Persistence Of Insecticides Research Articles

Estimation of insecticide persistence, biological activity and mosquito resistance to PermaNet® 2 long-lasting insecticidal nets over three to 32 months of use in Ethiopia

BackgroundInformation is needed on the expected durability of insecticidal nets under operational conditions. The persistence of insecticidal efficacy is important to estimate the median serviceable life of nets under field conditions and to plan for net replacement.MethodsDeltamethrin residue levels were evaluated by the proxy method of X-ray fluorescence spectrometry on 189 nets used for three to six months from nine sites, 220 nets used for 14-20 months from 11 sites, and 200 nets used for 26-32 months from ten sites in Ethiopia. A random sample of 16.5-20% of nets from each time period (total 112 of 609 nets) were tested by bioassay with susceptible mosquitoes, and nets used for 14-20 months and 26-32 months were also tested with wild caught mosquitoes.ResultsMean insecticide levels estimated by X-ray fluorescence declined by 25.9% from baseline of 66.2 (SD 14.6) mg/m2 at three to six months to 44.1 (SD 21.2) mg/m2 at 14-20 months and by 30.8% to 41.1 (SD 18.9) mg/m2 at 26-32 months. More than 95% of nets retained greater than 10 mg/m2 of deltamethrin and over 79% had at least 25 mg/m2 at all time periods. By bioassay with susceptible Anopheles, mortality averaged 89.0% on 28 nets tested at three to six months, 93.3% on 44 nets at 14-20 months and 94.1% on 40 nets at 26-32 months. With wild caught mosquitoes, mortality averaged 85.4% (range 79.1 to 91.7%) at 14-20 months but had dropped significantly to 47.2% (39.8 to 54.7%) at 26-32 months.ConclusionsInsecticide residue level, as estimated by X-ray fluorescence, declined by about one third between three and six months and 14-20 months, but remained relatively stable and above minimum requirements thereafter up to 26-32 months. The insecticidal activity of PermaNet® 2.0 long-lasting insecticidal nets in the specified study area may be considered effective to susceptible mosquitoes at least for the duration indicated in this study (32 months). However, results indicated that resistance in the wild population is already rendering nets with optimum insecticide concentrations less effective in practice.

Different tolerance of zooplankton communities to insecticide application depending on the species composition

Natural zooplankton communities are composed of many different species at different trophic levels in the aquatic food web. Several researchers have reported that in mesocosm/enclosure experiments, larger cladocerans tend to be more sensitive to carbamate insecticides than smaller ones (Daphnia > Moina, Diaphanosoma > Bosmina). In contrast, results from individual-level laboratory tests have suggested that large cladoceran species are more tolerant than small species. To clarify this inconsistency, we conducted a microcosm experiment using model zooplankton communities with different species compositions, where animals were exposed to lethal (near to the 24 h LC50, concentration estimated to kill 50% of individuals within 24-h for the small cladoceran Bosmina) and lower, sublethal concentrations of carbaryl. In the experiment, population densities of the small cladocerans (Bosmina and Bosminopsis) decreased subsequent to the applications of chemical, but no impacts were observed on the large cladoceran Daphnia. Our results supported the reports of previous individual level toxicity tests, and indicated that the sensitivity of zooplankton to the insecticide was unchanged by biological interactions but the response of population can be modified by compensation of population through hatching from resting eggs and/or the persistence of insecticide in the systems.

