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PubMed1961

Investigation into the problem of insecticide sorption by soils.

In view of the importance of the problem of insecticide sorption by certain soils, and its possible implications for mosquito control, an investigation was carried out on the influence of some factors governing sorption.In experiments with thin layers of homogenized soil/insecticide mixtures, in which no diffusion took place, changes in relative humidity had a pronounced influence on the effectiveness of the insecticide, a 10% increase in humidity doubling the toxicity. Further experiments using DDT labelled with (14)C showed that the effect of humidity was the only factor influencing toxicity.Studies with (14)C DDT and bio-assay determination of non-labelled DDT and dieldrin showed that movement of the insecticide in the soil was blocked at both very high and very low humidity, and that inward migration was found only at intermediate humidities. It was also observed that the migration of water in the soil caused the insecticide to move in the same direction.As at a high relative humidity the inward migration of the insecticide is blocked, and as the initial loss in effectiveness by sorption is counterbalanced by the greater availability of the remaining toxicant, the application of the usual field dosage of dieldrin (0.5 g/m(2)) remained effective for a considerable period. This would suggest that sorption might well be a problem in the field only when humidity is low. In work on means of reducing sorption under dry conditions, encouraging results were obtained with wettable powders based on ground solidified melts of dieldrin and certain resins.

Insecticides↗
PubMed1994

What role for insecticides in vector control programs?

Vector-borne diseases including dengue, yellow fever, Japanese encephalitis, malaria, leishmaniasis, and filariasis remain severe public health problems in most of the countries in which they are endemic. In some cases, their incidence is increasing and they are spreading to new geographic areas. For a number of the infections, the most effective manner of controlling their transmission is through control of their vectors. However, in some instances, such as dengue and Chagas' disease, there is no alternative. Most countries that are endemic for vector-borne diseases maintain vector control services, and most large tropical and semitropical cities also have pest control programs, mainly against pest mosquitoes. Virtually all of the vector and pest control programs depend on the use of insecticides formulated as larvicides, adulticides, baits, or insecticide impregnated bed nets. For many years, the development of new insecticides for use in public health programs was encouraged and supported by multilateral and bilateral health agencies, including the implementation of field trials in endemic areas. Due to the development of insecticide resistance, toxicologic and environmental considerations, and the cost of development and of registration, the number of compounds available for use has declined while the number of new insecticides submitted for laboratory and field trials to the World Health Organization has dwindled even more. The recrudescence of vector-borne diseases, the rapid pace of urbanization, lagging development of environmental services in many tropical cities, and difficulties encountered in ensuring the community's cooperation in its own protection through environmental measures make imperative the continued availability of pesticides for public health use. Since only the pesticide manufacturing industry has the combination of technical and financial resources to promulgate the research and development of new pesticides and pesticide groups, it is suggested that governments, bilateral, and multilateral organizations explore the manner in which they can assist industry in the development of new compounds and guarantee the continued availability of effective and safe pesticides for vector-control programs.

Animals↗
PubMed2004

Sustainability of reductions in malaria transmission and infant mortality in western Kenya with use of insecticide-treated bednets: 4 to 6 years of follow-up.

