The uptake,excretion and metabolism of the acylurea insecticide,flufenoxuron in spodoptera littoralis larvae,by feeding and topical application

Flufenoxuron is an acylurea insecticide which inhibits chitin synthesis. Its uptake, excretion and metabolism in larvae of Spodoptera littoralis have been measured. Larvae fed on a leaf disc sprayed at an application rate equivalent to 50 g ha^?1rapidly absorbed a maximum of 20% of ingested flufenoxuron, the remainder being voided in the frass. Excretion of absorbed compound was slow, with a 50% excretion time of approximately 20 h. Flufenoxuron was equally effective as an insecticide by topical application or ingestion. Following topical application, a total of 11% of the dose had penetrated into the body within 24 h. Of this 11%, half had travelled to the gut so that the rest of the body tissues contained no more than 6% of the toxicant dose. The concentration of flufenoxuron in the body tissues was maintained for at least 24 h, the surface residue probably acting as a reservoir. The compound was metabolically quite stable. Following topical application the 6 % of the applied dose which was absorbed into the body was still 92% as unchanged flufenoxuron 24 h after application. The enhanced toxicity of flufenoxuron to S. littoralis compared with earlier acylureas can probably be attributed to its slower metabolism and reduced excretion.     

Aqueous solubilities,photolysis rates and partition coefficients of benzoylphenylurea insecticides

The aqueous solubilities and octanol–water partition coefficients (K_OW) of the benzoylphenylurea (BPU) insecticides teflubenzuron, chlorfluazuron, flufenoxuron and hexaflumuron were determined in comparison with the more extensively studied diflubenzuron. Both teflubenzuron and hexaflumuron were substantially less water‐soluble (9.4 (± 0.3) µg litre⁻¹ and 16.2 (± 0.5) µg litre⁻¹ in water, respectively) than the value previously reported for diflubenzuron (89 (± 4) µg litre⁻¹ in water). Log K_OW values for diflubenzuron, teflubenzuron, hexaflumuron, flufenoxuron and chlorfluazuron were 3.8, 5.4, 5.4, 6.2 and 6.6, respectively, as determined using reverse‐phase HPLC. Photodegradation of hexaflumuron, teflubenzuron and diflubenzuron in water indicated hexaflumuron to be the most rapidly degraded of the three compounds at pH 7.0 (t_1/2 = 8.6 (± 0.4) h) and pH 9.0 (t_1/2 = 5 (± 1) h); diflubenzuron was the slowest of the three pesticides to degrade in pH 7.0 (17 (± 4) h) and pH 9.0 (8 (± 2) h) buffered water. In a solar simulator using river water buffered to pH 9.0, teflubenzuron, hexaflumuron and diflubenzuron half‐lives were 20 (± 4), 15 (± 2) and 12 (± 1) h, respectively; dark controls showed no loss of parent BPU over similar time periods. © 2000 Society of Chemical Industry     

Effect of insecticide—synergist combinations on the survival of Spodoptera exigua

The chitin synthesis inhibitors diflubenzuron and teflubenzuron have recently become ineffective for the control of Spodoptera exigua in floricultural crops. An extended laboratory test with second-instar larvae of S. exigua on Vicia faba plants was carried out to determine the influence of synergists on the biological activity of three benzoylphenyl ureas (BPUs). The co-application of piperonyl butoxide, an oxidase inhibitor, did not increase the activity of diflubenzuron, teflubenzuron or hexaflumuron. The best results were obtained with diethyl maleate, for suppressing glutathione S-transferase activity, and with dimethoate, as a hydrolase inhibitor. A joint application of diflubenzuron (at a concentration which resulted in 43% survival) with diethyl maleate or dimethoate gave only 6.2 and 8.9% surviving larvae, respectively. In addition, development to fourth-instar larvae was inhibited. The more stable teflubenzuron was synergized by both compounds to a much lesser extent than diflubenzuron. None of the synergists had a significant effect on the activity of hexaflumuron, which was the most potent insecticide of the three BPUs tested against S. exigua.     

