CI"13,C2H5 x= I -2 HO-~H H 00 H Fig. 8. Third-generation insecticides: Modifiers of insect development. A) Juvenile hormones. B) Feeding deterrants. Insecticides, their design and evaluation: Hummel 631 tent inhibitor of chitin synthetase in larvae clerodendroside and trichilins (111) (Fig. of Pieris brassicae (107) and other insects 8B). (121). Rationales and procedures for select- The most potent feeding deterrent ing diflubenzuron as the compound for known so far has been isolated from Neem ultimate development have been reviewed (dzadirachta indica), a native tree of the by Verloop and Ferrell (152). As in the lndo-African region. Among several dozen OP and carbamate series, Hammett's sigma compounds of similar structure, azadirach- constants and Hansch's 7r-constants were tin, a complex tetranortriterpenoid, is the helpful in identifying trends within a series most active component. As Schmutterer et of analogs. Biodegradability was an impor- al. (132) point out, topical application of 1 tant concern in selecting the difluoro- /~g azadirachtin per fourth-instar larva will analog. It has favorable environmental interfere with the molting process. In ad- properties (low bioaccumulation, moderate dition, azadirachtin sterilizes adult female persistance) and is safe to mammals. As insects. Obviously, Neem extracts have sev- with other growth regulators, its effects are eral distinguishable modes of action. For only visible several days after application. centuries, Neem tree products were im- Diflubenzuron is another example of the portant as insecticides in India and Africa. carefully designed and optimized members Partially purified Neem fruit extracts have of the third insecticide generation. It does been proposed for use in developing coun- not compete in price with the chlorinated tries as cheap but effective substitutes for hydrocarbons or the OP compounds. synthetic insecticides 003). Precocenes: Bowers et al. (17) discov- ered a new class of natural products with FOURTH-GENERATION growth regulating activity. Extracts of the INSECTICIDES greenhouse plant Ageratum houstontianum Chemical modification of behavior is produced developmental abnormalities in another recent approach to insect control. several insect species that paralleled pre- Chemical modifiers have in common that cocious molts obtained after ecdysterone they interfere with normal behavior pat- treatment. The compounds, named after terns and delay or prevent mating and pro- their biological effect, were isolated, chem- ducing offspring. Pheromones (75) (Fig. 9), ically identified, and synthesized; they are also known as ecomones, sex attractants, chromene derivatives (18) (Fig. 8A). Pre- and aggregants, are the prominent members cocenes are oxidized in vivo to highly reac- of the fourth-generation insecticides. They tive expoxides with known alkylating occur in a wide variety of insects (69,106), power (122). They exhibit specific toxicity 138,), spiders, mites, nematodes (14,125), to the sensitive neurosecretory cells where aquatic invertebrates (109), and animals they produce effects comparable to an al- including primates (127). Examples of latectomy. This results in precocious meta- insect pheromones given in Fig. 9 include morphosis to sterile or malformed adults silk moth (24,77) and honeybee (53), gypsy or to intermediates between larval, pupal, moth (9), boll weevil (151), pink bollworm and adult stages of development which are (68), bollworms (81), and corn rootworms incapable of reproduction. (57). In terms of specificity and sensitivity, Insect anti[eedants (Fig. 8B): Natural female moths produce the most striking products with feeding deterrent activity behavioral effects with their sex pheromones against insects have been described in sev- (124). However, some male butterflies have eral plant species; e.g., DIMBOA (80), a reversed tile sex roles and use pheromones compound from resistant hybrids of Zea as aphrodisiacs during courtship with fe- mays which discourages larval feeding of males. Social insects like termites, ants, and the corn earworm Heliothis zea and the bees communicate in their hives by phero- European corn borer Ostrinia nubilalis. mones, and bark beetles of both sexes use According to Lukefahr and Martin (88), pheromones for marking aggregation sites. the pigment gossypol confers resistance to Although chemically quite varied (70), cotton plants against the bollworm and all pheromones share a few common prop- tobacco budworm. Other antifeedant com- erties: volatility, specificity, potency, and pounds (61,71) have been identified as nontoxicity. Synthetic pheromones can be 632 Journal of Nematology, Volume 15, No 4, October 1983 LEPIDOPTERA COLEOPTERA i

BOMBYKOL HO I .,v--vvvvv o H BOMBYKAL,Siikrd6th IPSENOL cis- IPSDEN- 0 VERBENOL 0L o.c.c, ./~/x/vx/X~OH O LOOPL URE. C a bbage exo- Looper BREVICO- FRONTA- MYRCENE MIN LIN O~PARLURE, ~~ endo- Gypsy moth BREVICOMIN, 9. Bark beettes _ "'CH3 aggregation pheromones vv--v _ N O.C.CH3 i~C~ H H'I(CH2OH GOSSYPLURE, Pink 0 ('L. GRAND- Bottworm LURE, HYMENO- ii H~H0 0HC[~ Boltwee- vil male ~ pheromone '0H 0••TERAH H SULCATOL, QUEEN SUBSTANCE, Ambrosia beetle CH3_ Honey bee Female sex ~,7~OOCH3,~ Leaf .cut- ~'~~'OO~C2H$ pheromonesof corn ,,,n ~,^.rTAH ~t.u~,im E tlngant I Oo rootworm Trail pheromone ~ beetles Fig. 9. Fourth-generation insecticides: Modifiers of insect behavior. Insect pheromones of Lepidoptcra, Hymenoptera, and Coleoptera. used to manipulate insect behavior in such ternative explanations for the observed ef- a way that it will be maladaptive to the fects are disorientation of males by the individual organism (69,106,116,119,138). many artificial point sources which mask Mechanisms include the saturation of an- the females' natural signals. For a few tennal receptor sites with synthetic phero- selected agricultural pests (31,87,106), mones so that natural pheromones are not pheronlone applications resulted in con- detected, thereby preventing