Implications of bio-efficacy and persistence of insecticides when indoor residual spraying and long-lasting insecticide nets are combined for malaria prevention

BackgroundBio-efficacy and residual activity of insecticides used for indoor residual spraying (IRS) and long-lasting insecticide nets (LLINs) were assessed against laboratory-reared and wild populations of the malaria vector, Anopheles arabiensis in south eastern Tanzania. Implications of the findings are examined in the context of potential synergies and redundancies where IRS and LLINs are combined.MethodsBioassays were conducted monthly for six months on three LLIN types (Olyset® PermaNet 2.0®,and Icon Life®) and three IRS treatments (2 g/m2 pirimiphos-methyl, 2 g/m2 DDT and 0.03 g/m2 lambda-cyhalothrin, sprayed on mud walls and palm ceilings of experimental huts). Tests used susceptible laboratory-reared An. arabiensis exposed in cones (nets and IRS) or wire balls (nets only). Susceptibility of wild populations was assessed using WHO diagnostic concentrations and PCR for knock-down resistance (kdr) genes.ResultsIRS treatments killed ≥ 85% of mosquitoes exposed on palm ceilings and ≥ 90% of those exposed on mud walls, but up to 50% of this toxicity decayed within 1–3 months, except for DDT. By 6th month, only 7.5%, 42.5% and 30.0% of mosquitoes died when exposed to ceilings sprayed with pirimiphos-methyl, DDT or lambda-cyhalothrin respectively, while 12.5%, 36.0% and 27.5% died after exposure to mud walls sprayed with the same insecticides. In wire-ball assays, mortality decreased from 98.1% in 1st month to 92.6% in 6th month in tests on PermaNet 2.0®, from 100% to 61.1% on Icon Life® and from 93.2% to 33.3% on Olyset® nets. In cone bioassays, mortality reduced from 92.8% in 1st month to 83.3% in 6th month on PermaNet 2.0®, from 96.9% to 43.80% on Icon Life® and from 85.6% to 14.6% on Olyset®. Wild An. arabiensis were 100% susceptible to DDT, 95.8% to deltamethrin, 90.2% to lambda cyhalothrin and 95.2% susceptible to permethrin. No kdr gene mutations were detected.ConclusionsIn bioassays where sufficient contact with treated surfaces is assured, LLINs and IRS kill high proportions of susceptible An. arabiensis mosquitoes, though these efficacies decay gradually for LLINs and rapidly for IRS. It is, therefore, important to always add intact nets in sprayed houses, guaranteeing protection even after the IRS decays, and to ensure accurate timing, quality control and regular re-spraying in IRS programmes. By contrast, adding IRS in houses with intact LLINs is unlikely to improve protection relative to LLINs alone, since there is no guarantee that unfed vectors would rest long enough on the sprayed surfaces, and because of the rapid IRS decay. However, there is need to clarify these effects using data from observations of free flying mosquitoes in huts. Physiological susceptibility of An. arabiensis in the area remains 100% against DDT, but is slightly reduced against pyrethroids, necessitating caution over possible spread of resistance. The loss of LLIN toxicity, particularly Olyset® nets suggests that protection offered by these nets against An. arabiensis may be primarily due to physical bite prevention rather than insecticidal efficacy.

The susceptibility of five African Anopheles species to Anabaena PCC 7120 expressing Bacillus thuringiensis subsp. israelensis mosquitocidal cry genes.

BackgroundMalaria, one of the leading causes of death in Africa, is transmitted by the bite of an infected female Anopheles mosquito. Problems associated with the development of resistance to chemical insecticides and concerns about the non-target effects and persistence of chemical insecticides have prompted the development of environmentally friendly mosquito control agents. The aim of this study was to evaluate the larvicidal activity of a genetically engineered cyanobacterium, Anabaena PCC 7120#11, against five African Anopheles species in laboratory bioassays.FindingsThere were significant differences in the susceptibility of the anopheline species to PCC 7120#11. The ranking of the larvicidal activity of PCC 7120#11 against species in the An. gambiae complex was: An. merus <An. arabiensis <An. gambiae <An. quadriannulatus, where < indicates a statistically lower LC50. The LC50 of PCC 7120#11 against the important malaria vectors An. gambiae and An. arabiensis was 12.3 × 105 cells/ml and 8.10 × 105 cells/ml, respectively. PCC 7120#11 was not effective against An. funestus, with less than 50% mortality obtained at concentrations as high as 3.20 × 107 cells/ml.ConclusionsPCC 7120#11 exhibited good larvicidal activity against larvae of the An. gambiae complex, but relatively weak larvicidal activity against An. funestus. The study has highlighted the importance of evaluating a novel mosquitocidal agent against a range of malaria vectors so as to obtain a clear understanding of the agent’s spectrum of activity and potential as a vector control agent.

