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The glutathione S -transferases (GSTs) are important detoxifying enzymes in insects. Our previous studies found that the susceptibility of Chilo suppressalis to abamectin was significantly increased when the CsGST activity was inhibited by glutathione (GSH) depletory. In this study, the potential detoxification mechanisms of CsGSTs to abamectin were explored. Six CsGSTs of C. suppressalis were expressed in vitro . Enzymatic kinetic parameters including K m and V max of recombinant CsGSTs were determined, and results showed that all of the six CsGSTs were catalytically active and displaying glutathione transferase activity. Insecticide inhibitions revealed that a low concentration of abamectin could effectively inhibit the activities of CsGSTs including CsGSTd1, CsGSTe4, CsGSTo2, CsGSTs3, and CsGSTu1. However, the in vitro metabolism assay found that the six CsGSTs could not metabolize abamectin directly. Additionally, the glutathione transferase activity of CsGSTs in C. suppressalis was significantly increased post-treatment with abamectin. Comprehensive analysis of the results in present and our previous studies demonstrated that CsGSTs play an important role in detoxification of abamectin by catalyzing the conjugation of GSH to abamectin in C. suppressalis , and the high binding affinities of CsGSTd1, CsGSTe4, CsGSTo2, CsGSTs3, and CsGSTu1 with abamectin might also suggest the involvement of CsGSTs in detoxification of abamectin via the noncatalytic passive binding and sequestration instead of direct metabolism. These studies are helpful to better understand the detoxification mechanisms of GSTs in insects.

The review presents metabolic properties of Ivermectin (IVM) as substrate and inhibitor of human P450 (P450, CYP) enzymes and drug transporters. IVM is metabolized, both in vivo and in vitro, by C-hydroxylation and O-demethylation reactions catalyzed by P450 3A4 as the major enzyme, with a contribution of P450 3A5 and 2C9. In samples from both in vitro and in vivo metabolism, a number of metabolites were detected and as major identified metabolites were 3″-O-demethylated, C4-methyl hydroxylated, C25 isobutyl-/isopropyl-hydroxylated, and products of oxidation reactions. Ivermectin inhibited P450 2C9, 2C19, 2D6, and CYP3A4 with IC 50 values ranging from 5.3 μM to no inhibition suggesting that it is no or weak inhibitor of the enzymes. It is suggested that P-gp (MDR1) transporter participate in IVM efflux at low drug concentration with a slow transport rate. At the higher, micromolar concentration range, which saturates MDR1 (P-gp), MRP1, and to a lesser extent, MRP2 and MRP3 participate in IVM transport across physiological barriers. IVM exerts a potent inhibition of P-gp (ABCB1), MRP1 (ABCC1), MRP2 (ABCC2), and BCRP1 (ABCG2), and medium to weak inhibition of OATP1B1 (SLC21A6) and OATP1B3 (SLCOB3) transport activity. The metabolic and transport properties of IVM indicate that when IVM is co-administered with other drugs/chemicals that are potent inhibitors/inducers P4503A4 enzyme and of MDR1 (P-gp), BCRP or MRP transporters, or when polymorphisms of the drug transporters and P450 3A4 exist, drug-drug or drug-toxic chemical interactions might result in suboptimal response to the therapy or to toxic effects.

Glutathione S-transferases (GSTs) are a multifunctional protein superfamily that can catalyze the detoxification processes in an organism. In the present study, we determined the structure and function of GSTs in Chinese mitten crab (Eriocheir sinensis) by gene cloning, expression, and enzyme activity in order to investigate the metabolic detoxification of GSTs in the hepatopancreas and muscles under three pesticide (trichlorfon, β-cypermethrin and avermectin) stresses. Multiple sequence alignment analysis showed that all the three Es-GST genes possessed N-terminal, and C-terminal domain as well as G-binding sites, while Es-GST2 and Es-GST3 contained Mu-type GST-specific Mu-loop structures. Phylogenetic tree analysis revealed that the three Es-GSTs belonged to the Mu-type GST of crustaceans. The quantitative real-time PCR revealed that the three Es-GSTS were expressed in 9 tissues of Eriocheir sinensis, with highest expression in hepatopancreas and muscle. The expression of the three Es-GSTS significantly increased in the hepatopancreas and muscle under the three pesticide stresses compared to the control group, and a steady increase in GST activity was observed. The study showed that the three Es-GSTs belong to the Mu-type GST of the crustaceans and might play an important role in the metabolic detoxification in Eriocheir sinensis., Competing Interests: Declaration of competing interest The authors declare that they have no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

