Toxicity and Neurotoxic Effects of Monoterpenoids: in Insects and Earthworms Joel R

Entomology Publications Entomology

1-9-1991 Toxicity and Neurotoxic Effects of Monoterpenoids: In Insects and Earthworms Joel R. Coats Iowa State University, [email protected]

Laura L. Karr Iowa State University

Charles D. Drewes Iowa State University

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Abstract The insecticidal activity of several monoterpenoids from essential oils was evaluated against insect pests. Toxicity tests illustrated the bioactivity of d-limonene, α-terpineol, β-myrcene, linalool, and pulegone against insects, including the house fly, the German cockroach, the rice weevil, and the western corn rootworm. Bioassays were conducted to assess their toxicity via topical application, fumigation, ingestion, and ovicidal exposures. Growth, reproduction and repellency were also evaluated in the German cockroach. Non-invasive electrophysiological recordings were used with an earthworm to investigate neurotoxic effects of the monoterpenoids. Relevant monoterpenoid bioassay results in the literature are also discussed.

Disciplines Entomology | Environmental Health | Other Animal Sciences | Plant Biology | Plant Sciences

Comments Reprinted (adapted) with permission from Naturally Occurring Pest Bioregulators, 449(20); 305-316. Doi: 10.1021/bk-1991-0449.ch020. 1991 American Chemical Society.

This book chapter is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ent_pubs/377 Chapter 20

Toxicity and Neurotoxic Effects of Monoterpenoids In Insects and Earthworms

Joel R. Coats, Laura L. Karr1, and Charles D. Drewes

Department of Entomology and Department of Zoology, Iowa State University, Ames, IA 50011

The insecticidal activity of several monoterpenoids from essential oils was evaluated against insect pests. Toxicity tests illustrated the bioactivity of d-limonene, α-terpineol, ß-myrcene, linalool, and pulegone against insects, including the house fly, the German cockroach, the rice weevil, and the western corn rootworm. Bioassays were conducted to assess their toxicity via topical application, fumigation, ingestion, and ovicidal exposures. Growth, reproduction and repellency were also evaluated in the German cockroach. Non-invasive electrophysiological recordings were used with an earthworm to investigate neurotoxic effects of the monoterpenoids. Relevant monoterpenoid bioassay results in the literature are also discussed.

Many essential oils from plants possess biological activity against pests that could be harmful to the plant. Some exhibit acute toxicity, while others demonstrate repellent, antifeedant, or antioviposition effects or inhibition of growth, development or reproduction (1). Many of the fragrant volatile oils contain ten- carbon hydrocarbons, or their related alcohols, ketones, aldehydes, carboxylic acids, and oxides, and are termed monoterpenoids. Most Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 are considered secondary plant chemicals, with little direct metabolic importance, but with considerable coevolutionary Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org significance (2). Plant-insect interactions have been studied for many years, but a better understanding of these complex coadaptive relationships could provide a basis for using plant-derived chemicals in biorational approaches for better management of pest organisms (3). Botanical insecticides such as pyrethrins and rotenone have proven to be both safe and effective in controlling insect pests. An improved knowledge of the monoterpenoids (as well as sesquiterpenes, diterpenes, triterpenes, tetraterpenes) and their effects on insects contributes to unraveling the intricate interactions that have shaped the coevolution of insects and plants. It also provides leads for possible utility of these safe, degradable compounds in modern pest control, and, as more advanced genetic engineering capabilities develop, the potential for

Current address: DowElanco, Walnut Creek, CA 94598

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exploiting their efficacy by transferring genes to different crops or by selecting for the protective chemicals in breeding programs. Figure 1 shows the structures of 2 cyclic and 2 acyclic monoterpenoids. Knowledge of the spectrum of insecticidal activity is limited for most of the terpenoids. The results of d-limonene trials against a wide range of insect groups indicate that this important constituent of citrus oil is toxic to some life stages of some species via some routes of exposure (4,5,6,7). Its utility as a broad-spectrum insecticide, however, does not seem feasible. Spectrum of repellent activity has been evaluated for several types of terpenes, and reproductive effects have been described for some chemicals (1,8). Mechanisms of acute toxicity have not been elucidated for the monoterpenoids, but the onset of symptoms is usually rapid, manifested as agitation, hyperactivity, and quick knockdown (4,5.) • Our investigations have also included electrophysiological studies of the toxic effects of monoterpenoids on nerves.