Persistence of insecticides in ready-mix formulations and their efficacy against Lipaphis erysimi (Kalt) in cauliflower

Persistence of cypermethrin, deltamethrin, profenofos, and triazophos in cauliflower curd was studied, following application of two premix formulations viz: Roket 44EC (profenofos 40% + cypermethrin 4%) and Anaconda Plus 36EC (triazophos 35% + deltamethrin 1%) at recommended (1.0L ha(-1)) and double doses (2.0L ha(-1)). In the case of Roket 44EC, residues of cypermethrin dissipated with the half-life values of 1.5-2.1days, whereas residues of profenofos dissipated with the half-life of 2.9-3.3days on cauliflower curd. In the case of Anaconda, residues of triazophos and deltamethrin dissipated from curd with the half-life values of 2.6-3.0 and 2.2-2.6days, respectively. Both the combination mix significantly reduced the aphid population up to 14days after spray and increased the yield by 155-160% over control. Anaconda (2.0L ha(-1)) treated plots yielded the highest number of marketable curds. Based on risk assessment analysis, safe waiting period of 3 and 5days has been suggested for Roket 44EC and Anaconda Plus 36EC, respectively, in cauliflower at recommended dose of application.

Persistence and residue activity of deltamethrin on indoor residual spraying surfaces against malaria vectors in southeastern Iran

Objective To evaluate the efficacy of deltamethrin and find a relation between persistence and residue of this insecticide on the prevalent surfaces against malaria vectors in southeastern Iran. Methods After indoor residual spraying on prevalent surfaces in studied areas (plaster and mud as absorbent surfaces, wood as non absorbent surface and filter paper as control) for malaria control, conical tests as a bioassay method and chromatographic method as an analytical method were used for evolution of persistence and residue of deltamethrin insecticide. Results were investigated statistically by ANOVA and Tukey-HSD tests for determining relations or differences between residue and persistence of deltamethrin. Results According to the results, there was no significant difference between mortality rates from bioassay tests on different surfaces, and deltamethrin kept its utility to malaria vector control until 120 days after indoor residual spraying on these surfaces. In the case of residue, there was no significant relation between residue amounts and mortality rates on different surfaces, whereas this relation existed between residual amounts on filter papers and mortality rates from bioassay tests. Conclusions This study shows that measurement of residue in filter papers is a suitable tool for evolution and dictum of efficiency of deltamethrin insecticide in indoor residual spraying for malaria control.

Soil microbial degradation of neonicotinoid insecticides imidacloprid, acetamiprid, thiacloprid and imidaclothiz and its effect on the persistence of bioefficacy against horsebean aphid Aphis craccivora Koch after soil application

The neonicotinoids imidacloprid, imidaclothiz, acetamiprid and thiacloprid consist of similar structural substituents but differ considerably with respect to soil use. Therefore, the effects of soil microbial activity on the degradation and bioefficacy persistence of the four neonicotinoids were evaluated. In unsterilised soils, 94.0% of acetamiprid and 98.8% of thiacloprid were degraded within 15 days, while only 22.5% of imidacloprid and 25.1% of imidaclothiz were degraded over a longer period of 25 days. In contrast, in sterilised soils, the degradation rates of acetamiprid and thiacloprid were respectively only 21.4% and 27.6%, whereas the degradation rates of imidaclothiz and imidacloprid were respectively 9.0% and almost 0% within 25 days. The degradation products of imidacloprid and imidaclothiz were identified as olefin, nitroso or guanidine metabolites, the degradation product of thiacloprid was identified as an amide metabolite and no degradation product of acetamiprid was detected. A bioefficacy assay revealed that the bioefficacy and persistence of imidacloprid, imidaclothiz, acetamiprid and thiacloprid against horsebean aphid A. craccivora were related to their degradation rate and the bioefficacy of their degradation products in soil. Soil microbial activity played a key role in the bioefficacy persistence of neonicotinoid insecticides and therefore significantly affected their technical profile after soil application.