A series of new doramectin derivatives containing carbamate, ester and sulfonate were synthesized, and their structures were characterized by 1 H and 13 C nuclear magnetic resonance (NMR) and high-resolution mass spectrum (HRMS). Their insecticidal activities against oriental armyworm, diamondback moth, and corn borer were evaluated and compared with the parent doramectin and commercial avermectins, metolcarb, fenpropathrin. Among all compounds, three compounds (3a, 3g and 3h) showed excellent insecticidal effect. In particular, compound 3g containing cyclopropyl carbamate against oriental armyworm, diamondback moth, and corn borer, exhibited the most promising insecticidal activity with the final mortality rate of 66.67%, 36.67%, 40.00% at the concentration of 12.5 mg/L, respectively. The LC 50 values of 3g were 5.8859, 22.3214, and 22.0205 mg/L, showing 6.74, 2.23, 2.21-fold higher potency than parent doramectin (LC 50 values of 39.6907, 49.7736, and 48.6129 mg/L) and 6.83, 1.93, 3.36-fold higher potency than commercial avermectins (LC 50 values of 40.2489, 42.9922, and 73.9508 mg/L). Additionally, molecular docking simulations revealed that 3g displayed stronger hydrogen-bonding action in binding with the GABA receptor than parent doramectin, which were crucial for keeping high insecticidal activity. The present work demonstrated that these compounds containing alkyl carbamate group could be considered as potential candidates for the development of novel pesticides in the future., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

We previously observed that ivermectin-induced detoxification genes, including ATP binding cassette transporter C4 (PhABCC4) and cytochrome P450 6CJ1 (CYP6CJ1) were identified from body lice following a brief exposure to a sublethal dose of ivermectin using a non-invasive induction assay. In this current study, the functional properties of PhABCC4 and CYP6CJ1 were investigated after expression in either X. laevis oocytes or using a baculovirus expression system, respectively. Efflux of [ 3 H]-9-(2-phosphonomethoxyethyl) adenine ([ 3 H]-PMEA), a known ABCC4 substrate in humans, was detected from PhABCC4 cRNA-injected oocytes by liquid scintillation spectrophotometric analysis and PhABCC4 expression in oocytes was confirmed using ABC transporter inhibitors. Efflux was also determined to be ATP-dependent. Using a variety of insecticides in a competition assay, only co-injection of ivermectin and dichlorodiphenyltrichloroethane led to decreased efflux of [ 3 H]-PMEA. PhABCC4-expressing oocytes also directly effluxed [ 3 H]-ivermectin, which increased over time. In addition, ivermectin appeared to be oxidatively metabolized and/or sequestered, although at low levels, following functional expression of CYP6CJ1 along with cytochrome P450 reductase in Sf9 cells. Our study suggests that PhABCC4 and perhaps CYP6CJ1 are involved in the Phase III and Phase I xenobiotic metabolism of ivermectin, respectively, and may play an important role in the evolution of ivermectin resistance in lice and other insects as field selection occurs., (© 2017 The Royal Entomological Society.)

