Influence of parasitism by Chelonus insularis Cresson (Hymenoptera: Braconidae) on the susceptibility of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) to insecticides

The egg-larval parasitoid Chelonus insularis Cresson is a key parasitoid of the fall armyworm Spodoptera frugiperda (J. E. Smith) during the second field generation in the upper Magdalena River Basin, Colombia. In spite of selection pressure, the higher susceptibility of the second generation fall armyworm larvae to insecticides, compared with that of the first generation, suggests that the parasitism may be responsible for the apparent difference in susceptibility. Parasitized and non-parasitized 2nd-instar larvae of the fall armyworm were tested for susceptibility to chlorpyriphos, methomyl, cypermethrin, and Bacillus thuringiensis in the laboratory, using the leaf dip test. Parasitized larvae were up to 3.93 times more susceptible to chlorpyriphos, 3.71 times to methomyl, and 14.11 times to cypermethrin than non-parasitized larvae. The least effect of parasitism on susceptibility was found for B. thuringiensis. We discuss the negative influence of synthetic insecticide on the parasitoid population dynamics and its impact on insecticide resistance.

El parasitoide huevo-larva Chelonus insularis Cresson es un parasitoide clave del cogollero del maíz, Spodoptera frugiperda (J.E. Smith) durante la segunda generación de campo en el Valle Superior del Río Magdalena de Colombia. A pesar de la presión de selección se encuentra una susceptibilidad mayor a insecticidas en la segunda generación, comparado con la primera, siendo para esto la única diferencia aparente el parasitismo. Por esta razón, la susceptibilidad a clorpirifos, metomyl, cipermetrina y Bacillus thuringiensis de larvas parasitadas y no parasitadas del segundo instar del cogollero fueron evaluadas, utilizando el test de inmersión foliar. Las larvas parasitadas fueron 3,93 veces más suscptibles a clorpirifos, 3,71 veces más a metomyl y 14,11 veces más a cipermetrina que las no parasitadas. El menor efecto del parasitismo sobre la susceptibilidad se encontró con B. thuringiensis. Se discute la influencia negativa de las aplicaciones de insecticidas sintéticos sobre la dinámica poblacional del parasitoide, lo mismo que sus implicaciones en estudios de resistencia.

Chlorpyrifos induced alterations in levels of thiobarbituric acid reactive substances and glutathione in rat brain.

Chlorpyrifos, O,O'-diethyl-O-3,5,6-trichloro-2-pyridyl phosphorothionate, exposure in rats caused significant inhibition of acetylcholinesterae activity in different regions of brain and generated oxidative stress as evidenced by increase in the level of thiobarbituric acid reactive substances and decrease in the ratio of reduced to oxidized glutathione in all the three regions of brain. Malondialdehyde level was increased significantly in all regions of brain and the increase was dose dependent. Mid brain showed highest level of lipid peroxidation.

Insecticide cross-resistance spectra and underlying resistance mechanisms of Sri Lankan anopheline vectors of malaria.

Present status of insecticide resistance was investigated in two major vectors of malaria; An. culicifacies and An. subpictus, collected from a high malaria transmission area in Sri Lanka during 1996/1998. Adult and larval bioassays were carried out to obtain log-probit mortality lines for malathion, propoxur, permethrin and chlorpyrifos. Respective LD50 values were 4.45%, 0.002%, 0.16% and 0.001% for An. culicifacies and 0.66%, 0.004%, 0.004% and 0.04% for An. subpictus. Adults were also tested for WHO standard discriminating dosages of malathion, propoxur, permethrin, DDT, cypermethrin, deltamethlin and lambda cyhalothrin. Both populations were highly resistant to DDT. An. culicifacies was more resistant to malathion and An. subpictus was more resistant to chlorpyrifos. About 25% of both populations were resistant to permethrin. An. culicifacies was susceptible to propoxur, deltamethrin and lambda cyhalothrin and An. subpictus to cypermethrin and lambda cyhalothrin. Adult mosquitos were individually tested for their insecticide detoxifying enzyme activities and altered target-site, acetylcholinesterase. High general esterase activity indicated the presence of amplified esterase genes in both populations. Native gel electrophoresis resolved one elevated esterase isoenzyme, with high affinity to organophosphates, from each species. Malathion carboxylesterase mechanism was present in both populations. Higher glutathione-S-transferase activity was marked in An. subpictus. Synergistic studies showed the possible involvement of monooxygenases in resistance in both species. Acetylcholinesterase activity of approximately 80% of both populations was not inhibited by a standard dosage of propoxur. Low resistance to carbamates shows that the impact of agricultural pesticides is not significant in the development of resistance especially in An. culicifaies. Pyrethroids, other than permethrin, can be successfully used in vector control programs. Carbamates will be an alternative.

comparative study of inhibition of brain and plasma cholinesterase and hepatic and plasma aliesterases in young and adult rats following exposures to three organophosphorothionate insecticides

Young rats are more sensitive than adults to a variety of Organophosphorothionate insecticides [OPS], compounds which act in vivo by inhibition of Cholinesterase and aliesterases. Little is known, however, regarding age-related differences in biochemical responses to these toxicants. The time course of Cholinesterase and aliesterases inhibit and recovery in different tissues were compared in young [14 days of age] and adult [90-100 days of -age] rats after treatment with high sublethal intraperitoneal dosages of parathion, methyl parathion or chlorpyrifos. Young rats were more sensitive than adult in all cases [high sublethal doses for parathion, methyl parathion and chlorpyrifos; young = 0.5, 5 and 10 mg/kg, i.p.; adult = 4, 12 and 60 mg/kg, i.p... respectively]. In general, the maximal inhibition of brain regions and plasma Cholinesterase activity was not immediate with parathion and chlorpyrifos, in young and adult rats, reflecting the time required for bioactivation of the phosphorothionates as well as the effectiveness of the aliesterases to inactive much of the hepatically generated oxons. In contrast, brain regions and plasma Cholinesterase activities were rapidly inhibited following administration of methyl parathion in both age groups reflecting the low sensitivity of the aliesterases to methyl paraoxon. In general, maximal plasma and brain regions Cholinesterase inhibition was similar [greater than 80 percent] in both age groups but Cholinesterase activity recovered faster in young rats. Aliesterases were inhibited to a greater extent than acetyl cholinesterase at each sampling time with parathion and chlorpyrifos in young and adult rats where the reverse was true with methyl parathion .The very prolonged inhibition of esterase activities following chlorpyrifos treatment probably results from its substantially greater lipophilicity compared to the other compouritls, which would allow it to be stored and released for gradual bioactivation The data reported indicate that young rats are more sensitive to sublethal dosages from these compounds and that high sublethal doses exposure produce extensive plasma and different brain regions cholinesterase and plasma and liver aliesterases inhibition in both age groups. Significant inhibitor- related and age-related differences in the duration of cholinesterase and aliesterases inhibition can ensue, however, following such Organophosphorothionate insecticides exposures. Additionally, under defined experimental conditions plasma cholinesterase inhibition may be a useful quantitative index for the degree of brain cholinesterase inhibition following organophosphorous exposures