Field efficacy of wintertime insecticide applications against greenbugs, Schizaphis graminum (Rondani) (Hemiptera: Aphididae) on winter wheat ( Triticumaestivum L.)

Winter wheat producers in Oklahoma often combine an insecticide with a top-dress application of nitrogen during late fall and winter to control existing greenbug populations. We evaluated the efficacy of three classes of insecticides applied in cold weather field conditions ranging from −13.3 to 28.9 °C from 2002 to 2004 for greenbug control in winter wheat in replicated small plots. Insecticides provided control ranging from 0% to 98%, but generally responded similarly to temperature. Percent control of greenbugs resulting from applications of dimethoate, chlorpyrifos and λ-cyhalothrin was correlated with maximum, minimum and average daily temperatures that occurred two days post-treatment. While percent control was unique for each insecticide, a combined regression of percent control against average daily temperature predicted that a 95% level of control would be achieved when post-treatment temperatures exceeded 13.6 °C. Rainfall that occurred post-treatment affected the efficacy of dimethoate but did not affect the other insecticides. Insecticide persistence was extended under cooler temperatures, allowing acceptable control 14 days post-treatment if temperatures warmed. The decision to apply insecticides should be made based on existing treatment thresholds for greenbug, but if treatment with an insecticide is warranted, winter wheat producers can expect reasonable control of greenbugs from insecticides applied during the winter, as long as post-treatment temperatures exceed 13 °C within the following 14 days.

English

The maize stem borer Chilo partellus (swinhoe) is one of the most destructive pests of maize. This species is most important at altitudes below 1500 meters above sea level .It's magnitude of damage ranges from 26.7 to 80.4. In young plants the shoot can be killed, causing a dead heart. In older plants the upper part of the stem usually dies as a result of the boring of the caterpillars. Several contact and systematic insecticides are being used for its managemen. Therefore, it necessitated evaluating the relative persistence of insecticides and neem based formulations commonly used against this borer pest for devising its economically effective management programme.

A long‐lasting topical deltamethrin treatment to protect dogs against visceral leishmaniasis

To develop long-lasting, topical pour-on insecticides for dogs to control zoonotic visceral leishmaniasis, two deltamethrin-based formulations (emulsifiable concentrate [EC] and suspension concentrate [SC]) were tested for their efficacy against the phlebotomine sandfly Lutzomyia longipalpis Lutz & Neiva (Diptera: Psychodidae), vector of Leishmania infantum Nicolle (Kinetoplastida: Trypanosomatidae). The entomological outcomes tested were anti-feeding effect (proportion of female sandflies unfed), lethal effect (24-h female sandfly mortality) and these two effects combined, and the insecticide persistence time at 50% (residual activity, RA50) and 80% (RA80) efficacy. On initial application, the proportions of female flies that demonstrated anti-feeding activity or were killed were similar for both formulations, at 0.93 (95% confidence interval [CI] 0.856-0.977) vs. 0.81 (95% CI 0.763-0.858) (anti-feeding) and 0.86 (95% CI 0.787-0.920) vs. 0.76 (95% CI 0.698-0.817) (24-h mortality) for EC and SC, respectively. The RA(50) rates for anti-feeding and mortality caused by the EC formulation were 4.7 months (95% CI 4.18-5.84) and 2.5 months (95% CI 2.25-2.90), respectively, compared with 1.1 months (95% CI 0.96-1.15) and 0.6 months (95% CI 0.50-0.61), respectively, for the SC formulation. The RA(50) for the combined anti-feeding and mortality effects of EC was 5.2 months (95% CI 4.73-5.96), compared with only 0.9 months (95% CI 0.85-1.00) for the SC formulation. The four- to six-fold superior residual activity of the EC formulation was attributed to the addition of a solvent-soluble resin in the formulation which improved fur adhesion and acted as a reservoir for the slow release of the active ingredient. These results identify the potential of such a low-cost formulation to reduce the inter-intervention interval to 5-6 months, similar to that recommended for deltamethrin-impregnated dog collars or for re-impregnation of conventional bednets, both of which are currently used to combat Leishmania transmission. Finally, a novel bioassay was developed in which sandflies were exposed to fur from treated dogs, revealing no detectable tolerance (24-h mortality) in wild-caught sandflies to the insecticide formulations up to 8 months after the initiation of communitywide application of the insecticides to dogs.