Annual losses of honey bee colonies remain high and pesticide exposure is one possible cause. Dangerous combinations of pesticides, plant-produced compounds and antibiotics added to hives may cause or contribute to losses, but it is very difficult to test the many combinations of those compounds that bees encounter. We propose a mechanism-based strategy for simplifying the assessment of combinations of compounds, focusing here on compounds that interact with xenobiotic handling ABC transporters. We evaluate the use of ivermectin as a model substrate for these transporters. Compounds that increase sensitivity of bees to ivermectin may be inhibiting key transporters. We show that several compounds commonly encountered by honey bees (fumagillin, Pristine, quercetin) significantly increased honey bee mortality due to ivermectin and significantly reduced the LC50 of ivermectin suggesting that they may interfere with transporter function. These inhibitors also significantly increased honey bees sensitivity to the neonicotinoid insecticide acetamiprid. This mechanism-based strategy may dramatically reduce the number of tests needed to assess the possibility of adverse combinations among pesticides. We also demonstrate an in vivo transporter assay that provides physical evidence of transporter inhibition by tracking the dynamics of a fluorescent substrate of these transporters (Rhodamine B) in bee tissues. Significantly more Rhodamine B remains in the head and hemolymph of bees pretreated with higher concentrations of the transporter inhibitor verapamil. Mechanism-based strategies for simplifying the assessment of adverse chemical interactions such as described here could improve our ability to identify those combinations that pose significantly greater risk to bees and perhaps improve the risk assessment protocols for honey bees and similar sensitive species.

An abamectin (ABM)-degrading bacterium, Stenotrophomonas maltophilia ZJB-14120, was isolated and identified. This strain is capable of degrading 84.82% of ABM at an initial concentration of 200 mg/L over a 48 h incubation period. This strain showed efficient biodegradation ability (7.81 mg/L/h) to ABM and high tolerance (1000 mg/L) to all macrolides tested. In addition to ABM, emamectin, erythromycin and spiramycin can also be degraded by this strain. Modifications involving either reduction of the double bond between C22-C23 or replacement of the C25-group of ABM with a cyclohexyl group can completely inhibit biodegradation of ABM. The ABM-degrading capability of strain ZJB-14120 is likely to be intrinsic to its metabolism and could be inhibited by incubating with erythromycin, azithromycin, spiramycin or rifampicin. A new and successive degradation pathway was proposed based on metabolite analysis. Although there is evidence for metabolite inhibition, this strain has high ABM degradation activity and reusability. Further investigation showed that activated macrolide efflux pump(s) and an undetermined mechanism for regulating the intracellular ABM concentration are responsible for normal uptake of essential metabolites while pumping out excess harmful compounds. Strain ZJB-14120 may provide efficient treatment of water and soil contaminated by toxic levels of abamectin and emamectin., (Copyright © 2015 Institut Pasteur. Published by Elsevier Masson SAS. All rights reserved.)

As an effective insecticidal and nematicidal agent, avermectin (AVM) has been widely used in agricultural production and stock farming areas. Subsequently, the residues of AVM or its active metabolites in animal manure pose a toxic threat to non-target organisms in the environment. As the most characteristic epigenetic phenomena, DNA methylation status is a useful biological signal for the toxicity assessment of environmental chemical toxicants. In this study, analyses of the overall level of genomic DNA methylation were performed, and the expression levels of DNA methyltransferases (DNMTs), as well as demethylase methyl-CpG-binding domain protein 2 (MBD2), in pigeon brain tissues after subchronic exposure (with a AVM concentration of 20 mg/kg, 40 mg/kg and 60 mg/kg, respectively) to AVM for 30, 60 and 90 days were investigated. Global DNA hypomethylation and down-regulation of DNMT mRNA expression occurred in a dose-time-dependent manner in pigeon brains. The expression level of MBD2, which functions as a demethylase, was significantly enhanced in a dose-dependent but not time-dependent manner. In addition, the elevated expression level of MBD2 had a more robust effect on genomic DNA hypomethylation compared to changes in DNMT expression. Taken together, these results suggested that subchronic dose exposures of AVM could affect the global DNA methylation status, and this mechanism is closely related to changes in the expression levels of DNMTs and MBD2., (Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.)