CONTEXT: Insecticide-treated bednets reduce malaria transmission and child morbidity and mortality in short-term trials, but this impact may not be sustainable. Previous investigators have suggested that bednet use might paradoxically increase mortality in older children through delayed acquisition of immunity to malaria. OBJECTIVES: To determine whether adherence to and public health benefits of insecticide-treated bednets can be sustained over time and whether bednet use during infancy increases all-cause mortality rates in older children in an area of intense perennial malaria transmission. DESIGN AND SETTING: A community randomized controlled trial in western Kenya (phase 1: January 1997 to February 2000) followed by continued surveillance of adherence, entomologic parameters, morbidity indicators, and all-cause mortality (phase 2: April 1999 to February 2002), and extended demographic monitoring (January to December 2002). PARTICIPANTS: A total of 130,000 residents of 221 villages in Asembo and Gem were randomized to receive insecticide-treated bednets at the start of phase 1 (111 villages) or phase 2 (110 villages). MAIN OUTCOME MEASURES: Proportion of children younger than 5 years using insecticide-treated bednets, mean number of Anopheles mosquitoes per house, and all-cause mortality rates. RESULTS: Adherence to bednet use in children younger than 5 years increased from 65.9% in phase 1 to 82.5% in phase 2 (P<.001). After 3 to 4 years of bednet use, the mean number of Anopheles mosquitoes per house in the study area was 77% lower than in a neighboring area without bednets (risk ratio, 0.23; 95% confidence interval [CI], 0.15-0.35). All-cause mortality rates in infants aged 1 to 11 months were significantly reduced in intervention villages during phase 1 (hazard ratio [HR], 0.78; 95% CI, 0.67-0.90); low rates were maintained during phase 2. Mortality rates did not differ during 2002 (after up to 6 years of bednet use) between children from former intervention and former control households born during phase 1 (HR, 1.01; 95% CI, 0.86-1.19). CONCLUSIONS: The public health benefits of insecticide-treated bednets were sustained for up to 6 years. There is no evidence that bednet use from birth increases all-cause mortality in older children in an area of intense perennial transmission of malaria.

Animals↗
PubMed2001

Multi-residue determination of 41 insecticides in garlic by gas chromatography and ion trap mass spectrometry using the selective ion storage technique.

A method is described for determining 41 insecticide residues in garlic (Allium sativum L.), including organophosphorus, organochlorine, carbamate, and synthetic pyrethroid insecticides. These insecticides were extracted from samples with acetone and dichloromethane, and co-extractives removed using a charcoal/Celite/alumina column. Analysis was performed by gas chromatography with ion trap mass spectrometry in selective ion storage (SIS) mode. Retention times and specific ions (m/z values) were used to confirm insecticides. Recoveries for most insecticides (blank samples spiked at 0.05, 0.2 and 1 microg mL(-1) levels) ranged from 70% to 110%, the coefficient of variation (CV) of the method was <20% for every case, and the limit of detection (LOD), defined in terms of 3 times baseline noise, varied between 0.01 and 0.16 mg kg(-1), depending on the compound.

Food Contamination↗
PubMed2004

Insecticide-treated bed nets and curtains for preventing malaria.

BACKGROUND: Malaria is an important cause of illness and death in many parts of the world, especially in sub-Saharan Africa. There has been a renewed emphasis on preventive measures at community and individual levels. Insecticide-treated nets (ITNs) are the most prominent malaria preventive measure for large-scale deployment in highly endemic areas. OBJECTIVES: To assess the impact of insecticide-treated bed nets or curtains on mortality, malarial illness (life-threatening and mild), malaria parasitaemia, anaemia, and spleen rates. SEARCH STRATEGY: I searched the Cochrane Infectious Diseases Group trials register (January 2003), CENTRAL (The Cochrane Library, Issue 1, 2003), MEDLINE (1966 to October 2003), EMBASE (1974 to November 2002), LILACS (1982 to January 2003), and reference lists of reviews, books, and trials. I handsearched journals, contacted researchers, funding agencies, and net and insecticide manufacturers. SELECTION CRITERIA: Individual and cluster randomized controlled trials of insecticide-treated bed nets or curtains compared to nets without insecticide or no nets. Trials including only pregnant women were excluded. DATA COLLECTION AND ANALYSIS: The reviewer and two independent assessors reviewed trials for inclusion. The reviewer assessed trial methodological quality and extracted and analysed data. MAIN RESULTS: Fourteen cluster randomized and eight individually randomized controlled trials met the inclusion criteria. Five trials measured child mortality: ITNs provided 17% protective efficacy (PE) compared to no nets (relative rate 0.83, 95% confidence interval (CI) 0.76 to 0.90), and 23% PE compared to untreated nets (relative rate 0.77, 95% CI 0.63 to 0.95). About 5.5 lives (95% CI 3.39 to 7.67) can be saved each year for every 1000 children protected with ITNs. In areas with stable malaria, ITNs reduced the incidence of uncomplicated malarial episodes in areas of stable malaria by 50% compared to no nets, and 39% compared to untreated nets; and in areas of unstable malaria: by 62% for compared to no nets and 43% compared to untreated nets for Plasmodium falciparum episodes, and by 52% compared to no nets and 11% compared to untreated nets for P. vivax episodes. When compared to no nets and in areas of stable malaria, ITNs also had an impact on severe malaria (45% PE, 95% CI 20 to 63), parasite prevalence (13% PE), high parasitaemia (29% PE), splenomegaly (30% PE), and their use improved the average haemoglobin level in children by 1.7% packed cell volume. REVIEWERS' CONCLUSIONS: ITNs are highly effective in reducing childhood mortality and morbidity from malaria. Widespread access to ITNs is currently being advocated by Roll Back Malaria, but universal deployment will require major financial, technical, and operational inputs.