Methods

Topical applications to house flies (Musca domeatica) and German cockroaches (Blattella germanica), fumigation of German cockroaches and rice weevils {Sitophilus oryzae) and repellency to German cockroaches were conducted by methods described previously (4). The toxicity by ingestion and effects on growth and reproduction were evaluated by incorporation of the chemicals into the ground cat chow diet of the German cockroaches (9). Lavicidal and ovicidal activity against the western corn rootworm (Diabrotica vergifera vergifera) were tested in petri dishes of soil and on moist blotter paper, respectively (4). Repellency of terpenoids was evaluated with German cockroaches in choice tests, using pairs of plastic boxes (9 X 8.5 X 2cm) connected by plastic tubing. Treated filter paper was placed in one box, and a control (acetone treated) filter paper was put in the other box (4). Hedgeapple, bay leaves, and spearmint chewing gum were tested using the weight of products (in ug) per unit of volume of the box (in cnr) for determination of exposure concentrations (1 /jg/cnr = 1 ppm). Neurotoxicity was assessed in the earthworm Eisenia fetida using non-invasive electrophysiological techniques described previously (10). The earthworms were exposed in a vapor/contact method in which a small volume of terpenoid was delivered onto a moist filter paper in a vial which was then closed tightly (11). Periodically, worms were placed on an etched circuitboard recording grid for electrophysiological testing, and giant fiber activity Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 associated with the escape response was monitored (12.). The method

Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org has been used previously to examine sublethal effects of pesticides on earthworm neural activity (13).

Bioassay responses were calculated as LD50's, ED50's or ET50's using the trimmed Spearman-Karber method (14). Duncan's multiple range test and analysis of variance were used for the repellency trials, and the paired comparison t-test was used to analyze the food preference experiments.

Results and Discussion

I. Acute Toxicity. The utility of an insecticide has often been judged by its immediate and acute actions on pest species of insects. Terpenoids can effect toxicity symptoms very rapidly via contact or vapor exposures, including hyperactivity and tremors. Their degree of potency is significantly less than conventional synthetic organic insecticides, often by orders of magnitude. However, their actions can be very effective under circumstances that allow brief high-concentration uses of generally safe

Hedin; Naturally Occurring Pest Bioregulators ACS Symposium Series; American Chemical Society: Washington, DC, 1991. 20. COATS ETAL. Toxicity and Neurotoxic Effects of Monoterpenoids 307

pulegone

Fig. 1 - Structures of 4 monoterpenoids Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org

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chemicals (e.g., greenhouses, animal shampoos and dips, fumigations). Research has been limited to a few insect species and a few terpenoids, but the results of the topical treatments, fumigations, and repellency studies indicate that the monoterpenoids can exert substantial toxicity alone, or with a synergist, and demonstrate considerable repellent activity as well. Very little is known about their mode of action.

Topical Exposure. Dosing of female house fly females with five monoterpenoids yielded toxic effects when applied alone at high doses. d-Limonene was the most active of the five (Table I). Use of the synergist piperonyl butoxide enhanced the activity of d- limonene, pulegone, and linalool considerably, by 17, 21, and >14 fold, respectively. These results indicate that those three terpenoids' insecticidal activity is expressed more fully when the oxidative detoxification process is inhibited. It is not surprising that flies can detoxify them rapidly, considering the relatively simple hydrocarbon structures of the monoterpenoids. In the male German cockroach trials, pulegone was shown to be the most potent compound tested, but it required 260 ug/insect at the median lethal dose. d-Limonene and linalool also demonstrated slight toxicity (Table I). Myrcene, a-terpineol and I-limonene (not shown) were the least toxic. d-Limonene was slightly synergized, while linalool was not.

Table I. Acute toxicity of monoterpenoids to the house fly, Musca domestica, and the German cockroach, Blattella germanica, by topical application (24-h mortality data), alone or with piperonyl butoxide (p.b.)