Examination of stability of resistance and cross-resistance patterns to acylurea insect growth regulators in field populations of the diamondback moth,Plutella xylostella,from Malaysia

In teflubenzuron and teflubenzuron/chlorfluazuron laboratory-selected sub-populations of a Cameron Highlands (1988) strain of Plutella xylostella L., resistance to teflubenzuron reached much greater levels than chlorfluazuron but declined at a correspondingly greater rate to give similar resistance levels after 10–13 generations. There was little evidence of cross-resistance to flufenoxuron and none to hexaflumuron in these populations. Reselection of a Cameron Highlands population with teflubenzuron after 22 generations resulted in a significant (P < 0.05) increase in resistance after three generations. In two more recently collected strains from lowland Malaysia, Sawi (1991) and Serdang (1992) high levels of resistance to teflubenzuron were found, but only the former showed any apparent resistance to chlorfluazuron. There was also evidence of resistance to diflubenzuron in the Sawi strain (Serdang not examined). Resistance to teflubenzuron appeared to be unstable in unselected sub-populations of both lowland strains. Selection of the Sawi strain with teflubenzuron did not effect cross-resistance to chlorfluazuron, while attempted selection with chlorfluazuron had no effect on resistance to either compound. Selection of the Sawi strain with diflubenzuron increased resistance to teflubenzuron but had no effect on resistance to chlorfluazuron. In contrast to the Sawi strain, selection of the Serdang strain with teflubenzuron resulted in cross-resistance to chlorfluazuron. Synergist experiments with piperonyl butoxide (PB) and S,S,S-tributyl phosphorotrithioate (DEF) suggested that resistance to teflubenzuron in the lowland strains may involve both microsomal monooxygenases and esterases. The ovicidal activity of teflubenzuron, chlorfluazuron, hexaflumuron and flufenoxuron was consistently less than their activity against second-instar larvae, hexaflumuron, followed by teflubenzuron, showing the greatest relative ovicidal activity. Hexaflumuron was markedly more active than teflubenzuron against eggs from acylurea-selected Cameron Highlands populations and there was some evidence that resistance to teflubenzuron may be expressed at a lower level in eggs than in larvae in one such strain.     

Comparative laboratory evaluation of the acute and chronic toxicology of diflubenzuron,hexaflumuron and teflubenzuron against II instar desert locust (Schistocerca gregaria) (Orthoptera: Acrididae)

The acute and chronic toxicities of three benzoylphenyl ureas, diflubenzuron, hexaflumuron and teflubenzuron were assessed under laboratory conditions against two-day-old second (II) instar Desert Locust, Schistocerca gregaria (ForskÅl) nymphs. Following exposure by ingestion of a single precise dose applied to short pieces of spring barley, nymphs were monitored for two moults until the fourth (IV) instar. Analysis of acute response data gave three significantly different LD₅₀ statistics (P < 0·05), 68·0, 26·6 and 0·71 μg per nymph respectively for diflubenzuron, hexaflumuron and teflubenzuron. The probit regression slopes also differed significantly, indicating distinct tolerance distributions for the three compounds, the narrowest response being to diflubenzuron and the widest range of response being to teflubenzuron. The timing of death was found to vary between the compounds; most nymphs died during the first moult following treatment with either hexaflumuron or teflubenzuron. However, the majority of nymphs that died after exposure to diflubenzuron did so after completing the first moult after treatment, but before the second. The mean development times of nymphs during the II and especially third (III) instars were significantly longer (P < 0·05) than those of the controls following exposure to diflubenzuron and hexaflumuron. Teflubenzuron had no significant effect (P < 0·05) on the duration of the II instar. The potential of the three compounds to control S. gregaria populations in the field is discussed with particular reference to the timing and nature of acute and chronic responses.     

Cytochemical demonstration of the effects of the acylureas flufenoxuron and diflubenzuron on the incorporation of chitin into insect cuticle

Flufenoxuron (“Cascade”) is a novel acylurea with acaricidal and insecticidal properties. It acts in a similar manner to diflubenzuron (DFB) by impairing chitin incorporation into insect cuticle. Chitin can be localised cytochemically using the lectin, wheat germ agglutinin (WGA), which binds specifically to N-acetylglucosamine-containing polymers. WGA adsorbed to monodisperse colloidal gold (WGA-gold) was used as an electron-dense marker for localising chitin in the cuticle of sixth-instar Spodoptera littoralis. In the cuticle of control insects, the deposition zone and endocuticle were heavily labelled. In flufenoxuron- and DFB-treated insects, only the endocuticle (formed before treatment) was labelled, the deposition zone being devoid of label. These results show that flufenoxuron and DFB act in similar manner in reducing chitin incorporation in the cuticle of S. littoralis.     