The effect of three environmental conditions on the fitness of cytochrome P450 monooxygenase-mediated permethrin resistance in Culex pipiens quinquefasciatus.

BackgroundThe evolution of insecticide resistance and persistence of resistance phenotypes are influenced by the fitness of resistance alleles in the absence of insecticide pressure. Experimental determination of fitness is difficult, but fitness can be inferred by measuring changes in allele frequencies in appropriate environments. We conducted allele competition experiments by crossing two highly related strains of Culex pipiens quinquefasciatus mosquitoes. One strain (ISOP450) was permethrin resistant (due to P450-mediated detoxification) and one was a susceptible strain. Allele and genotype frequencies were examined for 12 generations under three environmental conditions: standard laboratory, temephos exposure (an insecticide to which the P450 detoxification mechanism in ISOP450 confers no resistance and which is commonly used in mosquito control programs) and cold temperature stress (mimics the colder temperatures within the habitat of this mosquito).ResultsA fitness cost was inferred for the P450 mechanism in the standard laboratory environment. A greater cost was associated with the temephos exposed environment, suggesting the temephos placed an additional stress on the P450 resistant mosquitoes. No observed cost was associated with the P450 resistance locus in the cold temperature environment, but there was a significant heterozygote advantage. In all environments the fitness of the resistant homozygotes was the lowest.ConclusionThe cytochrome P450-mediated permethrin detoxification resistance in Cx. p. quinquefasciatus can have an associated fitness cost in the absence of permethrin, relative to a susceptible allele. The strength of the cost varies depending on the environmental conditions. P450-mediated resistance is expected to decrease over time if the permethrin application is relaxed and to decrease at an even faster rate if permethrin is replaced with temephos. Additionally, these results indicate that a P450 resistance allele can persist (especially in heterozygotes) in colder temperatures and could potentially be carried into the Culex pipiens hybrid zone.

Initial activity and persistence of insecticides for the control of bollworms (Lepidoptera: Noctuidae) in cotton crops

We evaluated six insecticides for their initial activity against the cotton bollworms Helicoverpa armigera (Hübner), Diparopsis watersi (Rotschild), and Earias spp., in sub-Saharan Africa (Cameroon). Residual activity and effect of simulated rainfall were also assessed in the case of H. armigera. Bioassays were conducted by transferring larvae on leaf discs collected from sprayed plots. Thiodicarb, endosulfan, and to a lesser extent emamectin-benzoate and indoxacarb had high initial activity against H. armigera, regardless of larval instars. Spinosad and cypermethrin–profenofos (CP) mix were effective at controlling larvae of first and second instars but not larvae of third to fifth instars. All tested insecticides effectively controlled Earias larvae (87–98% mortality). Regarding D. watersi, indoxacarb and endosulfan (77 and 82% mortality respectively) were less effective than spinosad and thiodicarb (95–99% mortality). Persistence was quantified by the duration after which an insecticide kills less than 50% of H. armigera neonates. Rain had a significant detrimental effect on insecticide persistence, except in the case of thiodicarb and emamectin-benzoate. In rainy conditions, thiodicarb (17.2 d) was the most persistent insecticide, followed by emamectin-benzoate and spinosad (10.6 and 8.9 d), and endosulfan, indoxacarb, and CP (2.7–5.2 d). Indoxacarb should be recommended for controlling sporadic outbreaks of H. armigera due to its high efficacy and low persistence, while CP should be used to control D. watersi infestations. More persistent insecticides such as spinosad and emamectin-benzoate should be recommended to control continuous and mixed-species populations of bollworms in the field.