Bedding and Linens↗
PubMed1993

Insecticide action at the GABA-gated chloride channel: recognition, progress, and prospects.

Three billion (3 x 10(9)) pounds of hexachlorocyclohexanes, polychlorobornanes, and chlorinated cyclodienes (such as lindane, toxaphene, and endosulfan, respectively) were used to control pest insects before their mode of action was established as blocking the GABA-gated chloride channel. With the restricted use or demise of these polychlorocycloalkanes (each approximately 50 to approximately 75% by weight of chlorine), the GABAergic system is for now an underutilized target of insecticide action. Newer compounds with outstanding potency at this receptor and as toxicants to houseflies are suitably-substituted 2,6,7-trioxabicyclo[2.2.2]-octanes, particularly the bicycloorthobenzoates, and 1,3-dithianes, including those with no halogenated substituents. Picrotoxinin analogs and alkynylphenyl-silatranes also act at this target but are of lower insecticidal activity. [3H]n-Propyl-ethynylbicycloorthobenzoate ([3H]EBOB) is for now the best radioligand for this insecticidally-relevant binding site in insects. Macrocyclic lactones such as the avermectins and moxidectin act at a different binding site to disrupt chloride flux and they have a different spectrum of insecticidal activity and no cross resistance with cyclodienes in houseflies. The search for new insecticides has provided the incentive and probes for a better understanding of the insect GABAergic system.

Animals↗
PubMedDate not supplied

Structural analysis of the unique insecticidal activity of novel mungbean defensin VrD1 reveals possibility of homoplasy evolution between plant defensins and scorpion neurotoxins.

A variety of evolutionarily related defensin molecules is found in plants and animals. Plant gamma-thionins and scorpion neurotoxins, for instance, may be categorized in this functional group, although each class recognizes a distinct receptor binding site. Such molecules are also categorized into the superfamily of cysteine-rich proteins. Plant defensins were generally believed to be involved in antimicrobial or antifungal mechanisms and, unlike scorpion toxins, little is known about whether these molecules are also endowed with the function of insect resistance. We have previously reported the isolation of a cDNA encoding a small cysteine-rich protein designated VrD1 (VrCRP) from a bruchid-resistant mungbean, which is apparently the first discovered plant defensin exhibiting in vitro and in vivo both insecticidal and antifungal activities. Our previous data also successfully demonstrated that VrD1 is toxic to E. coli and able to completely arrest the growth of Sf-21 insect cells at low concentration. However, the molecular and structural basis of this unique insecticidal activity of VrD1 is not clear. Therefore, in the present study, we use structural approach and phylogenic analysis to investigate the evolutionary and functional relations for such unique insecticidal activity. From our results, it is suggested that VrD1, in addition to gamma-thionins and several amylase inhibitors, is highly homologous to scorpion toxins, especially the short toxins. Moreover, based on the observation from our homology structures, VrD1 may utilize a newly found cluster of basic residues to achieve its insecticidal function, whereas all the other plant gamma-thionins were known to use a previously identified basic cluster conserved for gamma-thionins. Considering the general feature of short scorpion toxins to act on insect cell membranes with K(+)- or Cl(-)-channels as molecular targets, our analysis of interaction and recognition modes provides reasonable correlations between this newly found basic cluster and the insecticidal activity of VrD1, which is also comprehended as a possible link for "homoplasy evolution" between plant and animal defensin molecules.

