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Spodoptera exigua (Hubner), is a polyphagous pest of vegetables and field crops. A field population of S. exigua from Lodhran was found resistant to conventional insecticides. It gave 65, 66, 92, 73, 34 and 29-fold resistance to deltamethrin, Cypermethrin, chlorpyrifos, profenofos, abamectin and spinosad respectively compared with Lab-PK susceptible population. Field population was divided into two sub-populations. One was left unselected and the second (Del-SEL) selected for five generations at the dose equal to LC50 Bioassays at G5 for Del-SEL strain gave resistance ratios of 976, 421, 118, 30, 15 and 17-fold for deltamethrin, Cypermethrin, chlorpyrifos, profenofos, abamectin and spinosad, respectively compared with Lab-PK. Resistance was found stable for all the insecticides in the field population but it was more stable in Del-SEL strain for deltamethrin, Cypermethrin and chlorpyrifos than profenofos, abamectin and spinosad when reared without exposing to deltamethrin (G5 - G10). It indicated that cross resistance occurred between deltamethrin, Cypermethrin and chlorpyrifos. There was no effect of delta selection on the toxicity of three insecticides i.e., profenofos, abamectin and spinosad, which indicated lack of cross resistance with deltamethrin. Deltamethrin resistance in Del-SEL strain of S. exigua was suppressed with the synergists such as piperonyl butoxide (PBO) and .S.S.S'-tributylphosphorotrithioate (DEF), suggesting the involvement of monooxygenase and esterase in the development of resistance in S. exigua. Due to the stability and cross resistance with other insecticides deltamethrin should be replaced with insecticides having different mode of action possibly with new chemistries in integration with other techniques like catch and kill method with light traps, altering cropping pattern i.e., rotation of host plants with non host plants etc. [ABSTRACT FROM AUTHOR]

SfT6 has been identified in a subtracted cDNA library of Spodoptera frugiperda larval midgut transcripts as a serine-protease gene downregulated within 24 h of intoxication with Bacillus thuringiensis Cry1Ca1 protein. In the present study, the specific role of SfT6 during Cry1Ca1 intoxication was investigated by RT-PCR and in vivo RNA interference. Quantitative real-time RT-PCR analysis showed SfT6 mRNA levels in the midgut tissue were significantly reduced after injecting or feeding 4th-instar larvae with specific long-size dsRNA. Gut juice-mediated in vitro protoxin activation and susceptibility for Cry1Ca1 were investigated in Sft6-knockdown larvae and compared with control treated with nonspecific dsRNA. Our results demonstrate SfT6 plays a determinant role in Cry1Ca1 toxicity against S. frugiperda since a decreased expression caused a reduced protoxin activation by larval gut juice and reduced susceptibility of insects to toxin in bioassays. We propose SfT6 downregulation occurring at the early stages of Cry1Ca1 intoxication is part of a complex and multifaceted defensive mechanism triggered in the insect gut to withstand B. thuringiensis pathogenesis. [ABSTRACT FROM AUTHOR]

The mode of action of the entomopathogenic bacterium Bacillus thuringiensis ( Bt) remains a matter of debate. Recent reports have claimed that aseptic lepidopteran hosts were not susceptible to Bt and that inoculation with mid-gut bacteria restores pathogenicity. These claims are controversial because larvae were rendered aseptic by consuming antibiotics, although the effect of these antibiotics on Bt was not examined. We tested the generality of the mid-gut bacteria hypothesis in the diamondback moth, Plutella xylostella using properly controlled experiments that investigated the effect of antibiotic consumption and absence of gut microbiota separately. We found that purified Bt toxin and spore/toxin mixtures were fully pathogenic to larvae reared aseptically. Persistence of antibiotics in larval tissues was implicated in reducing host mortality because larval consumption of the antibiotic rifampicin reduced the pathogenicity of rifampicin-sensitive Bt strains but not rifampicin-resistant strains. Inoculating larvae with Enterobacter sp. Mn2 reduced the mortality of larvae feeding on Bt HD-1 and the presence of a culturable gut microbiota also reduced the pathogenicity of the Bt toxin Cry1Ac, in agreement with other studies indicating that an intestinal microbiota can protect taxonomically diverse hosts from pathogen attack. As ingestion of antibiotics suppresses host mortality the vegetative growth of Bt in the host must be important for its pathogenicity. Furthermore, claims that aseptic larvae are not susceptible to Bt must be supported by experiments that control for the effect of administering antibiotics. [ABSTRACT FROM AUTHOR]

1 Foliar nitrogen concentration, which can be manipulated in crop plants by fertilizer supply, has long been recognized as a major factor in phytophagous insect abundance and performance. More recently, the type of fertilizer supplied has been shown to influence the abundance of some herbivore species. The diamondback moth Plutella xylostella is a global pest of Brassica crops. Although it has been the subject of numerous studies on host-plant resistance and pest control, few studies have addressed the effect of abiotic factors, such as nutrient supply, on its performance and behaviour. 2 We assessed oviposition preference, larval feeding preference and larval performance of P. xylostella on two cultivars of Brassica oleracea. Plants were grown using two fertilizer types, John Innes fertilizer and an organic animal manure, at high and low concentrations. 3 Plutella xylostella laid more eggs on cultivar Derby Day than Drago. Derby Day was also the cultivar on which larval performance was maximized. However, differences in larval performance between cultivars were only found when plants were grown in compost with John Innes fertilizer, and not when fertilized with animal manure. 4 Foliar nitrogen concentration was greater in plants grown in high fertilizer treatments but did not differ between cultivars. The concentrations of three glucosinolate compounds (glucoiberin, sinigrin and glucobrassicin) were greater in the high fertilizer treatments. Glucosinolate concentrations were higher in the Drago than the Derby Day cultivar. 5 These results are discussed in relation to the preference-performance hypothesis, and the assessment of plant resistance differences between cultivars using different types of fertilizer. [ABSTRACT FROM AUTHOR]

The article reports on the cross-resistance between a Bacillus thuringiensis Berliner (Bt) crystal (Cry) toxin and a non-Bt insecticides in diamondback moth. The selection of a subpopulation of a Plutella xylostella SERD4 population with pyrethroid deltamethrin and a second subpopulation with Bt toxin Cry1Ac had increased its resistance to Cry1Ac and deltamethric. A complementation test between these subpopulations suggested presence of a common genetic loci controlling resistance to insecticides. In conclusion, the selection of resistance phenotype that results to simultaneous selection of the other could be due to a mechanism that acts against both classes of insecticides.

1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that host-plant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer – lepidopteran [ Plutella xylostella (L.)] – parasitoid [ Cotesia plutellae (Kurdjumov)], system. 2. The effects of host-plant suitability on herbivore performance and parasitism were examined. The bottom-up effect of plant suitability on host-parasitoid immune responses was then evaluated using measures of cellular and humoral effectors. 3. Host-plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence. 4. Poor host-plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized. 5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid. 6. These results demonstrated that plant-mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella, nor to host-plant effects on superparasitism. 7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host-plant-mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts. [ABSTRACT FROM AUTHOR]