Chitin Synthesis Inhibitor Research Articles

Transovarial transmission of novaluron in Choristoneura rosaceana (Lepidoptera: Tortricidae)

AbstractThe obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae), has become a significant pest of tree fruit production in North America within the past 25 years. Control of the OBLR has historically relied upon broad-spectrum insecticides like organophosphates, carbamates, and pyrethroids. However, with evidence of resistance development, newer chemistries have been developed to combat this pest. The effects of novaluron, a chitin synthesis inhibitor, were studied to determine if reduced egg hatch occurs after exposure to adults. The transovarial effects of this compound were tested through laboratory bioassays, looking at decreased egg hatch and also presence of novaluron in egg masses following adult exposure. Results from the study demonstrated a decrease in egg hatch after adult exposure. Analysis of egg mass using HPLC also demonstrated novaluron present in the eggs laid by treated adults. Along with the direct ovicidal and larvicidal properties of novaluron, this transovarial activity provides an important contribution to the overall control seen in the field.

Evaluation of etoxazole against insects and acari in vegetables in China

Etoxazole, 2-(2,6-difluorophenyl-4-[4-(1,1-dimethylethyl)-2-ethoxy-phenyl]-4,5-dihydrooxazole, an organofluorine chitin synthesis inhibitor, was assayed for its bioactivities against several major insect and acarus pests and compared to several other pesticides: two chitin synthesis inhibitors, hexaflumuron and chlorfluazuron; a pyrethroid, permethrin; an organophosphate, acephate; a carboximide, hexythiazox; and a tetrazine, clofentezine. The LC 50 of etoxazole was calculated using probit analysis of the concentration-dependent mortality data against susceptible and resistant strains of the beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae); diamondback moth, Plutella xylostella L. (Plutellidae); bean aphid, Aphis craccivora Koch (Hemiptera: Aphididae); and carmine spider mite, Tetranychus cinnabarinus (Boisduval) Boudreaux (Trombidiformes: Tetranychidae). The resistant strains were found to be resistant against all tested pesticides except etoxazole. The bioactivity of etoxazole was many times that of the other tested insecticides and acaricides widely used in vegetable crops in China. On the basis of our research, etoxazole can be expected to be extensively used on vegetable crops in China.

BIOCHEMICAL IMPACTS OF THREE CHITIN SYNTHESIS INHIBITORS AND THEIR BINARY MIXTURES WITH CHLORPYRIFOS OR BETA-CYFLUTHRIN ON Spodoptera littoralis (BOISD.)

The joint action of the binary mixtures of chlorpyrifos (organophosphorus) or beta-cyfluthrin (pyrethroid) plus three chitin synthesis inhibitors(flufenoxuron, chlorfluazuron and lufenuron) on the 4th instar larvae of the laboratory strain of the cotton leafworm Spodoptera littoralis (Boisd.) at LC25 level were evaluated. In general, according to co-toxicity factor data clearly indicated that all tested mixtures decreased the toxicity except for the combination between chlorpyrifos and lufenuron, it produced additive effect (-19.15). The tested mixtures of Beta-cyfluthrin with all tested insect growth regulators (IGRs) were high antagonism (ranging between - 55.56 and - 84.13) than the tested mixtures of IGRs with chlorpyrifos. The effect of both tested individual insecticides and it’s mixtures on the activity of esterases (α- & β-esterase), transaminase (AST and ALT) chitinase, alkaline phosphatase, total proteins, total lipids and glucose was determined colorimetrically.