mating. A]- trol levels that are economically competi- Insecticides, their design and evaluation: Hummel 633 tive with conventional second-generation The oceans cover over 71% of the pest control measures. Their environmen- earth's surface. Comparatively little is tal benignity is among the best of all in- known about this huge ecosystem and its re- secticides. sources. Nereistoxin, a defensive substance Pheromone structural optimization has produced by the marine annelid Lumbri- reached a high state of perfection through conereis heteropoda, served as the model natural selection over many insect genera- compound tor the synthesis of the insecti- tions. The analytical chemist can, however, cide cartap (Fig. 7). Its metabolic activa- improve pheromone mixtures by quanti- tion in vivo generates nereistoxin, a synap- tatively defining minor components (81). tic blocking agent that binds, like nicotine, The synthetic chemist can identify less to ACh receptors. costly starting materials and new routes for Other new structures from diverse ma- production of pheromones of higher purity. rine organisms are currently being inten- The ecologist can further improve the effi- sively investigated (45,126,131). Potent an- cacy of refined pheromone mixtures by de- tifungal, antibacterial, and antitumor drugs veloping better monitoring traps and by have already been found. defining tile best time of day and season for Contributions from the neurosciences: their deployment (69). A promising target for new insecticides is Novel formulation technology: Im- the cholin acetyltransferase. Inhibitors of proved insecticides and behavior modifiers this enzyme would impede the synthesis can be attained by providing new com- of ACh (134). At ttle other end of the bio- pounds and formulations and by develop- chemical chain, tile interaction of ACh ing new modes of application. Recent with its receptor would provide a target for progress has been made in the field of interference. The natural products nico- microencapsulation techniques (25). Com- tine and nereistoxin point in the direction pounds like OP esters and certain carba- research may take (73). mates previously considered hazardous to The glutamate transmitter system is the applicator can now be handled without only superficially understood (104). More special precautions. Also, the formulated research is needed to identify naturally oc- material has increased stability against oxi- curring glutamate analogs and to design dation, hydrolysis, and photolysis and will possible synthetic substances that focus on allow the user to realize savings by more that system (60). judicious and sparing application. Con- Neurotransmitters like ACh, GABA, trolled release of pheromones from hollow and catecholamines produce postsynaptic fibers and laminated sheets (84,85) repre- effects quite rapidly. ACh excites the post- sents progress in application technology as synaptic membrane across the 100-A-wide well. synaptic cleft (136) within 40 msec and The final success of fourth-generation must be inactivated by ACHE, one of the insecticides depends on close cooperation most active enzymes, before another excita- and integration of the efforts of several dis- tory event can take place. In contrast, newly ciplines. This in'ice must be paid for the discovered neurotransmitter peptides with ever increasing sophistication of approach. molecular weights of about 1,000 Daltons It is a characteristic of all biorational strat- are similar to the mammalian luteotropic egies (38,93). hormone releaser hormone (LHRH). They act slowly, within 10 min, and by diffusion PROMISING LEADS over distances of 10 /ml (7). inactivation Novel natural products: Toxins are ef- sets in as slowly as activation takes place. fectively used by insects in their own de- Toxicological interference with this cell fense or as offensive weapons in their search communication system seems possible, and for food and shelter (12). Dendrolasin, investigations in this area could produce iridomyrmecin, and cantharidin (30) all new types of insecticides (22,35,117). A have considerable activity. But analog syn- lead already has been provided in the field thesis has not yet produced useful insect of specific antibodies (115). control agents based on the leads suggested In this futuristic area, the pleas by by nature. Barend Ter Haar (6) and Corbett (36) for 634 Journal of Nematology, Volume 15, No 4, October 1983 interdisciplinary cooperation provide the upon better knowledge of their modes of key for success. Classical toxicology must action. Target sites investigated so far in- join the neurosciences in its quest for new clude 1) the neuromuscular junction anti knowledge. If results from this research the peripheral nervons system, 2) the CNS, find practical applications, the compounds 3) cuticle formation, 4) reproduction, and will justifiably be labeled "fifth-generation 5) behavior of host and mate finding. Other insecticides" because they will represent sites for increased future attention should new approaches with respect to their dis- include 1) manipulation of ionic nerve covery, their mode of action, their design, channels other than Na ÷, 2) novel acylating and their evaluation. agents for AChE (other than phosphory- lating and carbainylating agents known to- CONCLUSIONS day), 3) anti-JH, 4) cholinergic receptor Short appraisals of the developmental agonists, 5) cholinacetyl transferase inhibi- histories of some discussed insecticides are tors, 6) glutamate analogs, and 7) anti- summarized in Table 1. The majority of bodies as insecticides (8). insecticides were developed by the "proven scheme" of random synthesis; screening and LITERATURE CITED structure optimization were productive but I. Abou-Donia, M. B. 1981. Organophosphorus time consuming and expensive. Cases of ester-induced delayed neurotoxicity. Ann. R. rational design and enlightened research, Pharmacol. 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