γ-Aminobutyric acid (GABA) receptors (GABARs) are an important target for existing insecticides such as fiproles. These insecticides act as noncompetitive antagonists (channel blockers) for insect GABARs by binding to a site within the intrinsic channel of the GABAR. Recently, a novel class of insecticides, 3-benzamido-N-phenylbenzamides (BPBs), was shown to inhibit GABARs by binding to a site distinct from the site for fiproles. We examined the binding site of BPBs in the adult housefly by means of radioligand-binding and electrophysiological experiments. 3-Benzamido-N-(2,6-dimethyl-4-perfluoroisopropylphenyl)-2-fluorobenzamide (BPB 1) (the N-demethyl BPB) was a partial, but potent, inhibitor of [(3)H]4'-ethynyl-4-n-propylbicycloorthobenzoate (GABA channel blocker) binding to housefly head membranes, whereas the 3-(N-methyl)benzamido congener (the N-methyl BPB) had low or little activity. A total of 15 BPB analogs were tested for their abilities to inhibit [(3)H]BPB 1 binding to the head membranes. The N-demethyl analogs, known to be highly effective insecticides, potently inhibited the [(3)H]BPB 1 binding, but the N-methyl analogs did not even though they, too, are considered highly effective. [(3)H]BPB 1 equally bound to the head membranes from wild-type and dieldrin-resistant (rdl mutant) houseflies. GABA allosterically inhibited [(3)H]BPB 1 binding. By contrast, channel blocker-type antagonists enhanced [(3)H]BPB 1 binding to housefly head membranes by increasing the affinity of BPB 1. Antiparasitic macrolides, such as ivermectin B1a, were potent inhibitors of [(3)H]BPB 1 binding. BPB 1 inhibited GABA-induced currents in housefly GABARs expressed in Xenopus oocytes, whereas it failed to inhibit l-glutamate-induced currents in inhibitory l-glutamate receptors. Overall, these findings indicate that BPBs act at a novel allosteric site that is different from the site for channel blocker-type antagonists and that is probably overlapped with the site for macrolides in insect GABARs., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The effects of different ages and dosages on the plasma disposition and hair concentration profile of ivermectin following pour-on administration in goats. J. vet. Pharmacol. Therap.34, 70-75. The effects of different ages and dosages on the plasma disposition and hair degradation of ivermectin (IVM) were investigated following pour-on administration in goats. Twenty-eight female Saanen goats allocated into two groups of 14 animals according to their ages as young (5-6 months old) and old (12-24 months old) groups. Each age group was divided into two further of seven goats and administered pour-on formulation of IVM topically at the in recommended dosage rate of 0.5 mg/kg bodyweight The recommended cattle dosages rate of 0.5 mg/kg or at the higher dosage of 1.0 mg/kg. Blood samples were collected at various times between 1 h and 40 days. In addition, hair samples (>0.01 g) were collected using tweezers from the application sites and far from application sites of the all animals throughout the blood sampling period. The plasma and hair samples were analyzed by high performance liquid chromatography (HPLC) using fluorescence detection following solid and liquid phase extractions, respectively. Dose- and age-dependent plasma disposition of IVM were observed in goats after pour-on administration. In addition, relatively high concentration and slow degradation of IVM in hair samples collected from the application site and far from the application site were observed in the present study. The differences between young and old goats are probably related to differences in body condition and/or lengths of haircoat. The systemic availability of IVM following pour-on administration is relatively much lower than after oral and subcutaneous administrations but the plasma persistence was prolonged. Although, the longer persistence of IVM on hairs on the application site may prolong of efficacy against ectoparasites, the poor plasma availability could result in subtherapeutic plasma concentrations, which may confer the risk of resistance development in for internal parasites after pour-on administration in goats., (© 2010 Blackwell Publishing Ltd.)

Abamectin is widely used in agriculture as an insecticide and in veterinary as an anti-parasitic agent, and has caused great environmental pollution by posing potential risk to non-target soil invertebrates and nearby aquatic systems. A bacterium designated GB-01, which was capable of degrading abamectin, was isolated from soil by enrichment culture method. On the basis of morphological, physiological and biochemical characteristics, combined with phylogenetic analysis of 16S rRNA gene, the bacterium GB-01 was identified as Burkholderia cepacia-like species. The bacterium GB-01 was able to utilize abamectin as its sole carbon source for growth, and could degrade more than 90% of abamectin at initial concentrations of 50 and 100 mg l(-1) in mineral salt medium in 30 and 36 h, respectively. The longer degradation cycle was observed with abamectin concentrations higher than 100 mg l(-1). Optimal growth temperatures and pH values with highest degradation rate were 30-35 degrees C and 7-8, respectively. Two new degradation products were identified and characterized by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) based mass spectral data and a plausible partial degradation pathway of abamectin was proposed. This is the first report in which an abamectin-degrading Burkholderia species isolated from soil was identified and characterized.