Control failure likelihood and spatial dependence of insecticide resistance in the tomato pinworm, Tuta absoluta

BACKGROUND: Insecticide resistance is a likely cause of field control failures of Tuta absoluta, but the subject has been little studied. Therefore, resistance to ten insecticides was surveyed in seven representative field populations of this species. The likelihood of control failures was assessed, as well as weather influence and the spatial dependence of insecticide resistance. RESULTS: No resistance or only low resistance levels were observed for pyrethroids (bifenthrin and permethrin), abamectin, spinosad, Bacillus thuringiensis and the mixture deltamethrin + triazophos (<12.5‐fold). In contrast, indoxacarb exhibited moderate levels of resistance (up to 27.5‐fold), and chitin synthesis inhibitors exhibited moderate to high levels of resistance (up to 222.3‐fold). Evidence of control failures was obtained for bifenthrin, permethrin, diflubenzuron, teflubenzuron, triflumuron and B. thuringiensis. Weather conditions favour resistance to some insecticides, and spatial dependence was observed only for bifenthrin and permethrin. CONCLUSION: Insecticide resistance in field populations of the tomato pinworm prevails for the insecticides nowadays most frequently used against them—the chitin synthesis inhibitors (diflubenzuron, triflumuron and teflubenzuron). Local selection favoured by weather conditions and dispersal seem important for pyrethroid resistance evolution among Brazilian populations of T. absoluta and should be considered in designing pest management programmes. Copyright © 2011 Society of Chemical Industry     

Persistence of insect growth regulators against the rice weevil,sitophilus oryzae,in grain commodities

Ten insect growth regulators (IGRs): four juvenile hormone analogs (JHAs) — fenoxycarb, MV-678, R-20458 and dofenapyn, and six molt inhibitors (MIs) — triflumuron, chlorfluazuron, hexaflumuron, teflubenzuron, Dowco 439 and flufenoxuron; and the organophosphate malathion, were evaluated for residual activity in stored grain for 2 years against the rice weevil,Sitophilus oryzae (L.). Triflumuron, flufenoxuron, teflubenzuron, chlorfluazuron, hexaflumuron and fenoxycarb, each at a concentration as low as 1 ppm a.i., were superior to 10 ppm malathion throughout the experiment. The six MIs, each tested at 50 and 100 ppm a.i., gave complete control for 2 years. Moreover, a concentration as low as 1 ppm a.i. of triflumuron, teflubenzuron, flufenoxuron and chlorfluazuron had good residual activity for 1 year post-treatment and provided almost complete grain protection during this period. Among the JHAs, fenoxycarb had the best residual activity. It gave almost complete control (96.6%) ofS. oryzae at 10 ppm for 18 months post-treatment. The other JHAs at the four test concentrations failed to produce this level of control. The viability of the wheat grain was not affected by any of the insecticidal treatments, with 90% germination recorded in treated and control, grains at all intervals tested.     

Toxicity of the chitin synthesis inhibitors,diflubenzuron and its dichloro-analogue,to Spodoptera littoralis larvae

The activities of the chitin synthesis inhibitors, diflubenzuron and PH 60–38, against Spodoptera littoralis larvae were assayed by feeding treated alfalfa or poisoned wheat bran baits, by allowing the larvae to imbibe sucrose-containing aqueous dispersions of the compounds, and by injection into larvae. PH 60–38 was less active than diflubenzuron. On alfalfa, diflubenzuron had to be fed for at least 2 days to prevent formation of normal pupae and emergence of adults. For very big (480–540 mg) larvae, feeding diflubenzuron at concentrations of 50 mg/litre for 2 days or 2.5 mg/litre for 3 days prevented adult emergence. For 200–250 mg larvae, this was achieved by feeding concentrations of 100 mg/litre for 2 days, 5 mg/litre for 3 days or 3.5 mg/litre for 4 days. In all larvae > 150 mg, mortality in feeding experiments occurred in the prepupal or the pupal stage. Only with 30–50 mg and 100–150 mg larvae was there considerable mortality during moults between larval instars, the larvae being unable to liberate themselves from the old larval skins and head capsules. Diflubenzuron incorporated into wheat bran baits at concentrations of from 2.5 to 10 000 μg/g killed approximately 70–90% of the insects. When imbibed, diflubenzuron was much less toxic as a wettable powder than as a liquid formulation but the two formulations were equitoxic when injected into the larvae.     