The Activity of Some Detoxification Enzymes in Spodoptera Littoralis (Boisd.) Larvae (Lepidoptera – Noctuidae) Treated With Two Different Insect Growth Regulators

The susceptibility to IGRs, moulting hormone agonist, mimic (tebufenozide) and chitin synthesis inhibitors match (lufenuron) was studied in the 2 nd and 4 th larval instars of the cotton leafworm Spodoptera littoralis (Bosid). Obtained results revealed that match was the more toxic against 2 nd larval instars at sub-lethal concentrations LC25 and LC50 (0.3 and 0.6 ppm, respectively) than mimic (1.1 and 1.5ppm, respectively). But LC90 level was the same in the two IGRs. With respect to 4 th larval instar, match induced the higher toxic effect at all sub lethal doses than mimic. Biochemical analysis showed that match and mimic exhibited a severe reduction in the activities of the detoxification enzymes, acid phosphatase and esterases (α and β), as compared to the control. The enzymatic activities were inhibited with the increase in the time posttreatment and also with the increase in dose. Therefore, the tested IGRs, tebufenozide and lufenuron , may be not detoxify by these enzymes.

Effect of chlorfluazuron and pyriproxyfen on the antennal morphology, pheromone production and response of surviving adults of Tribolium castaneum treated at the LC50 level during the pupal stage

Initial experiments were carried out to determine the dosage mortality response of the rust-red flour beetle Tribolium castaneum to two insect growth regulators, the chitin synthesis inhibitor chlorfluazuron and the juvenile hormone analogue pyriproxyfen. The response was measured as the proportion of adults developing from individuals treated during the pupal stage. For males, at the LC50 level, chlorfluazuron was more toxic than pyriproxyfen with LC50 values of 10.6 and 12.6 ppm respectively. For females, pyriproxyfen was more potent than chlorfluazuron with LC50 values of 7.1 and 8.3 ppm.Subsequent experiments were carried out using adults that had survived after treatment at the LC50 level during the pupal stage. Both responses to pheromone and the production of pheromone, by adults of both sexes that had been treated during the pupal stages with pyriproxyfen, were significantly more affected than those treated with chlorfluazuron.Both treatments caused abnormalities in the antennae of adults of both sexes.

Molting Process in the Formosan Subterranean Termite (Isoptera: Rhinotermitidae)

Abstract This study describes the behavioral and histological changes that take place during ecdysis in the Formosan subterranean termite. The molting process was described in four distinct phases, starting with the peristaltic contraction of the abdomen to the complete shedding of the exuvium. Although individual termites still managed to go through the molting process when isolated from their nestmates, it required more time for the molting individual to complete the process than when aided by its nestmates. Histological observations were made on termites during the intermolt period, the premolting or fasting period, the pre-ecdysis and the ecdysis periods, and on newly molted individuals. Symbiotic protozoans were voided at the beginning of the premolting/fasting period. The detachment and reattachment of the muscles of the abdominal segments occurred during pre-ecdysis, and the leg muscle detachment and reattachment occurred during ecdysis. During pre-ecdysis, the abdominal cuticle had a wrinkled texture and two layers of cuticles were observed, one of which was the newly formed cuticle underneath the old one. Finally, the old tracheae were shed from the tracheal system and were pulled out from the spiracular openings of the mesothorax with the help of the nestmates. We concluded that, as social insects, the presence of nestmates during the molting of individual termites reduced the time necessary to complete the ecdysis, and the histological description of the molting process provides a template for further studies on the effect of chitin synthesis inhibitors on ecdysis in termites.

Effectiveness of Slow-Release Tablet Formulations of the IGR Diflubenzuron and the Bioinsecticide Spinosad Against Larvae ofAedes aegypti(L.)

The larvicidal effectiveness of slow-release tablet formulations of the chitin synthesis inhibitor Dudim® (diflubenzuron) and the bioinsecticide Natular™ (spinosad) against mosquito larvae of Aedes aegypti (L.) has been evaluated. The results showed that the test formulations provided long-term residual control against the larvae. Effective control giving 90–100 % inhibition of adult emergence was achieved for 10 weeks post-treatment for diflubenzuron and 7 weeks for spinosad. In addition, larval treatments with slow-release diflubenzuron formulations led to a marked prolongation in the time needed for blood meal digestion and a reduction in the reproductive potential of adult survivors. On the other hand, larval treatments with slow-release formulations of spinosad tablets affect neither the time of blood meal digestion nor the reproductive capacity of mosquito adults that emerged from surviving larvae.