Treatment LDc«(/ig/insect)8 female house tlv mal e German cockroach

d-Limonene 90(70-130) 700(610-810) 1 d-Limonene:5 p.b. 5.2(3.7-7.5) 300(180-560) Pulegone 166(131-201) 260(230-300) 1 Pulegone: p.b. 7.7(5.4-11) Linalool >100 550(410-730) 1 Linalool: p.b. 32(23-44) 1 Linalool: p.b. 7.2(4.1-12) 610(530-700) Myrcene 360(300-430) >1,580 a-Terpineol 310(260-380) 1,070(680-1,690) Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020

Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org 95% confidence intervals

Contact toxicity trials with d-limonene against adult cat fleas illustrated extremely fast knockdown time and mortality. A synergistic ratio of 3.2 was observed when piperonyl butoxide was also used on treated filter papers (5). Southern pine beetles, Dendroctonus frontalis, were assayed for susceptibility to 16 terpenoids from pine oleoresin (15). The

LD^0 of d-limonene was 0.47 /ig/insect, while i-limonene was slightly less toxic (0.55 /jg/insect), as was myrcene (0.62 jig/insect). The most toxic chemical tested was limonene dioxide (0.24 /ig/insect), indicating that oxidation of limonene may be an activation process (15). Dose-mortality studies with constituents of lyophilized lemon oil demonstrated notable toxicity to the adult cowpea weevil,

Callosobruchus maculatus. The LD5Q of the oil was 16.4 /ig/insect,

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while three potent compounds isolated from the oil by thin-layer

chromatography had LD50's from 4.73 to 2.66 /ig/insect (16). Nematicidal activity of some essential oils and their major constituents has been reported (17.) • Eugenol showed the most efficacy against 3 species of nematodes, while linalool, geraniol, and menthol were more effective against the root-knot nematode.

Fumigation. The monoterpenoids, as a family, are volatile which makes them potentially quite useful as fumigants. Most are also pleasantly odiferous and of low toxicity to mammals, properties which also are consistent with fumigation usages on produce, grain, clothing, buildings, ships and soil. Laboratory trials against adult rice weevils illustrate the relative potencies of five monoterpenoids in Table II.

Table II. Acute toxicity of monoterpenoids to the rice weevil (Sitophilus oryzae) and the German cockroach (Blattella germanica) by fumigation (24-h mortality data)

a D Treatment LC50 (ppm) ' rice weevil German cockroach0

cf-Limonene 19(13-27) 23(17-31) Pulegone 3.1(2.7-3.5) 9.6(1-113) 9 17(10-30) 6 4.5(0.6-36) Linalool 14 12 Myrcene >100 >100 a-Terpineol >100 >100

a95% confidence intervals ^g/liter cboth sexes included (50:50) except where noted

The most toxic of the five in a vapor form was pulegone with an

LC50 of 3.1 ppm (mg/liter of air). Linalool and cf-limonene were also effective, while myrcene and a-terpineol were ineffective. Against German cockroaches, pulegone was, again, the most efficacious, followed by linalool and limonene (Table II). Male cockroaches were four times as susceptible as the females to pulegone. Vapor-exposure assays for adult cat fleas also showed that cf- limonene was effective at inducing rapid knockdown and mortality as a fumigant (5) • The larvae were also relatively susceptible to cf- Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 limonene vapors, while the eggs were less susceptible and pupae

Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org were relatively tolerant of this chemical. Vapors of monoterpenes from pine were evaluated for bioactivity against the Western pine beetle, Dendroctonus brevicomis. The four-day bioassay determined that limonene was the most toxic among the five compounds tested (7).

Larvicidal and Ovicidal Activity. Acute toxicity testing of terpenoids on immature stages of insects has been limited to a few studies. Larvae (third instar) of the western corn rootworm, Diabrotica vergifera vergifera, were assayed in treated soil. The 48-h LCCQ for cf-limonene was 12.2 /ig/g soil (4), which is approxi• mately 10-fold less potent than the standard organophosphorus and carbamate compounds currently used for rootworm control (19). The eggs of that species were exposed to treated blotter paper to

assess contact ovicidal activity. The LC50's at 28 days for cf- limonene and linalool were 1.8% active ingredient (a.i.) and 0.26% a.i., respectively. These 2 monoterpenoids were about 3 orders of

magnitude less potent than chlordimeform (LC50 = 0.0006% a.i.).