Insecticide dissipation from soil and plant surfaces in tropical horticulture of southern Benin, West Africa

In Sub-Saharan Africa, horticulture provides livelihood opportunities for millions of people, especially in urban and peri-urban areas. Although the vegetable agroecosystems are often characterized by intensive pesticide use, risks resulting therefrom are largely unknown under tropical horticultural conditions. The objective of this study therefore was to study the fate of pesticides in two representative horticultural soils (Acrisol and Arenosol) and plants (Solanum macrocarpon L.) after field application and thus to gain first insight on environmental persistence and dispersion of typical insecticides used in vegetable horticulture in Benin, West Africa. On plant surfaces, dissipation was rapid with half lives ranging from 2 to 87 h (alpha-endosulfan < beta-endosulfan < deltamethrin). Soil dissipation was considerably slower than dissipation from plant surfaces with half-lives ranging from 3 (diazinon) to 74 d (total endosulfan), but persistence of pesticides in soil was still reduced compared to temperate climates. Nevertheless, for deltamethrin and endosulfan, a tendency for mid-term accumulation in soil upon repeated applications was observed. The soil and plant surface concentrations of the metabolite endosulfan sulfate increased during the entire trial period, indicating that this compound is a potential long-term pollutant even in tropical environments.

Persistence of New Insecticides and Their Efficacy Against Insect Pests of Okra

Persistence and efficacy of bifenthrin (25 and 50 g ai ha(-1)), fipronil (50 and 100 g ai ha(-1)) and indoxacarb (70 and 140 g ai ha(-1)) has been studied in okra fruits. The initial deposits varied from 0.259-0.382 microg g(-1) at low and 0.461-0.688 microg g(-1) at high rate of application. The residues persisted upto 10 days with half-life of 1.32-1.58 days for bifenthrin, 0.65-1.12 days for fipronil and 0.58-1.02 days for indoxacarb. Based on ADI, the suggested waiting period was 1 day for bifenthrin and indoxacarb and 3 days for fipronil. All the insecticides were found effective against leafhopper and shoot and fruit borer.

A Probabilistic Assessment of Malathion Spray Exposures (PAMSE) for the Prevention of West Nile Virus

P-553 Introduction: To assist public health decision-makers manage the threat of West Nile virus (WNV) in British Columbia, we developed a probabilistic model quantifying the exposure of different individuals in a community to malathion, an insecticide used to kill the adult mosquitoes that carry WNV. Methods: The Probabilistic Assessment of Malathion Spray Exposures (PAMSE) model was developed to capture parametric and structural uncertainties in determinants of exposure. Probabilistic variables and distributions were defined and propagated throughout model equations to calculate probability distribution of doses received by exposure pathway and age. PAMSE is spatially explicit and dynamic. A variety of population characteristics such as age, weight, height, proportion of time spent on outdoor activities, and dietary intake are accounted for. Uncertainties and variation in application method, insecticide dispersion, insecticide persistence in the environment, and exposure pathways were estimated and propagated in calculations. PAMSE was developed in Analytica, a software modeling environment with graphical representation of parameter dependencies and model structures. PAMSE framework can be used to assess a variety of circumstances to provide quantitative insights for informed decisions in pesticide control measures to limit vector-borne diseases. The model calculates probability distributions of total dose over a 40-day simulation, and the percentage of population whose exposure would exceed the target Margin of Exposure. Results: Children under 6 are at the greatest risk for exceeding malathion's Margin of Exposure. Children under 2 are at the greatest risk of inhalation exposure, and children 2–6 are also at risk of dermal exposure. Discussion: PAMSE model has used information provided by risk assessments in a more realistic framework reflecting multiple spray swaths, range of half-lives, and variety of population demographics and behaviors. PAMSE shows that malathion spraying in compliance with Canadian guidelines can still leave more than 5% of children under the age of 6 at risk of excessive exposure to malathion. This pattern of excess exposure has not been noted in the modeling literature, likely due to prior quantifications not including an extensive set of spatial and population variabilities and uncertainties as represented in PAMSE. Finally, the model revealed a simple strategy to reduce dermal and oral exposure. Spraying malathion in moist micro-environments (such as watered lawns and gardens) significantly increases its half-life. Public service messages aimed at reducing moist environmental conditions prior to spray events may help reduce personal exposures.