Lufenuron Suppresses the Resistance of Formosan Subterranean Termites (Isoptera: Rhinotermitidae) to Entomopathogenic Bacteria

Pesticides can negatively affect insect immunity. Although studies show that Formosan subterranean termites, Coptotermes formosanus Shiraki, are resistant to microbial infections, the effects of pesticides on disease resistance is not well studied. In this study, C. formosanus previously fed lufenuron was exposed to each of the three entomopathogenic bacteria, Pseudomonas aeruginosa (Schroeter) Migula, Serratia marcescens Bizio, and Bacillus thuringiensis Berliner subsp. israelensis. We found that termite mortality was significantly higher and synergistic in the combination of lufenuron and P. aeruginosa compared with treatment of lufenuron or P. aeruginosa alone. Other bacteria and lufenuron combinations were not quite as effective. Interestingly, only in treatments without lufenuron did termites show carcass-burying behavior. The results indicate that lufenuron, a chitin synthesis inhibitor, can suppress Formosan subterranean termite resistance to P. aeruginosa. Possible suppression mechanisms are discussed.

Ovicidal activity of three insect growth regulators against Aedes and Culex mosquitoes

Interspecific variations in the susceptibility of freshly and embryonated eggs of Aedes albopictus, Ae. aegypti, Ae. atropalpus and Culex pipiens were tested against three classes of insect growth regulators (IGRs) including ecdysone agonist (azadirachtin), chitin synthesis inhibitor (diflubenzuron) and juvenile hormone analog (pyriproxyfen) at 0.001, 0.01, 0.1 and 1.0ppm concentrations. Egg hatching inhibition was dose dependent, the highest being at 1.0ppm concentration for freshly laid eggs of Ae. albopictus (pyriproxyfen: 80.6%, azadirachtin: 42.9% and diflubenzuron: 35.8%). Aedes aegypti showed lower egg hatching inhibition when exposed to pyriproxyfen (47.3%), azadirachtin (15.7%) and diflubenzuron (25.5%). Freshly laid eggs of Cx. pipiens were most susceptible to diflubenzuron. Aedes atropalpus eggs were tolerant to all three classes of IGRs. Embryonated eggs of Ae. albopictus, Ae. aegypti, Ae. atropalpus and Cx. pipiens were resistant to pyriproxyfen, azadirachtin and diflubenzuron than freshly laid eggs. The median desiccation time (DT50) of Ae. atropalpus eggs was maximum (5.1h) as compared to Ae. aegypti (4.9h), Ae. albopictus (3.9h) or Cx. pipiens (1.7h) eggs. Insignificant relationship between the rates of desiccation and egg hatching inhibition suggests other factors than physical providing eggs the ability to tolerate exposures to various IGRs. Egg hatching inhibition was due to the alteration in embryonic development caused by IGRs. Changes in the egg shell morphology and abnormal egg hatching from the side of the egg wall instead of operculum, was observed at higher concentrations of diflubenzuron. Morphological and physiological variations in eggs may be the key factor to influence the ovicidal efficacy of IGRs. The present data provide a base line for the improvement of the ovicidal efficacy of the insecticide and its formulation.