Amino Acid Sequence↗
PubMed2005

The effect of insecticide on adipokinetic hormone titre in the insect body.

The effect of an insecticide, permethrin (Ambush 25 EC), on the titre of two adipokinetic hormones in the central nervous system (CNS) and haemolymph of the firebug Pyrrhocoris apterus (L) was tested. Doses of 2.5, 12.5 and 25 ng of the insecticide elicited no significant effect on the titre of the adipokinetic peptides in CNS, but caused their dramatic elevation in the haemolymph of both macropterous and reproductive brachypterous bugs. Changes in diapausing brachypterous bugs were minimal and not significant. Using high-performance liquid chromatography (HPLC) and competitive ELISA, both adipokinetic peptides of P apterus (Pyrap-AKH and Peram-CAH-II) were identified and quantified in the CNS of all experimental groups. The ratios of the two peptides in the CNS of the bugs used were affected by insecticide treatment, but in all three groups of bugs the Pyrap-AKH level remained dominant. The insecticide-treated bugs were compared with those exposed to forced running (shaking) and dark shock. Both factors also acted as stressors and enhanced the titre of adipokinetic peptides in the haemolymph but had no effect on hormone titre in CNS. The results indicate an involvement of adipokinetic peptides in the response of insect to various stressors including insecticides.

Animals↗
PubMed2006

Evaluation of volatile low molecular weight insecticides using Drosophila melanogaster as a model.

Volatile insecticides, otherwise known as fumigants, have numerous uses in pest control and integrated pest management. Fumigants have been broadly applied in the protection of stored agricultural commodities and also in the elimination of structural pest infestations. However, fumigants have so far been mostly overlooked for use in management of public health pests such as flies and mosquitoes. This paper reports information on the development of a volatility bioassay for use in evaluating volatile insecticides against dipteran insects and on the use of this bioassay to compare the relative efficacy of 30 candidate compounds to Drosophila melanogaster Meig. The findings revealed seven highly effective insecticidal compounds, which include three heterobicyclics (menthofuran, thiophene and coumaran) and four formate esters [butyl, hexyl and heptyl formate and ethylene glycol diformate (EGDF)]. Ethyl formate, a compound previously identified as being a highly effective fumigant against coleopteran stored product pests, caused toxicity in Drosophila but was not as effective as other tested formates. Two volatile fumigant insecticides, dichlorvos (DDVP) and methyl isothiocyanate (MITC), were also tested as positive controls. Both of these volatile standards were significantly more toxic than the seven most effective of the novel compounds. Finally, regression analyses revealed that volatility is not entirely predictive of acute toxicity and that other structural features should be considered when designing and synthesizing volatile insecticidal compounds for future study.

Animals↗
PubMed2001

Effects of organosilicon pyrethroid-like insecticides on nerve preparations of American cockroaches and crayfish.

Quaternary organosilicon pyrethroid-like ethers (five compounds) and alkanes (three compounds) were used for neurophysiological tests. Their activities in inducing repetitive firing in the central nervous cord of the American cockroach (Periplaneta americana) were evaluated by an extracellular recording method. The ethers were more active than the corresponding alkanes. The ability of the compounds to cause conduction blockage was also measured using the same nerve preparations, but the effects were too weak to allow definitive activity values to be determined. The compounds prolonged the sodium tail-current in the crayfish giant axon under voltage clamp conditions. The rate of decay of the tail-current changed in parallel with the reported insecticidal activity against P americana. These findings indicated that tail-current activity was the most useful nerve parameter in predicting insecticidal activity. Regression analysis of the numerical data together with those reported for other alkanes revealed that the higher the tail-current activity, the higher the insecticidal activity when a structural feature and the hydrophobicity were considered separately. The insecticidal activity of the ethers was about seven-fold higher than that of the alkanes with equivalent tail-current activity and hydrophobicity. Variations in insecticidal activity were parabolically correlated with the hydrophobicity [(log P)opt = 9.1] when other factors were similar.