Influence and persistence of phorate and carbofuran insecticides on microorganisms in rice field

An experiment was conducted in microplots (4 m × 4 m) with two insecticides, phorate and carbofuran at rates of 1.5 and 1.0 kg a.i. ha −1 respectively, to investigate its effect on the population and distribution of bacteria, actinomycetes and fungi as well as the persistence of the insecticidal residues in rhizosphere soils of rice ( Oryza sativa L., variety IR-50). Application of the insecticides stimulated the population of bacteria, actinomycetes and fungi in the rhizosphere soils, and the stimulation was more pronounced with phorate as compared to carbofuran. Both the insecticides did not have marked effect on the numbers of Streptomyces and Nocardia in the rhizosphere soils. However, the growth of Bacillus, Escherichia, Flavobacterium, Micromonospora, Penicillium, Aspergillus and Trichoderma with phorate and that of Bacillus, Corynebacterium, Flavobacterium, Aspergillus and Phytophthora with carbofuran were increased. On the other hand, the numbers of Staphylococcus, Micrococcus, Fusarium, Humicola and Rhizopus under phorate and Pseudomonas, Staphylococcus, Micrococcus, Klebsiella, Fusarium, Humicola and Rhizopus under carbofuran were inhibited. Both the insecticides persisted in the rhizosphere soil for a short period of time and the rate of dissipation of carbofuran was higher than that of phorate in the soil depicting the half-life ( T 1/2) 9.1 and 10.4 days, respectively.

Residual toxicities and persistence of organophosphorus insecticides mixed with carbaryl as structural treatments against three liposcelidid psocid species (Psocoptera: Liposcelididae) infesting stored grain

Four organophosphorus insecticides, azamethiphos, fenitrothion, chlorpyrifos-methyl, and pirimiphos-methyl were each mixed with carbaryl and evaluated as structural treatments on concrete and galvanized steel panels (0.3×0.3 m) against adults of three liposcelidid psocids: Liposcelis bostrychophila Badonnel, L. entomophila (Enderlein), and L. paeta Pearman. Residual toxicities and persistence of these insecticide mixtures were assessed at 30±1°C, 70±2% r.h., and a photoperiod of 12:12(L:D) h from 1 day after treatment (0 week) and at weeks 1, 2, 4, 6, and 8, and then every 4 weeks up to week 40. Mortality was recorded at exposure periods of 6 h and then every 24 h until end-point was achieved. Significant enhancement in residual toxicities was observed in azamethiphos plus carbaryl against L. entomophila, and chlorpyrifos-methyl plus carbaryl and pirimiphos-methyl plus carbaryl against all three species on steel surfaces, compared with the individual effectiveness of these insecticides reported earlier. We conclude that combining carbaryl with either azamethiphos, chlorpyrifos-methyl, or pirimiphos-methyl as a structural treatment will provide long-term protection (up to 40 weeks) against infestation by all three psocid species on steel storage structures, whilst carbaryl with fenitrothion will give shorter protection (up to 8 weeks). On concrete structures, however, only azamethiphos plus carbaryl will provide long-term protection against L. bostrychophila (up to 28 weeks) and L. paeta (up to 16 weeks). None of the four insecticide mixtures studied would provide long-term protection against an L. entomophila infestation on concrete surfaces.