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Citrus oils and several individual components of them were tested against larvae of the Caribbean fruit fly, Anaatrepha auapenaa (6_). Citral was the most efficaceous monoterpenoid, followed by limonene, then a-pinene and a-terpineol. One-hour immersion in a 40% (a.i.) solution of the most potent compound resulted in 50% mortality during the larval development period. The eggs of the Caribbean fruit fly have also been evaluated for ovicidal effects of terpenoids(6). a-Terpineol was the most effective of those tested (1% a.i. caused 100% mortality), followed by citral and limonene. a-Pinene had no effect. Studies on the house fly, Musea domes tica, revealed the considerable acute larvicidal activity of several terpenoids, especially carvacrol and cf-limonene (19). Several metamorphosis inhibition effects were also observed, including inhibition of pupal ecdysis, uneclosed pupae, and deformed adults. Numerous compounds exhibited potency in these developmental mortalities: camphene, carvacrol, carvone, cineole, citral, citronellal, citronellol, eugenol, farnesol, geraniol, limonene, linalool, 13- phellandrene, and a-pinene. Egg hatch was also inhibited by exposure to the terpenoids; the most effective ovicidal compounds were carvacrol, citronellal and B-phellandrene, with no acutely toxic effects noted to the embryos, but rather show inhibition of development of the embryo and inability to eclose from the egg (19). The larvae of the cat flea, Ctenocephalidea felia, were

susceptible to cf-limonene (LD5Q of 226 /ig/enr of filter paper), and some modest synergism was observed when piperonyl butoxide was

employed (LC5Q of 157 uq/cxx?). The vapors were also toxic to the flea larvae. Pupae were less susceptible than eggs, larvae, or adults. Eggs of the cat flea were exposed to cf-limonene at 65 ug/ctvr and mortality was 100%. A test of its toxicity to them in a vapor chamber showed 60% mortality at 130 uq/ctt? (5) . It is evident that some terpenoids possess efficacy against insect eggs, but the degree of potency is modest at best. It was reported that larvae of 3 species of mosquitoes were susceptible to cf-limonene but no data were provided (5.).

Repellencv. A number of other plant-derived terpenoids have been demonstrated to be repellent to various insects. Many compounds in this class have been proven to be attractants to certain insects (1). Limonene at 1 or 10 mg/box repelled the cockroaches, to the untreated box, in significantly (p<0.05) greater numbers than the 0.001 mg rate or the controls (4). Pulegone and linalool at the 10 mg/box rate repelled significantly more individuals than the 0.0001 mg rate or the controls. Myrcene and a-terpineol did not demonstrate any repellency. Natural pyrethrins were significantly Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 different from the controls at the 0.1 mg/box rate.

Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org Three other substances were bioassayed for repellency based on their purported household usefulness: hedgeapple, bay leaves, and spearmint chewing gum. Figure 2 shows that at the highest exposure rate, the hedgeapple and bay leaves effectively repelled a high percentage of the cockroaches, but were considerably less potent than pyrethrins. A recent study of repellency of some terpenoids and phenolics showed thymol and carvacrol were more effective for deterring oviposition by female Aedes aegypti mosquitoes than N,N-diethyl-m- toluamide (DEET). Eucalyptol (1,8-cineole) and p-cymene were considerably less repellent than DEET (20). House fly attractant and deterrent properties have been exhibited by numerous terpenoids. The cf isomer of limonene was repellent while the 2-limonene was an attractant. Carvone was attractive at low concentrations and repellent at high levels (jL£). A similar pattern has also been observed in German cockroaches for low and high concentrations of cf-limonene (4).

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bay leaves osage orange pyrethrins spearmint gum Material

••• 1000 ppm 100 ppm Y/////A 10 ppm (10 pyreth.) (1 pyreth.) (.1 pyretrO

Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 Fig. 2 - Repellency of hedgeapple, bay leaves, spearmint gum, and pyrethrins to German cockroaches. Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org

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Citrus oil components were evaluated for attractive or repellent activity toward the red scale, Aonidiella aurantii. Most hydrocarbon terpenes were attractive, while the alcohols, including linalool, were repellent. One aldehyde, citral, was attractive, while the other, citronellal, was repellent (21). Antifeedant properties of the herb tansy have been observed in the Colorado potato beetle, Leptinotarsa decemlineata, along with some larval mortality (22.). The major active constituents of oil of tansy include the monoterpenoids borneol, camphor, and thujone (23).

Effects on Insect Growth and Development. Chronic feeding studies were conducted with German cockroaches to determine if monoterpenoids have any influence on their survival or growth rate. Cohorts of newly eclosed nymphs were used in experiments that assessed the time of development to adult. Mortality was recorded weekly. The greatest effect was noted for those consuming diet treated with pulegone. A concentration of 10% in the diet caused 23% mortality after 1 week and 30% mortality at 2 weeks, compared to 50% mortality from 1% triflumuron. Linalool and cf-limonene caused no mortality above control rates after up to 4 weeks of treatment. Effects on growth rate were recorded for all monoterpenoids tested. Although mortality was only noted in the pulegone trials, linalool at a high dose (25%) caused a slight lengthening of time to adult. The other four terpenoids all resulted in dramatically more rapid growth and development. Linalool and cf-limonene effects are presented in Table III. Myrcene and a-terpineol had similar but less dramatic effects (9). A feeding preference study showed that untreated food was greatly preferred, confirming that the increased growth rate was not due to a feeding attractant or arrestant effect. At 1% active ingredient in the diet, pulegone was by far the most active inhibitor of feeding. A hormonal effect is one possible explanation of the increase in growth rate.