Toxicological assessment of the anti-salmon lice drug diflubenzuron on Atlantic cod Gadus morhua

Increasing use of the chitin synthesis inhibitor diflubenzuron against the ectoparasitic salmon louse Lepeophtheirus salmonis in marine aquaculture has raised concerns over its environmental impacts. This study evaluated how diflubenzuron affects Atlantic cod Gadus morhua, a fish species often found near Atlantic salmon Salmo salar farms, focusing on uptake kinetics and hepatic transcriptional responses. Two experiments were conducted, one time-series trial in which the fish were given a daily dose (3 mg kg-1 fish) of diflubenzuron for 14 d followed by a 3 wk depuration period, and one dose-response trial with increasing concentrations (3, 10 and 50 mg kg-1 fish). The highest diflubenzuron concentrations were found in the liver at Day 15. No detectable levels of diflubenzuron were found in liver or muscle 3 wk after the end of the treatment. At the molecular level, small effects of diflubenzuron treatment on gene transcription were observed. In the time-series experiment, the strongest effects were seen at Day 8, with 2 transcripts being upregulated (bclx2 and cpt1a) and 8 transcripts being downregulated (gstp1, gstm1, gstt1, ugt1a, nat2, cat, p53 and slc16a9a). Five transcripts (cyp3a, cpt1a, ptgs2, elovl5 and mapk1) responded significantly to diflubenzuron exposure in the dose-response experiment. This study shows that diflubenzuron can be taken up by Atlantic cod, that it is rapidly cleared from the body and that when present this pharmaceutical causes only small effects on the expression of genes involved in detoxification pathways. Taken together, our data suggest that accumulated diflubenzuron at the levels studied would have a relatively small effect on wild Atlantic cod.

Biochemical studies on the effect of Chitin synthesis inhibitor, (flufenoxuron) and SpliNPV on the cotton leaf worm Spodopteralittoralis Bosid (Lepidoptera:Noctuidae)

In this studyflufenoxuron as insect growth regualtor and SpliNPV were used to investigate their toxicity on Spodopteralittoralis. Both 2 nd and 4 th larval instars of S. littoralis were exposed to the different concentrations of flufenoxuron and SpliNPV. The biochemical effect of IGR and SpliNPV on both 2 nd and 4 th larvae instar show changes in protein electrophoretic pattern in the current study reveals differences between the untreated samples and treated one, some proteins were missed or expressed at different stages which may be responsible for all the obtained deformation. The effect of these compounds on biochemical activities were revealed after studying electrophoretic protein (fractionated protein) using PAGE in order to determine their effects on the vital system of S. littoralis, which may indicate the formation of new proteins responsible for stimulating the immune system of the insect as a result of entering foreign objects inside the body of the insect.

Lethality of reduced-risk insecticides against plum curculio (Coleoptera: Curculionidae) in blueberries, with emphasis on their curative activity

Historically, management of plum curculio, Conotrachelus nenuphar (Herbst), in highbush blueberries has focused on post-bloom broad-spectrum insecticide applications targeting the adults. Here, the efficacy of different classes of insecticides against various stages of C. nenuphar was compared, and a prebloom treatment with the chitin synthesis inhibitor novaluron in combination with a post-bloom insecticide application was tested. Novaluron decreased the number of oviposition scars and eggs on fruit and reduced larval emergence by >60% when applied prebloom. Post-bloom applications of the oxadiazine indoxacarb and the organophosphate phosmet, but not the neonicotinoid acetamiprid, showed significant adulticidal activity. The neonicotinoids acetamiprid and thiamethoxam and phosmet showed significant curative activity on C. nenuphar larvae when applied topically to infested fruit, whereas the pyrethroid fenpropathrin, indoxacarb and novaluron were weaker curative agents. Residue profiles showed that acetamiprid and phosmet residues had the highest levels while fenpropathrin and novaluron had the lowest levels of fruit penetration. In blueberries, novaluron showed anti-ovipositional/ovicidal activity, indoxacarb and phosmet showed adulticidal activity, while the neonicotinoids and phosmet showed best curative (larvicidal) control on C. nenuphar. A prebloom novaluron application in combination with a post-bloom treatment with an adulticidal/larvicidal insecticide is recommended for optimal multi-life-stage management of C. nenuphar. © 2013 Society of Chemical Industry.