Action Potentials↗
PubMed2001

Insecticidal and binding activities of N3-substituted imidacloprid derivatives against the housefly Musca domestica and the alpha-bungarotoxin binding sites of nicotinic acetylcholine receptors.

N3-substituted imidacloprid congeners containing C1-C6 alkyl groups or various analogous groups, and their corresponding nitromethylene analogues, were used in this study. Their insecticidal activity against the housefly, Musca domestica, and their binding activity toward the nicotinic acetylcholine receptor were determined. The insecticidal test was conducted using the synergists piperonyl butoxide and propargyl propyl phenylphosphonate. The binding assay was performed with housefly head membrane preparations using radio-labelled alpha-bungarotoxin. Both insecticidal and binding activities were drastically lowered by the introduction of alkyl/allyl groups at the imidazolidine NH sites of both nitroimino and nitromethylene compounds. The binding activity of N3-substituted nitromethylene analogues was much higher than that of the corresponding nitroimino analogues. However, the insecticidal activity of both series of compounds with a given substituent was nearly identical. The insecticidal activity correlated positively with the binding activity after taking into account the structural difference of the nitroimino and nitromethylene moieties and a structural feature of the N3-substituents.

Animals↗
PubMed2003

Effect of the combined treatment of insecticides and an attractant for the control of Phloeotribus scarabaeoides, a pest of Olea europea.

Different insecticides have been tested for the control of the olive bark beetle, Phloeotribus scarabaeoides Bern. This scolytid can be managed at two points in its biological cycle: in pruned logs, where it excavates reproduction galleries, or in living trees, after emergence from the logs, where it digs feeding galleries. In mortality laboratory bioassays, the efficiency of organophosphorus insecticides has been ranked as follows: chlorpyrifos + dimethoate < formothion < methidathion. Formothion and methidathion, the two most efficient, were sprayed on olive logs together with a pyrethroid insecticide, deltamethrin, and a formulation which combined an organophosphorus (fenitrothion) and a pyrethroid (cypermethrin) insecticide. Deltamethrin inhibited the excavation of new reproduction galleries and induced a repellent effect on the olive pest. In contrast, none of the organophosphorus insecticides or the combination, fenitrothion + cypermethrin, were able to control the olive bark beetle. In olive trees, deltamethrin controlled this olive pest without showing the repellent effect observed for logs. Ethylene, a plant hormone, has been reported as an attractant for the olive bark beetle. The use of dispensers which released ethylene increased the number of P. scarabaeoides approaching the treated olive trees, thus favouring its use in a lure-and-trap control system.

Animals↗
PubMed1995

Relationship of aquatic natural organic material characteristics to the toxicity of selected insecticides.

Toxicities of the commercial insecticide formulations of azinphos methyl, chlorpyrifos, fenvalerate, and methyl parathion were evaluated in streamwater samples containing natural organic material (NOM) using a modification of a bacterial bioluminescence assay. Toxicity reduction of azinphos methyl was significantly (P < 0.05) correlated with water sample nonvolatile total solids (NVTS) concentration. Toxicity reductions of fenvalerate and of methyl parathion were significantly correlated to E4/E6 absorbance ratio of NOM, and to the proportion of NOM having molecular weight greater than 3700 Da. Toxicity reductions of fenvalerate and methyl parathion had significant correlations to the concentration of NVTS in water samples. Linear regressions of insecticide EC50 values to these NOM characteristics were used to mathematically model the effect of NOM on insecticide toxicity. Insecticide logKOW values were significantly correlated to the slopes of the EC50-total organic carbon (TOC) concentration regression lines. This confirms that the level of association of some nonionic organic contaminants with NOM is a function of the contaminant's water solubility. No single NOM characteristic was the best predictor of toxicity for all insecticides examined. However, these findings suggest that the influence of NOM on toxicity results primarily from differences in TOC content and abundance of larger molecular size NOM.

Animals↗
PubMed2001

Acute toxicity of locust insecticides to two indigenous invertebrates from Sahelian temporary ponds.