Persistence and activity towards Sitophilus zeamais (Coleoptera: Curculionidae) of pirimiphos-methyl sprayed at different temperatures on maize

The air temperature in storage units in tropical areas frequently exceeds 50°C during the warmest periods of the day. Since protectant insecticides are sprayed on grains under these conditions, such high temperatures may interfere with the insecticidal activity. To assess this possibility we sprayed maize grains with pirimiphos-methyl 500 EC (0.8 ml c.p./l and 1.5 l/t) at different temperatures (25°C, 30°C, 35°C, 40°C, 45°C, and 50°C) and 55% r.h. The grains were then maintained at 27±1°C and 55±5% r.h. throughout the experiment which lasted 90 days. Residues of pirimiphos-methyl on the sprayed grains were analyzed every 30 days. Grain samples were assessed every 15 days after the insecticide spraying for their effect on the maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae). Insect mortality was evaluated after 24 and 48 h of exposure to the treated grain. The level of pirimiphos-methyl residue on maize grains decreased with an increase in storage period and air temperature during the insecticide spraying (varying from 11.0±0.1 to 1.1±0.1 ppm, corresponding to the residue levels at 25°C on the day of the spraying and after spraying at 50°C and 90 days of storage). The same trend was observed for mortality of the maize weevil, which dropped from 95.4±13.3% to 2.5±2.5% after 90 days storage after insecticide spraying at 50°C. These results indicate that temperature at spraying can affect insecticide persistence and activity during storage.

Effects of fertilizer on insecticides adsorption and biodegradation in crop soils.

Recent organic fertilizer treatments (cow manure, pig slurry, composts, or green manure) simultaneously increase insecticide adsorption onto soil and the insecticide soil persistence, indicating a mechanism of slow release of insecticide into soil by the organic matter. This occurred in sugar beet crops with aldicarb, thiofanox and imidacloprid; also, in leek, cauliflower and brussels sprouts crops with chlorpyrifos and chlorfenvinphos. In contrast, organic fertilizer treatments applied once or repeatedly in the past, have no significant influence on adsorption or persistence of insecticides; the same is observed for the old soil organic matter, when its soil concentrations change in limited ranges

Further comparison of the persistence in clay loam of single and repeated annual applications of some granular insecticides

Abstract Granular formulations of chlorethoxyphos, DOWCO 429X, fensulfothion, phorate, tefluthrin, terbufos and trimethacarb were applied in incorporated bands to duplicate 2 m2 field plots of clay loam. Insecticide concentrations were determined in samples taken from treated bands at 0, 1, 2, 3, 4, 6, 8, 10, 12, 16 and 20 wk. Plots were thoroughly cultivated the following spring and the insecticide applications and concentration measurements were repeated in a second and third year. In the third year, initial applications were also made to plots containing previously untreated soils. To measure variability in persistence not associated with previously treated soil, terbufos was initially applied to duplicate plots each year. Insecticide‐generated enhanced microbial degradative activity in the soils was assessed periodically in the laboratory. Insecticide persistence in the field plots was evaluated on the basis of the 0–8 wk first order half lives and average concentration of residual insecticide measured at 8, 10 and 12 wk. Insecticide persistence decreased in the following order based on the average half life or residual concentration for all year/treatment combinations: tefluthrin >> trimethacarb = DOWCO 429X > terbufos > chlorethoxyphos = fensulfothion > phorate. Enhanced microbial degradative activity for chlorethoxyphos, DOWCO 429X, fensulfothion and trimethacarb and the sulfoxide/sulfone metabolites of terbufos, phorate and fensulfothion was identified in the soils within 6 wk of initial treatment. Differences in the persistence of the insecticides applied to previously untreated and previously‐treated soils did not correlate well with the presence of enhanced microbial degradative activity. Results are compared with those of a previous study and the possible role of enhanced soil microbial degradative activity on granular formulations in commercial agriculture is discussed.