Effect of chitin-synthesis inhibitors on stored-product beetles

The fecundity of four important coleopteran stored-grain pests, Rhyzopertha dominica (Fab.), Sitophilus oryzae (L.), Oryzaephilus surinamensis (L.) and Tribolium castaneum (Herbst.) was assessed after exposure to wheat treated with five benzoylphenyl ureas (BPUs), members of the chitin-synthesis inhibitor group of insect growth regulators (IGRs). Chlorfluazuron was found to control all four species more effectively than triflumuron, teflubenzuron, flufenoxuron or diflubenzuron. Two weeks of exposure of parents to 0.5 mg kg ?1 of all the BPUs, except diflubenzuron, inhibited almost all F1 production and all the F2 production of each species. The fecundities of T. castaneum and O. surinamensis recovered almost to their untreated levels after two weeks of exposure to 1 mg kg ?1 of BPUs followed by two weeks on untreated wheat, but the effect of the exposure persisted for at least two weeks on untreated wheat in adult S. oryzae and to a lesser extent in R. dominica. The mean F1 response of the four species after two weeks of exposure to four doses showed clearly that the four newer BPUs were all similar in their overall effectiveness and significantly more effective than diflubenzuron, although diflubenzuron was equally effective against S. oryzae. At 0.1 mg kg ?1 triflumuron was less effective against O. surinamensis, while teflubenzuron and flufenoxuron were not very effective against the rice weevil, S. oryzae. Chlorfluazuron provided the most effective control of all four stored-product pest species, including the weevil, S. oryzae. The BPUs show particular potential for use in the stored-product industry because low dosages will effectively control the rice weevil in addition to the other major pest species.     

Hazards and uptake of chitin synthesis inhibitors in bumblebees Bombus terrestris

This research project examined the potential hazards of a major class of insect growth regulators (IGRs) to survival, reproduction and larval growth in bumblebees Bombus terrestris L. Eight chitin synthesis inhibitors (CSIs) were tested: buprofezin, cyromazine, diflubenzuron, flucycloxuron, flufenoxuron, lufenuron, novaluron and teflubenzuron. These different IGRs, which are important in the control of pest insects in greenhouses, were applied via three different routes of exposure under laboratory conditions: dermal contact, and orally via the drinking of sugar/water and via pollen. The compounds were tested at their respective maximum field recommended concentrations (MFRC) and also in dose-response assays to calculate LC(50) values. In general, none of the CSIs showed acute worker toxicity. However, there was a dramatic reduction in brood production, especially after oral treatment with pollen and sugar/water. Conspicuously, egg fertility was reduced in all treatments with diflubenzuron and teflubenzuron. In addition to egg mortality, the worker bumblebees removed larvae from the treated nest, and in most cases these individuals were dead first-second instars. Under a binocular microscope, such larvae showed an abnormally formed cuticle leading to mechanical weakness and death. In another series of experiments using (14)C-diflubenzuron and (14)C-flufenoxuron, cuticular penetration in workers was studied for a better understanding of the differences in toxicity. With (14)C-diflubenzuron, transovarial transport and accumulation in the deposited eggs supported the strong reproductive effects. Overall, the present results suggest that CSIs should be applied with caution in combination with bumblebees. The compatibility of each compound to be used in combination with B. terrestris is discussed in relation to calculated LC(50) values, routes of uptake and effects.     

Cross-resistance between acylurea insect growth regulators in a strain of Plutella xylostella L. (Lepidoptera: Yponomeutidae) from Malaysia