During desert locust plagues large amounts of insecticides are used for control operations. Drift from these treatments and accidental overspraying may contaminate small surface waters such as temporary ponds. The present study describes methods for static acute toxicity tests with two abundant organisms that occur in temporary ponds in the African Sahel region: the fairy shrimp Streptocephalus sudanicus Daday (Branchiopoda, Anostraca, Streptocephalidae) and the backswimmer Anisops sardeus Herrich-Schåffer (Hemiptera, Notonectidae). The organisms were captured in the field and 48-h static toxicity tests were conducted in the laboratory. The assays were used to screen the toxicity of 11 formulated synthetic insecticides used in desert locust control and of spores of the mycopesticide Metarhizium anisopliae var. acridum. Most of the synthetic insecticides tested were highly toxic to both organisms (LC50 or EC50<1 mg/L). Exceptions were the toxicity of diflubenzuron to A. sardeus (moderately toxic: 1<LC50<10 mg/L), and that of fenitrothion (moderately toxic) and malathion (slightly toxic: 10<EC50<100 mg/L) to S. sudanicus. M. anisopliae var. acridum was moderately toxic to S. sudanicus and only slightly toxic to A. sardeus. EC50 values of the insecticides for S. sudanicus were not significantly correlated with L(E)C50 values for Daphnia magna collected from the literature. For A. sardeus a significant correlation with D. magna was found, but even in this data set the two species had widely differing sensitivities to some insecticides.

Animals↗
PubMed2001

Role of human GABA(A) receptor beta3 subunit in insecticide toxicity.

The gamma-aminobutyric acid type A (GABA(A)) receptor is the target for the major insecticides alpha-endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [(3)H]ethynylbicycloorthobenzoate ([(3)H]EBOB). This study tests the hypothesis that GABA(A) receptor subunit composition determines the sensitivity and selectivity of insecticide toxicity. Human receptor subtypes were expressed individually (alpha1, alpha6, beta1, beta3, and gamma2) and in combination in insect Sf9 cells. Binding parameters were similar for [(3)H]EBOB in the beta3 homooligomer, alpha1beta3gamma2 heterooligomer, and native brain membranes, but toxicological profiles were very different. Surprisingly, alpha-endosulfan, lindane, and fipronil were all remarkably potent on the recombinant beta3 homooligomeric receptor (IC50 values of 0.5-2.4 nM), whereas they were similar in potency on the alpha1beta3gamma2 subtype (IC50 values of 16-33 nM) and highly selective on the native receptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/beta3 or alpha1beta3gamma2/beta3 was as follows: fipronil > lindane > 19 other insecticides including alpha-endosulfan and picrotoxinin > 4 trioxabicyclooctanes and dithianes (almost nonselective) > tetramethylenedisulfotetramine, 4-chlorophenylsilatrane, or alpha-thujone. Specificity between mammals and insects at the target site (fipronil > lindane > alpha-endosulfan) paralleled that for toxicity. Potency at the native receptor is more predictive for inhibition of GABA-stimulated chloride uptake than that at the beta3 or alpha1beta3gamma2 receptors. Therefore, the beta3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity.

Binding Sites↗
PubMed1975

[Residues of organochlorine insecticides in sediments of a high estuary of Fleuve St-Laurent].

In order to determine the distribution and levels of organochlorine insecticide residues in sediments of the upper estuary of the St. Lawrence River, we have carried out the first analyses in Quebec of these compounds in samples from different parts of this region. The recovery of these residues was conducted by continuous Soxhlet extraction of the samples with 100 ml of a mixture of acetone and n-hexane (50:50). The insecticide-containing extract was cleaned up by column chromatography on activated florisil. Elution with 100 ml of n-hexane and then 150 ml of a mixture of n-hexane and ether (85:15) gave two fractions. The analyses were performed by gas liquid chromatography using an electron-capture detector with a radioactive 63Ni source. In the two series of samplings (June and July 1973), DDT and its metabolites were detected in appreciable amounts at nearly all sampling sites while the other insecticides: alpha-BHC, gamma-BHC, heptachlor and dieldrin, were found only in a few and at very low concentrations: beta-BHC, aldrin and heptachlor epoxide were absent at all sites. In general, the insecticide levels at the various sites show a slight variation in time. However, in some, the concentration of organochlorine insecticides is much lower than in others; in these cases it is presumed that this is due to a greater rate of sedimentation in the absence of current and also to local agriculture and sediment texture.