Effect of triflumuron, a chitin synthesis inhibitor, on Aedes aegypti, Aedes albopictus and Culex quinquefasciatus under laboratory conditions

BackgroundResistance to traditional insecticides represents a threat to the control of disease vectors. The insect growth regulators (IGR) are a potential alternative to control mosquitoes, including resistant populations. The chitin synthesis inhibitors (CSI) are IGRs, which interfere with the insect molting process and represent one major class of compounds against Aedes aegypti populations resistant to the larvicide organophosphate temephos. In the present study, we evaluated the efficacy of the CSI triflumuron on Culex quinquefasciatus, Aedes albopictus and against several Ae. aegypti field populations.MethodsThe efficacy of triflumuron, against Cx. quinquefasciatus and Ae. albopictus was evaluated with laboratory strains through dose–response assays. Additionaly, this CSI was tested against seven Ae. aegypti field populations exhibiting distinct resistance levels to both temephos and the pyrethroid deltamethrin. Aedes aegypti populations were exposed to both a dose that inhibits 99% of the adult emergence of mosquitoes from the susceptible reference strain, Rockefeller, (EI99 = 3.95 μg/L) and the diagnostic dose (DD), corresponding to twice the EI99.ResultsOur results indicate that triflumuron was effective in emergence inhibition (EI) of Cx. quinquefasciatus (EI50= 5.28 μg/L; EI90= 12.47 μg/L) and Ae. albopictus (EI50= 1.59 μg/L; EI90= 2.63 μg/L). Triflumuron was also effective against seven Ae. aegypti Brazilian populations resistant to both temephos and deltamethrin. Exposure of all the Ae. aegypti populations to the triflumuron EI99 of the susceptible reference strain, Rockefeller, resulted in complete inhibition of adult emergence, suggesting no cross-resistance among traditional insecticides and this CSI. However, a positive correlation between temephos resistance and tolerance to triflumuron was observed.ConclusionThe results suggest that triflumuron represents a potential tool for the control of disease vectors in public health. Nevertheless, they point to the need of constant monitoring of the susceptibility status of vector populations to CSIs.

The sulfonylurea receptor Sur is dispensable for chitin synthesis inDrosophila melanogasterembryos

Chitin produced by membrane-inserted chitin synthases is an important constituent of the arthropod cuticle and midgut peritrophic matrix. Chitin synthesis inhibitors are common insecticides in pest control. As the target of sulfonylurea-derived insecticides such as diflubenzuron, the ABC transporter sulfonylurea receptor (Sur) has been postulated to be an essential cofactor of chitin synthesis. However, direct evidence for this assumption is missing. Here, a study has been made of the phenotype of Drosophila melanogaster larvae suffering completely eliminated Sur function. Taken together, it is found that cuticle architecture is normal and chitin amounts are not diminished in the cuticle of these animals, indicating that Sur is dispensable for chitin synthesis. The data obtained suggest that there must exist another sulfonylurea-sensitive ABC transporter that either instead of Sur is the true sulfonylurea-sensitive transporter involved in chitin synthesis or is able to substitute Sur function during cuticle formation. Identification and characterisation of this factor is pivotal for understanding the mode of action of sulfonylurea as insecticide.

Comparative effects of insecticides with different mechanisms of action onChrysoperla externa(Neuroptera: Chrysopidae): Lethal, sublethal and dose-response effects