The activities of the acylurea insect growth regulators, chlorfluazuron, teflubenzuron and difubenzuron, and the neurotoxic macrocyclic lactone, abamectin were assessed against a laboratory susceptible (FS) strain and a field (Cameron Highlands, Malaysia (CH)) strain of the diamondback moth, Plutella xylostella L. using a leaf-dip bioassay at 20°C. The time taken to achieve end-point mortality was found to vary considerably (9–17 days), being fastest with abamectin against the FS strain and slowest with difubenzuron against the CH strain. The order of activity (LC₅₀ at F6/7) against second-instar larvae of both strains was: abamectin > chlorfluazuron = teflubenzuron ? difubenzuron. Subsequent assays (F14) with the acylureas, flufenoxuron and hexaflumuron against the FS strain suggested that the former was slightly more active than chlorfluazuron or teflubenzuron, the latter slightly less active. The CH population was found to be 12.6-, 6.7-, 6.4- and 2.3-fold less sensitive to difubenzuron, teflubenzuron, chlorfluazuron and abamectin respectively than the FS strain. Selection of sub-populations of the CH strain with chlorfluazuron (CHL-SEL) and teflubenzuron (TEF-SEL) for six generations (F6-11), resulted in LC₅₀ resistance ratios of 109- and 315-fold respectively when compared with the FS strain, equivalent to an 18- and a 46-fold increase in resistance compared with the unselected CH strain. Marked cross-resistance was also demonstrated between chlorfluazuron and teflubenzuron in both sub-populations. However, there was no evidence of cross-resistance to dijlubenzuron and abamectin and little or no cross-resistance to flufenoxuron and hexaflumuron. Resistance to chlorfluazuron and teflubenzuron appeared to be relatively unstable in the TEF-SEL compared with the CHL-SEL sub-population (over 6–9 generations). However, reselection of the TEF-SEL population with chlorfluazuron (F18–20) led to a very rapid increase in resistance to chlorfluazuron and particularly teflubenzuron. For the latter compound, resistance factors of about 1000000 were obtained (F19, 21). Such values are probably only semi-quantitative, as above a certain level of resistance feeding bioassays with acylureas (compounds which are active to a significant extent by ingestion) are likely to become rate-limiting.     

The effect of CME 134 onSpodoptera Littoralis Eggs and larvae

CME 134, a new benzoylphenyl urea chitin synthesis inhibitor, was less active than diflubenzuron and BAY SIR 8514, when tested againstSpodoptera littoralis eggs by a dipping method. AgainstS. littoralis larvae the compound was tested by feeding treated alfalfa, topical application and contact with crystalline residues on glass, followed by observation until the adult stage. With both 200–250 and 360–440-mg larvae 100% mortality was obtained by one-day feeding of alfalfa treated with 0.15 ppm a.i. Topical application to 100- and 200-mg larvae showed CME 134 to be about five and nine times more active than BAY SIR 8514 and diflubenzuron, respectively. These differences were even much greater in the contact tests. Cotton field plots were sprayed with either CME 134 or diflubenzuron formulations, leaves were collected at different intervals and fed for one day toS. littoralis larvae in the laboratory. 0.0009% a.i. CME 134 residues gave complete kill of 30–50-mg larvae after 5 and 20 days, and 86% kill after 28 days of aging. With 0.003 and 0.009% a.i., complete kill was obtained in 200–250-mg larvae until 50 days after spraying.     

Inhibition of chitin synthesis in an insect cell-line

An epithelial cell-line from Chironomus tentans, able to synthesise chitin in considerable amounts, was used as a test system to study the influence of tunicamycin (I₂₀=750 nM), the benzoylphenylurea derivative SIR 8514 (I₅₀=100 nM), oligomycin (I₅₀=200 nM), avermectin (I₅₀=5 μM) and nikkomycin Z (I₅₀,=50 μM) on the rate of glucosamine incorporation into chitin. Chitin synthesis was diminished by application of 20-hydroxyecdysone.     

Mechanisms of resistance to diflubenzuron in the house fly, Musca domestica (L.)

The mechanisms of resistance to the chitin synthesis inhibitor diflubenzuron were investigated in a diflubenzuron-selected strain of the house fly (Musca domestica L.) with > 1000 × resistance, and in an OMS-12-selected strain [O-ethyl O-(2,4-dichlorophenyl)phosphoramidothioate] with 380 × resistance to diflubenzuron. In agreement with the accepted mode of action of diflubenzuron, chitin synthesis was reduced less in larvae of the resistant (R) than of a susceptible (S) strain. Cuticular penetration of diflubenzuron into larvae of the R strains was about half that of the S. Both piperonyl butoxide and sesamex synergized diflubenzuron markedly in the R strains, indicating that mixed-function oxidase enzymes play a major role in resistance. Limited synergism by DEF (S,S,S-tributyl phosphorotrithioate) and diethylmaleate indicated that esterases and glutathione-dependent transferases play a relatively small role in resistance. Larvae of the S and R strains exhibited a similar pattern of in vivo cleavage of ³H- and ¹⁴C-labeled diflubenzuron at N₁C₂ and N₁C₁ bonds. However, there were marked differences in the amounts of major metabolites produced: R larvae metabolized diflubenzuron at considerably higher rates, resulting in 18-fold lower accumulation of unmetabolized diflubenzuron by comparison with S larvae. Polar metabolites were excreted at a 2-fold higher rate by R larvae. The high levels of resistance to diflubenzuron in R-Diflubenzuron and R-OMS-12 larvae are due to the combined effect of reduced cuticular penetration, increased metabolism, and rapid excretion of the chemical.     