The avermectin analogue doramectin (CHC-B1), which is produced in mutants that have an altered biosynthesis pathway of avermectin, is one of the most effective agricultural pesticides and antiparasitics. We report here the construction of a bkdF olmA double-deletion mutant lacking one of the branched-chain alpha-keto acid dehydrogenase encoding genes (bkdF) and the oligomycin PKS encoding gene cluster (olmA) in Streptomyces avermitilis 76-05. We then characterized the production of various antibiotics in cultures of the deletion mutant. In a fermentation medium supplemented with cyclohexanecarboxylic acid, this double mutant produced doramectin and its analogues but no oligomycin. The mutant proved to be genetically stable, without any antibiotic resistance markers inserted into its chromosome, and could potentially become an industrial doramectin-producing strain after further improvement.

Discovery of the CYP107Z subfamily of cytochrome P450 oxidases (CYPs) led to an alternative biocatalytic synthesis of 4''-oxo-avermectin, a key intermediate for the commercial production of the semisynthetic insecticide emamectin. However, under industrial process conditions, these wild-type CYPs showed lower yields due to side product formation. Molecular evolution employing GeneReassembly was used to improve the regiospecificity of these enzymes by a combination of random mutagenesis, protein structure-guided site-directed mutagenesis, and recombination of multiple natural and synthetic CYP107Z gene fragments. To assess the specificity of CYP mutants, a miniaturized, whole-cell biocatalytic reaction system that allowed high-throughput screening of large numbers of variants was developed. In an iterative process consisting of four successive rounds of GeneReassembly evolution, enzyme variants with significantly improved specificity for the production of 4''-oxo-avermectin were identified; these variants could be employed for a more economical industrial biocatalytic process to manufacture emamectin.

The aims of this study were to investigate the content of emamectin in blood, mucus and muscle following field administration of the recommended dose, and correlation with sea lice infection on the same fish (elimination study). The tissue distribution of tritiated emamectin benzoate after a single oral dose in Atlantic salmon was also investigated by means of whole-body autoradiography and scintillation counting (distribution study). In the elimination study, concentrations of emamectin benzoate reached maximum levels of 128, 105 and 68 ng/g (p.p.b.) for blood, mucus and muscle respectively, on day 7, the last day of administration. From day 7, the concentration in the blood declined until concentration was less than the limit of detection on day 77. The concentration was higher in mucus compared with plasma (P < 0.05) except on days 7 and 21. The concentration of emamectin benzoate decreased gradually from the end of treatment (day 7) to day 70 with half-lives of 9.2, 10.0 and 11.3 days in muscle, plasma and mucus respectively. The distribution study demonstrated a high quantity of radioactivity in mucous membranes (gastrointestinal tract, gills) throughout the observation period (56 days). Activity was high in the epiphysis, hypophysis and olfactory rosette throughout the study. The highest activity was observed in the bile, indicating this to be an important route for excretion. The distribution study confirmed the results from the elimination study with respect to concentrations in blood, skin mucous and muscle.

Incubation test on the degradation dynamics of Abamectin in soil showed that the half-life of its non-biodegradation plus microbial biodegradation, non-biodegradation, and microbial biodegradation was 34.8, 277.3 and 49.9 d, respectively, and its degradation in soil was mostly by microbes. A dominant bacterium which could effectively degrade Abamectin was isolated from test soil, and identified as Stenotrophomonas maltrophilia by 16S rDNA. The crude enzyme extracted from the dominant bacteria had a Michaelis-Menten's constant 6.78 nmol x ml(-1) and a maximum rate 81.5 nmol x min(-1) x mg(-1).