The comprehensive knowledge that the delayed systemic and reproduction side effects can be even more deleterious than acute toxicity, has caused a shift in focus toward sublethal effects assessment on physiology and behavior of beneficial insects. In this study, we assessed the risks posed by some insecticides with different mode of action through lethal and delayed systemic sublethal effects on the pupation, adult emergence, and reproduction of the chrysopid Chrysoperla externa (Hagen, 1861; Neuroptera: Chrysopidae), an important predator in pest biological control. The maximum field recommended dose (MFRD) and twice (2×MFRD) for chlorantraniliprole, tebufenozide, and pyriproxyfen were harmless to C. externa. In contrast, all the tested chitin synthesis inhibitors (CSIs) were highly detrimental to the predator, despite of their lack of acute lethal toxicity. Therefore, the safety assumed by using IGRs toward beneficial insects is not valid for chrysopids. Dose-response data showed that although all CSIs have a similar mechanism of action, the relative extent of toxicity may differ (novaluron > lufenuron > teflubenzuron). For CSIs, the delayed systemic effects became obvious at adult emergence, where the predicted no observable effect dose (NOED) was 1/2048 of the MFRD for novaluron (0.085 ng/insect), and 1/256 of the MFRD for both lufenuron (0.25 ng/insect) and teflubenzuron (0.6 ng/insect). Finally, this work emphasized the significance of performing toxicity risk assessments with an adequate posttreatment period to avoid underestimating the toxicities of insecticides, as the acute lethal toxicity assays may not provide accurate information regarding the long-range effects of hazardous compounds.

Experimental Study on Diflubenzuron: Degradation in Freshwater and Bioconcentration in Mosquitofish Following Chronic Exposure

Diflubenzuron (DFB, trade name dimilin ® 25 WP) is a chitin synthesis inhibitor widely used against forest insect pests in Algeria. Prior to implementation of these products as chemical agents for mosquito control, knowledge on their potential effects on non-target organisms and its behaviour in water are needed. Therefore, the present study was focused on DFB and aimed to use an HPLC procedure developed previously in order to obtain information on its degradation in freshwater and its bioconcentration in adult females of the fish Gambusia affinis (Cyprinodondiformes, Poeciliidae), which is one of the best candidates for biological control programs against mosquitoes. The adult females were exposed to dimilin ® (initial concentration 312 ng a.i./ml) for 28 days and residues analysis determined at different exposure times (0, 7, 14, 21 and 28 days). The concentration of DFB in freshwater decreased with exposure time while the amount of residues detected at the surface of the fish body increased progressively to reach a maximum at day 14 (162.7 ± 14.0 ng/fish) and declined thereafter during the exposure period. In addition, DFB incorporation into body increased with decreasing DFB concentration in water at each exposure time. The following average distribution was noted at the end of experiment (28 days): about 33% of the applied concentration was detected on the surface of fish body and was recovered by simple rinsing, and about 67% was found inside the fish body. A degradation in water and surface of fish occurred starting day 14 during the experimental period. Thus, about 40% of the initial concentration was degraded in freshwater after 28 days. The results are discussed to develop a better understanding of the degradation of dimilin in water and their potential effect on non-target organisms for its application for controlling mosquito.

Toxicological and biochemical studies of lufenuron, chlorfluazuron and chromafenozide against Pectinophora gossypiella (Saunders)

In the present study, one day old eggs laboratory colony of Pectinophora gossypiella, were treated with estimated LC50 values of lufenuron 5%, chlorfluazuron 5% and chromafenozide 80% (3.471, 4.189 and 122.703 ppm respectively), to study their effects on percentage of hatchability and duration of subsequent larval, pupal, immature stages and longevity, fecundity and fertility of resulted adults. The obtained results clear that the percentage of hatchability of treated eggs were 49.6, 51.0 and 53.0 for the three tested compounds, respectively oppose to 97% in untreated control. The incubation period of the hatched eggs was 6.5, 4.7, 6.2 and 3.2 days at three (IGRS) and control, respectively. Also, the treatments affected the subsequent stages it caused significant effects on larval and pupal duration, weight and malformations. On the other hand, the adult stage resulted from treated eggs was highly affected by the three (IGRS). All the compounds especially chlorfluazuron 5% caused increasing in adult longevity and reduced the respective fecundity and fertility compared to control. The biochemical effects of the tested insect growth regulators as chitin synthesis inhibitor (CSI) against larvae resulted from (treated one-day old eggs) was studied. The obtained results indicated that the tested IGRS reduced the glucose, protein and carbohydrate contents. Also, the tested IGRSelicited inhibitory effect on alanine amino transferase (ALT) and aspirate amino transferase (AST).