Canada↗
PubMed1996

Heterogeneity of the glutathione transferase genes encoding enzymes responsible for insecticide degradation in the housefly.

One of the four glutathione-S-transferases (GST) that is overproduced in the insecticide-resistant Cornell-R strain of the housefly (Musca domestica) produces an activity that degrades the insecticide dimethyl parathion and conjugates glutathione to lindane. In earlier work, it was shown that the resistant Cornell-R carries an amplification, probably a duplication, of one or more of its GST loci and that this amplification is directly related to resistance. Using polymerase chain reaction (PCR) amplification with genomic DNA, multiple copies of the gene encoding the parathion-degrading activity (called MdGst-3) were subcloned from both the ancestral, insecticide-susceptible strain BPM and from the insecticide-resistant Cornell-R. In BPM, three different MdGst-3 genes were identified while in Cornell-R, 12 different MdGst-3 sequences were found that, though closely related to ancestral genes, had diverged by a few nucleotides. This diversity in MdGst-3 genomic sequences in Cornell-R is reflected in the expressed sequences, as sampled through a cDNA bank. Population heterozygosity cannot account for these multiple GST genes. We suggest that selection for resistance to insecticides has resulted in not only amplification of the MdGst-3 genes but also in the divergence of sequence between the amplified copies.

Animals↗
PubMed1999

Multiple origins of cyclodiene insecticide resistance in Tribolium castaneum (Coleoptera: Tenebrionidae).

The number of origins of pesticide resistance-associated mutations is important not only to our understanding of the evolution of resistance but also in modeling its spread. Previous studies of amplified esterase genes in a highly dispersive Culex mosquito have suggested that insecticide resistance-associated mutations (specifically a single-gene duplication event) can occur a single time and then spread throughout global populations. In order to provide data for resistance-associated point mutations, which are more typical of pesticide mechanisms as a whole, we studied the number of independent origins of cyclodiene insecticide resistance in the red flour beetle Tribolium castaneum. Target-site insensitivity to cyclodienes is conferred by single point mutations in the gene Resistance to dieldrin (Rdl), which codes for a subunit of a gamma-aminobutyric acid (GABA) receptor. These point mutations are associated with replacements of alanine 302 which render the receptor insensitive to block by the insecticide. We collected 141 strains of Tribolium worldwide and screened them for resistance. Twenty-four strains contained resistant individuals. After homozygosing 23 of these resistance alleles we derived a nucleotide sequence phylogeny of the resistant strains from a 694-bp section of Rdl, encompassing exon 7 (which contains the resistance-associated mutation) and part of a flanking intron. The phylogeny also included six susceptible alleles chosen at random from a range of geographical locations. Resistance alleles fell into six clades and three clades contained both resistant and susceptible alleles. Although statistical analysis provided support at only the 5-6% level, the pattern of variation in resistance alleles is more readily explained by multiple independent origins of resistance than by spread of a single resistance-associated mutation. For example, two resistance alleles differed from two susceptible alleles only by the resistance-associated mutation itself, suggesting that they form the susceptible ancestors and that resistance arose independently in several susceptible backgrounds. This suggests that in Tribolium Rdl, de novo mutations for resistance have arisen independently in several populations. Identical alleles were found in geographically distant regions as well, also implying that some Rdl alleles have been exported in stored grain. These differences from the Culex study may stem both from differences in the population genetics of Tribolium versus that of mosquitoes and differences in mutation rates associated with point mutations versus gene duplication events. The Tribolium data therefore suggest that multiple origins of insecticide resistance (associated with specific point mutations) may be more common than the spread of single events. These findings have implications for the way in which we model the evolution and spread of insecticide resistance genes and also suggest that parallel adaptive substitutions may not be uncommon in phyletic evolution.

Animals↗