Table III. Effects of two monoterpenoids on growth rate of German cockroaches

Mean days from hatch to adult stage (% active ingredient in diet)

Chemical/dose Control(0%) 0.1% 1% 10%

Linalool 131 116 120 108 cf-Limonene 144 131 123 113 Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org Earlier work investigated growth effects from monoterpenoids dosed topically on last-instar larvae of the house fly (19). Development and metamorphosis failures occurred with several compounds. Cineole was the most active inhibitor of pupal eclosion (40% inhibition at a dose of 10 /jg/insect) and of imaginal differentiation, within those uneclosed pupae. Camphene, eugenol, and a-pinene were also active. Inhibition of pupal ecdysis was highest in insects treated with camphene, although carvone and several others were also effective. Effects of pulegone on the survival, growth and development of armyworm larvae have been investigated by Brattsten (1). Growth of the fall armyworm was markedly inhibited, and considerable mortality was noted at 0.1% in the diet.

Effects on Insect Reproduction. The reproductive cycle in insects is a complex process, often sensitive to perturbation by low doses of toxicants or hormones. In light of distinct differences between

Hedin; Naturally Occurring Pest Bioregulators ACS Symposium Series; American Chemical Society: Washington, DC, 1991. 20. COATS ETAL. Toxicity and Neurotoxic Effects of Monoterpenoids 313

insect and mammalian reproductive systems and endocrine controls, it should be possible to exploit those differences for biorational control of insect pests. Few investigations have been conducted on the effects of monoterpenoids on insect reproductive function or success. The German cockroach was not very susceptible to disruption of reproduction by d-limonene, linalool, myrcene, and a-terpineol (9). Exposure of males or females to lifetime feeding, to topical application, or to vapor exposure caused no effect to minimal effects on reproduction. However, topical treatment of their oothecae resulted in considerable embryotoxicity. d-Limonene was the most toxic of the group, with linalool also affecting the development and survival of the cockroach embryos (Table IV).

Table IV. Embryotoxic effect of monoterpenoids in German cockroach following topical treatment of the ootheca

Mean % of oothecae Chemical Dosefuch producing offspring cf-Limonene 0 90.0 210 73.3 420 70.0 840 16.7 Linalool 0 90.0 215 86.7 430 90.0 860 70.0 Myrcene 0 90.0 198 80.0 395 63.3 790 46.7 a-Terpineol 0 90.0 235 70.0 470 53.3 940 80.0

Studies on the southern armyworm, Spodoptera eridania, proved that pulegone fed to larvae was effective in reducing the number of eggs laid by the adults (1). Housefly reproduction was affected deleteriously by several monoterpenoids. Those most potent at inhibiting egg hatch following adults' exposure to treated surfaces included linalool, carvacrol, fl-phellandrene, and citronellal. cf-Limonene was moderately active and I-limonene was only

Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 slightly effective (19). A study of 31 essential plant oils on reproduction inhibition was conducted in the rice weevil. Many of

Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org the oils were active in reducing population reproduction, although 2 oils acted to stimulate growth and increase populations (24). The involvement of monoterpenoids in the juvenile hormone synthesis pathway and the role of juvenile hormone in mating and vitellogenesis support the reproductive impact of these terpenoids on insects.

Neurotoxic Effects. Symptoms of acute poisoning of insects by monoterpenoids are similar to those effected by some neurotoxic compounds. Cockroaches and house flies both exhibited overt hyperactivity, loss of coordination, and tremors. In cat fleas, trembling and paralysis of legs were also noted, followed by convulsions and death (5). Our attempts to study the neurotoxic actions of cf-limonene in American cockroach nerve preparations yielded somewhat erratic results, apparently due to difficulties of delivering and maintaining a specific concentration of this highly volatile and saline-insoluble chemical to the nerve. The non• invasive neuroassay for earthworms had been utilized successfully