Effects of insect growth regulators on the mosquito-parasitic nematode Romanomermis iyengari

Pyriproxyfen, a juvenile hormone analogue, diflubenzuron, a chitin synthesis inhibitor, and azadirachtin, an ecdysone agonist, are three insect growth regulators (IGRs) considered as selective and effective insecticides for mosquitoes. Romanomermis iyengari (Welch) is a mosquito-parasitic mermithid that can provide biological control against many medically important mosquito species. The compatibility of these two control tactics was tested by evaluating the sublethal effects of exposure to IGR on nematode developmental stages (preparasitic, parasitic, and preparasitic + parasitic) using Culex pipiens larvae as the host. Sublethal concentrations of IGRs were 90 % emergence inhibition of host mosquito. Preparasitic exposure to pyriproxyfen, azadirachtin, and diflurbenzuron had no effect on infectivity, parasite load, sex ratio, or male size but reduced nematode female length and increased male sex ratio at one parasite/larva. When IGRs treatments were made against the parasitic and preparasitic + parasitic stages, pyriproxyfen and azadirachtin reduced R. iyengari infectivity, parasite load, and male nematode length, whereas pyriproxyfen exposure increased male sex ratio and reduced the female R. iyengari length. Thus, IGRs have significant negative impacts on different stages of mosquito mermithid that can destabilize the balance of host-parasite population interaction. Therefore, IGRs should be used with caution in mosquito habitats where these parasites have established.

Biochemical characterization of chitin synthase activity and inhibition in the African malaria mosquito, Anopheles gambiae

Chitin synthase (CHS) is an important enzyme catalyzing the formation of chitin polymers in all chitin containing organisms and a potential target site for insect pest control. However, our understanding of biochemical properties of insect CHSs has been very limited. We here report enzymatic and inhibitory properties of CHS prepared from the African malaria mosquito, Anopheles gambiae. Our study, which represents the first time to use a nonradioactive method to assay CHS activity in an insect species, determined the optimal conditions for measuring the enzyme activity, including pH, temperature, and concentrations of the substrate uridine diphosphate N-acetyl-d-glucosamine (UDP-GlcNAc) and Mg(++) . The optimal pH was about 6.5-7.0, and the highest activity was detected at temperatures between 37°C and 44°C. Dithithreitol is required to prevent melanization of the enzyme extract. CHS activity was enhanced at low concentration of GlcNAc, but inhibited at high concentrations. Proteolytic activation of the activity is significant both in the 500 ×g supernatant and the 40 000 ×g pellet. Our study revealed only slight in vitro inhibition of A. gambiae CHS activity by diflubenzuron and nikkomycin Z at the highest concentration (2.5 μmol/L) examined. There was no in vitro inhibition by polyoxin D at any concentration examined. Furthermore, we did not observe any in vivo inhibition of CHS activity by any of these chemicals at any concentration examined. Our results suggest that the inhibition of chitin synthesis by these chemicals is not due to direct inhibition of CHS in A. gambiae.

Ultrastructural Observations on the Gonads and Neurosecretory Cells ofSchistocerca gregariaAfter Treatment with Lufenuron (CGA-184699)

Abstract Topical application of a series of concentrations of lufenuron, a chitinsynthesis inhibitor, on the neck membrane of newly molted fifth-instar desert locusts, Schistocerca gregaria, gave various ultrastructural changes in ovarioles, including a disintegrated follicular epithelial cell layer, vacuolated cytoplasm and the appearance of lysosomal bodies. In males, electron micrographs showed loosely disrupted testicular tissues with vacuolated testicular epithelia. In both sexes the neurosecretory cells in the pars intercerebralis exhibited trapped neurosecretion without normal liberation into their connecting nerves and the mitochondria appeared to be losing their cristae.