Differential effects of the acylurea insect growth regulator teflubenzuron on the adults of two endolarval parasitoids of Plutella xylostella,Cotesia plutellae and Diadegma semiclausum

The acylurea chitin synthesis inhibitor, teflubenzuron (‘Nomolt’® SC) showed no apparent activity against adult males of Diadegma semiclausum Hellén or Cotesia plutellae Kurdj. when applied to foliage at a rate ( = 50 g a.i. ha^?1) approximating to 2 × field usage. In contrast, the pyrethroid cypermethrin (‘Cymbush’® EC) showed appreciable foliar activity (at rates = 10–50 ga.i. ha^?1) to D. semiclausum but showed low activity against C. plutellae. When single adult female D. semiclausum were pre-treated with teflubenzuron (= 50 g a.i. ha^?1) and then exposed to 5. 10. 25, 50 or 100 second-instar host larvae, the number of purasitoid cocoons formed was significantly reduced compared with controls except at the lowest host density (P 0-05). However, there was no evidence that teflubenzuron had a host density-dependent effect on parasitism by D. semiclausum. Teflubenzuron had no significant effect on host parasitism in an equivalent experiment with C. plutellae (P > 0-05) except at the median host density. When batches of ten host larvae were introduced at hourly intervals to female D. semiclausum pre-treated with teflubenzuron (= 50 g a.i. ha^?1), the number of cocoons formed in the first two batches was not significantly different from untreated controls (P > 0-05), whereas teflubenzuron significantly reduced the number of cocoons in the next three batches of host larvae (P < 0-05). This suggests that teflubenzuron may affect chitin synthesis in developing eggs within female D. semiclausum but has little or no effect on pre-formed eggs which are more likely to be oviposited first. Teflubenzuron had no significant (P > 0-05) effect on parasitism by C. plutellae when presented with batches of host larvae. When female D. semiclausum and C. plutellae were placed in the same host arena, the former showed a 3-fold greater rate of parasitism. Pre-treatment with teflubenzuron significantly reduced parasitism by D. semiclausum (P > 0-05) but had no significant effect with C. plutellae (P > 0-05) although the latter species failed to exploit the reduction in parasitism by D. semiclausum. The differential response of D. semiclausum and C. plutellae to insecticides is discussed in relation to the relative rates of establishment of these species in Malaysia.     

Egg-sterilizing effect of benzoylphenylureasvia the adult stage of the nitidulid beetleCarpophilus hemipterus

The effect of a 24-h exposure of adults ofCarpophilus hemipterus to artificial diets treated on the surface with different concentrations of benzoylphenylureas, on the hatchability of eggs laid during the subsequent 14 days, was investigated. The order of persistence of sterilization at 5 ppm was chlorfluazuron (IKI-7899), 12 days > XRD-473, 10 days > diflubenzuron, 6 days > teflubenzuron (CME 134), 4 days. Three of the substances (chlorfluazuron, diflubenzuron and teflubenzuron) had no direct ovicidal effect whenC. hemipterus eggs were dipped in 1000 ppm dilutions, but the larvae that hatched from the treatments died within 2 days.     