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to characterize and quantify neurotoxic effects of insecticides (10) , and it offered advantages that could provide a superior technique for studying the neurotoxicity of monoterpenoids. Results from short-term contact exposure of earthworms, Elsenla fetida, to various terpenoids are presented in Table V. The methods of exposure and recording have been presented elsewhere (11) . Neurophysiological symptoms that were detected after 30 min. include: (a) decreased velocity of impulse conduction in medial (MGF) and lateral (LGF) giant nerve fibers along the worm (Figure 3), (b) decreased sensitivity to a touch stimulus, as indicated by difficulty or failure in evoking giant nerve fiber spikes in response to a light tactile stimulus, and (c) decreased amplitude or absence of the muscle electrical response that normally accompanies each MGF spike. A rebounding of action potentials was also observed in the MGF for some of the chemicals. These neurophysiologcal symptoms were also accompanied by clear-cut behavioral and morphological symptoms (Figure 4) which included various combinations of clitellar swelling, ataxia and general limpness of the body. The neurotoxic, behavioral, and morphological effects were fully reversible at sublethal concentrations. These results suggest that the terpenoids, as a group, are neuroactive as indicated by adverse electrophysiological effects on earthworm escape reflex pathways, as well as impaired postural or locomotory function. However, the data are as yet insufficient to determine either the exact site(s) of terpenoid action or whether these compounds share a common mode of action.

Table V. Sublethal effect of short-term contact exposure to 5 /til of monoterpenoids in the earthworm Elsenia fetida. (c = control; cf-lim = cf-limonene; pul = pulegone; lin = linalool; myr = myrcene)

SYMPTOM TREATMENT (30 min exposure) 0 = absent + = present c cf-lim pul lin mvr

1. Decreased MGF velocity8 0 + + + 0 2. Decreased LGF velocity8 0 0 + + + 3. Decreased sensitivity 0 0 0 + 0 to touch stimulus 4. Loss of MGF-mediated 0 + + + + muscle potential 5. Clitellar swelling 0 + 0 0 + 6. Ataxia 0 + + + 0 7. Limpness of bodv 0 0 + + 0 Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020

Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org Velocity is < 90% of pretreatment velocity

Conclusions

It is clear that a wide range of monoterpenoids possess some degree of insecticidal acitivity. The insecticidal role of most will be limited to specialty uses. Currently, limonene, linalool, and citronellal occupy places in the competitive and dynamic pesticide market. The most promising potential for exploitation of these molecules may lie in the synthesis of derivatives and analogs through directed synthesis (25,). The biological effects summarized here, i.e., the repellency, acute toxicity, fumigant activity, reproductive toxicity, and neurotoxicity reflect the wide spectrum of activities possible, each caused by an interaction of monoterpenoid at an active site in the insect. Mode-of-action studies on neurotoxicity and growth and reproductive effects are necessary to elucidate the specific toxicological bases for the

Hedin; Naturally Occurring Pest Bioregulators ACS Symposium Series; American Chemical Society: Washington, DC, 1991. 20. COATS ETAL. Toxicity and Neurotoxic Effects of Monoterpenoids 315

1.10

o o "5 1,

c 0

1 0.90 C o o > 0.80 -*-» _g a> a: MGF 0.70 15 30 45

Exposure Concentration (ppm)

o -A - 0.25 0.5 1.0 hr hr hr hr

Fig. 3 - Effects of cf-limonene concentration and exposure duration on relative conduction velocity in MGF (reproduced from 11, by permission, copyright Academic Press, 1990). Publication Date: January 9, 1991 | doi: 10.1021/bk-1991-0449.ch020 Downloaded by IOWA STATE UNIV on March 18, 2016 | http://pubs.acs.org

Fig. 4 - Silhouettes, from video recordings of normal untreated earthworm (left), worm at 30 min postexposure to 6.3 ppm cf-limonene (center), and worm at 5 min postexposure to 100 ppm cf-limonene (right).

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insecticidal activity. Quantitative structure-activity relationships must be derived to optimize effectively those toxicant-receptor interactions. Metabolism studies are required to understand the role of any bioactivations, e.g., epoxidation, that may occur in vivo in the insect pests. Although many questions remain, the monterpenoids posses considerable potential as insecticides of the future.

Acknowledgment

This chapter is Journal Paper No. J-14132 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa; Project No. 2306. Partial funding for this project was also provided by Pet Chemical Inc.

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Hedin; Naturally Occurring Pest Bioregulators ACS Symposium Series; American Chemical Society: Washington, DC, 1991.