Foraging cockroaches ingest insecticide baits, translocate them, and can cause mortality in untreated cockroaches that contact the foragers or ingest their excretions. Translocation of eight ingested baits by adult male Blattella germanica (L.) was examined in relation to the type of the active ingredient, formulation, and foraging area. Ingested boric acid, chlorpyrifos, fipronil, and hydramethylnon that were excreted by adults in small dishes killed 100% of first instars within 10 d and >50% of second instars within 14 d. Residues from these ingested baits were also highly effective on nymphs in larger arenas and killed 16-100% of the adults. However, when the baits and dead cockroaches were removed from the large arenas and replaced with new cockroaches, only residues of the slow-acting hydramethylnon killed most of the nymphs and adults, whereas residues of fast acting insecticides (chlorpyrifos and fipronil) killed fewer nymphs and adults. Excretions from cockroaches that ingested abamectin baits failed to cause significant mortality in cockroaches that contacted the residues. These results suggest that hydramethylnon is highly effective in these assays because cockroaches that feed on the bait have ample time to return to their shelter and defecate insecticide-laden feces. The relatively high concentration of hydramethylnon in the bait (2.15%) and its apparent stability in the digestive tract and feces probably contribute to the efficacy of hydramethylnon. To control for differences among baits in inert ingredients and the amount of active ingredient, we compared 1% chlorpyrifos with 1% hydramethylnon in identical baits. Again, hydramethylnon residues provided greater secondary kill, but the results highlighted the importance of the inert ingredients. We conclude that, in the absence of cannibalism and necrophagy, translocation of baits and secondary kill are most effective with slow acting insecticides in palatable baits that can traverse the digestive tract and be deposited within and around the cockroach aggregation.

Avermectin B1a (1) is a potent anthelmintic, insecticide, miticide and chloride channel activator on interaction with a specific nerve membrane site analyzed by binding assays with [3H]1. Candidate photoaffinity probes were prepared replacing the dioleandrosyl substituent with potential isosteric esters and ethers approximating the original overall atom length and terminating in a phenyl moiety substituted with azido, iodo or hydroxy. Several of the candidates met the goal of high potency on mouse, housefly and fruit fly brain chloride channels with IC50 values of 7-57 nM in competing for the [3H]1 binding site.

The metabolism of 3H/14C-labeled 4"-deoxy-4"-epimethylaminoavermectin B1a (MAB1a) benzoate, the major homologue (>/=90%) of the avermectin insecticide emamectin benzoate, was studied in laying chickens. Ten Leghorn hens (Gallus domesticus) were orally dosed once daily for 7 days (1 mg/kg of body weight/day). Eggs and excreta were collected daily, and eggs were subsequently separated into whites and yolks. Chickens were euthanized within 20 hr after the last dose, and liver, kidney, heart, muscle, fat, ovaries, gizzard, gastrointestinal tract and contents, and carcass were collected. Approximately 70 and 6% of the total administered dose were recovered in the excreta plus gastrointestinal tract and contents and in the tissues plus eggs, respectively. Two novel metabolites, i.e. the 24-hydroxymethyl derivative of the parent compound (24-hydroxymethyl-4"-deoxy-4"-epimethylaminoavermectin B1a) and the N-demethylated derivative of 24-hydroxymethyl-4"-deoxy-4"-epimethylaminoavermectin B1a (24-hydroxymethyl-4"-deoxy-4"-epiaminoavermectin B1a), were identified. In addition, eight fatty acid conjugates of each of these two metabolites, comprising 8-75% of total radioactive residues in tissues and eggs, were isolated and identified. Although this represents some of the most extensive in vivo fatty acid conjugation to a xenobiotic reported to date, potential human exposure to MAB1a residues from consumption of chicken would be extremely low, because the dosage level in this study was approximately 1000-fold greater than the MAB1a residue levels seen in crops and because the majority of the applied dose was recovered in the excreta. Based on these findings, the avian biotransformation of MAB1a differs substantially from the mammalian biotransformation.

A high-affinity avermectin binding site from Drosophila melanogaster head membranes was subjected to target size analysis by radiation inactivation in order to determine the functional unit size. Using the [3H]ivermectin binding assay to assess ligand binding activity, the target size was determined to be 44.3 +/- 3.9 kDa. This result suggests that the size of the functional unit required for high-affinity ligand binding is a monomer. The membrane-associated acetylcholinesterase present in the Drosophila head membranes was also analyzed by radiation inactivation and served as a control. By monitoring enzymatic activity, the functional unit size of the Drosophila acetylcholinesterase was determined to be 70 +/- 9.7 kDa. This corresponds to the molecular weight of a dimer composed of a 55 kDa subunit and a 16-18 kDa subunit.