Comparative studies on acylurea insect growth regulators and neuroactive insecticides for the control of the armyworm Spodoptera exempta walk

Leaf-dip bioassays with commercial pesticide formulations were used throughout this study. Third-instar larvae of Spodoptera littoralis were bioassayed for comparative purposes. The end-point mortality against the third-instar larvae (L3) of two Spodoptera exempta populations (Bangor and Gent) and a S. littoralis population (NRI) was found to occur at 72 h and 120 h for neuroactive compounds (carbamate, organophosphate and pyrethroid insecticides) and acylureas respectively. Overall, the acylureas were found to be more toxic (up to 240-fold at the LC₅₀ level) than the most active of the neuroactive compounds tested, the pyrethroids. This difference was greatest against S. littoralis, which was markedly less sensitive to the latter compounds. The Gent population of S. exempta was also found to be significantly less sensitive (7-folt at LC₅₀) to the pyrethroid, lambda-cyhalothrin, than the Bangor strain. Two of the most toxic neuroactive and non-neuroactive compounds, the pyrethroid, cypermethrin and the acylurea, teflubenzuron were chosen for further studies with S. exempta (Gent) and S. littoralis. Relatively little difference in the toxicity of teflubenzuron was observed against different larval instars (L2, L3 and L4) of S. exempta when compared with cypermethrin. Choice-chamber-experiments on feeding behaviour showed that when L3 larvae of either species were exposed to both treated and untreated leaves. teflubenzuron had no significant effect on larval distribution, the percentage of each leaf disc consumed or larval weight gain when compared with untreated controls. In contrast, a marked dose-dependent repellent response to cypermethrin-treated leaves was shown by S. littralis after 6 h. together with corresponding reductions in the percentage of the treated leaf consumed and in larval weight gain. Similarly, when L3 S. exempta were exposed only to pesticidetreated leaves, cypermethrin significantly reduced premoult larval weight gain, whereas tefiubenzuron had no premoult effect. However, following exposure of L3 S. exempta and S. littoralis to teflubenzuron for 48 h prior to moulting, L4 which survived the moult showed reduced weight gain compared with controls, the reduction being more pronounced on maize than on artificial diet. This antifeedant effect increased in proportion to the concentration of teflubenzuron originally administered. The effect of exposure time to teflubenzuron was examined by placing L3 S. exempta and S. littoralis on leaves treated with the approximate LC₅₀ or LC₉₅ dose for 6–48 h. The exposure time required to produce appreciable mortality (relative to LC values) within the treated population was found to be 24 h.     

Contact activity of diflubenzuron against Spodoptera littoralis larvae

The effect of diflubenzuron as a residue on glass or applied topically to Spodoptera littoralis larvae was investigated. Diflubenzuron was active as a residue on glass against 100 and 200 mg larvae; the toxicity of residues was identical whether a dispersable formulation or a wettable powder or the technical substance were used. By topical application, diflubenzuron had an ED₅₀ for cumulative percentage mortality up to the adult stage of 004 and 0066 μg/larva for 100 and 200mg larvae respectively. Neither the site of the topical application nor whether the larvae were kept singly or in groups of ten after the treatment had an influence on toxicity. The data indicate that diflubenzuron has contact toxicity to at least one insect species as well as the known stomach poison action.     

Temperature effects on the action of acylurea insecticides against tobacco hornworm (Manduca sexta) larvae

The toxic effects of six acylurea insecticides on larvae of the tobacco hornworm were investigated at each of four environmental temperatures (20, 25, 30 and 35°C). This spans the range of temperatures which the insects can tolerate. For all the acylureas tested, mortality increased with temperature when either newly hatched or fourth-instar larvae were given insecticide in their food. Sub-lethal growth inhibition also became more pronounced at progressively higher environmental temperatures. This temperature dependence of acylurea action was not due to altered uptake of the insecticide, since there was no significant variation with temperature in the amount of [¹⁴C]flufenoxuron taken up by fifth-instar larvae when given a single meal containing labelled insecticide. Additionally, mortality of fourth-instar larvae given a single intra-haemocoelic injection of flufenoxuron was significantly greater at higher temperatures, implying that temperature affects a process that occurs after insecticide uptake. The intrinsic ability of acylureas to inhibit chitin synthesis is temperaturesensitive, since flufenoxuron inhibited the incorporation of [¹⁴C]N-acetylalucosamine into chitin by proleg epidermis in vitro significantly less well at 20°C than at the higher temperatures tested. However, there was no significant variation between the effectiveness of in-vitro chitin synthesis inhibition at 25, 30 and 35°C. These data show that the effectiveness of acylurea insecticides is subject to strong temperature effects in the range of temperatures likely to be experienced in the field.