The effects of faecal drug residues following the administration of anthelmintics in the form of sustained-release boluses, on dung-colonising Coleoptera and Diptera are reported. In blind field trials, pats of standard weight and size were prepared from the dung of cattle treated with an ivermectin (Ivomec SR Bolus, MSD Agvet) or a fenbendazole (Panacur Bolus, Hoechst) sustained-release bolus, and from a third control group of cattle that received no treatment. Pats were recovered after 7, 14, 21 and 42 days in the field and searched for invertebrates. There were no differences in the numbers of adult beetles found in the pats from the three treatment groups. Pats made from the dung of ivermectin-treated animals contained no larval Diptera Cyclorrhapha and significantly fewer larval Scarabaeidae than pats made from the dung of the other two groups. Furthermore, larval Scarabaeidae in the ivermectin pats were inhibited in their development. The pats from fenbendazole-treated animals contained similar numbers of larval Scarabaeidae and Diptera to the pats from untreated animals throughout the trial. At 42 days, the solid matter of the control and fenbendazole-containing cow pats were reduced to a crumbling, granular texture, while the pats from the ivermectin-treated animals were solid and compacted. Pitfall trapping, using traps baited with dung from the three groups, showed no significant difference between the numbers of adult Scarabaeidae attracted, though a trend towards higher numbers attracted to the dung of both anthelmintic-treated groups was evident. The results provide evidence of the toxic effects of excreted ivermectin on key dung-colonising families of insects, and show that fenbendazole lacks such toxic effects.

The influence of complex nitrogen-containing substrates (Difko yeast extract, EKD nutrient yeast extract, soy bean flour and cotton seed meal), ammonium salts and some amino acids (alanine, methionine, valine, isoleucine and threonine) on the biosynthesis of avermectins in the cultures of two mutants of Streptomyces avermitilis was studied. It was shown that an excess of the nitrogen compounds in the fermentation medium induced a decrease in the antibiotic biosynthesis and the relative content of the group B avermectins.

Abamectin (avermectin B1) and ivermectin (22,23-dihydroavermectin B1) are high molecular weight hydrophobic compounds, active against a variety of animal parasites and insects. Numerous environmental fate and effects studies have been carried out in the development of these two compounds as antiparasitic agents and for abamectin as a crop protection chemical. They were found to be immobile in soil (Koc > or = 4000), rapidly photodegraded in water (degradation half-life (t1/2) in the summer 0.5 days or less) and as thin films on surfaces (t1/2 < 1 day), and aerobically degraded in soil (ivermectin in soil/feces mixtures (t1/2) = 7-14 days; avermectin B1a in soils, t1/2 = 2-8 weeks) to less bioactive compounds. Abamectin is not taken up from the soil by plants, nor is it bioconcentrated by fish (calculated steady-state bioconcentration factor of 52, with rapid depuration). Daphnia magna is the fresh water species found to be most sensitive to ivermectin and abamectin (LC50 values of 0.025 and 0.34 ppb respectively); fish (e.g. rainbow trout) are much less sensitive to these compounds (LC50 values of 3.0 ppb and 3.2 ppb, respectively). In the presence of sediment, toxicity toward Daphnia is significantly reduced. The metabolism and degradation of ivermectin and abamectin result in reduced toxicity to Daphnia. Abamectin and ivermectin possess no significant antibacterial and antifungal activity. They display little toxicity to earthworms (LC50 values of 315 ppm and 28 ppm in soil for ivermectin and abamectin, respectively) or avians (abamectin dietary LC50 values for bobwhite quail and mallard duck of 3102 ppm and 383 ppm, respectively), and no phytotoxicity. Residues of the avermectins in feces of livestock affect some dung-associated insects, especially their larval forms. This does not delay degradation of naturally formed cattle pats under field conditions; however, in some cases, delays have been observed with artificially formed pats. Based on usage patterns, the availability of residue-free dung and insect mobility, overall effects on dung-associated insects will be limited. As abamectin and ivermectin undergo rapid degradation in light and soil, and bind tightly to soil and sediment, they will not accumulate and will not undergo translocation in the environment, minimizing any environmental impact on non-target organisms resulting from their use.