The bacterium Bacillus thuringiensis produces ICPs (insecticidal crystal proteins) that are deposited in their spore mother cells. When susceptible lepidopteran larvae ingest these spore mother cells, the ICPs get solubilized in the alkaline gut environment. Of approx. 140 insecticidal proteins described thus far, insecticidal protein Cry1Ac has been applied extensively as the main ingredient of spray formulation as well as the principal ICP introduced into crops as transgene for agricultural crop protection. The 135 kDa Cry1Ac protein, upon ingestion by the insect, is processed successively at the N- and C-terminus by the insect midgut proteases to generate a 65 kDa bioactive core protein. The activated core protein interacts with specific receptors located at the midgut epithilium resulting in the lysis of cells and eventual death of the larvae. A laboratory-reared population of Helicoverpa armigera displayed 72-fold resistance to the B. thuringiensis insecticidal protein Cry1Ac. A careful zymogram analysis of Cry1Ac-resistant insects revealed an altered proteolytic profile. The altered protease profile resulted in improper processing of the insecticidal protein and as a consequence increased the LC50 concentrations of Cry1Ac. The 135 kDa protoxin-susceptible insect larval population processed the protein to the biologically active 65 kDa core protein, while the resistant insect larval population yielded a mixture of 95 kDa and 68 kDa Cry1Ac polypeptides. N-terminal sequencing of these 95 and 68 kDa polypeptides produced by gut juices of resistant insects revealed an intact N-terminus. Protease gene transcription profiling by semi-quantitative RT (reverse transcription)-PCR led to the identification of a down-regulated HaSP2 (H. armigera serine protease 2) in the Cry1Ac-resistant population. Protease HaSP2 was cloned, expressed and demonstrated to be responsible for proper processing of insecticidal protoxin. The larval population displaying resistance to Cry1Ac do not show an altered sensitivity against another insecticidal protein, Cry2Ab. The implications of these observations in the context of the possibility of development of resistance and its management in H. armigera to Cry1Ac through transgenic crop cultivation are discussed.

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: Very high cypermethrin and fenvalerate resistance frequencies were recorded in Helicoverpa armigera (Hu bner) populations in central India during the 1993È94, 1994È95 and 1995È96 cropping seasons. Synergism assays and biochemical analyses of detoxiÐcation enzyme levels indicated that monooxygenases and esterases were important metabolic mechanisms mediating pyrethroid resistance. Piperonyl butoxide-(PBO) and profenofos-suppressible pyrethroid resistance were correlated with enhanced levels of cytochrome P450 and general esterases respectively. Enzyme assay data indicated that high cytochrome P450 levels generally coincided with low esterase activity and vice versa. Similarly, synergist bioassays showed that PBO-insensitive resistance was frequently associated with profenofos-sensitive resistance and vice versa. Oxidaseand esterase-mediated mechanisms evidently alternated in a reciprocal manner, with perceptible shifts in relative importance occurring during mid-October in all three seasons and in late January in 1995. Apart from metabolic mechanisms, a synergist-insensitive resistance mechanism (believed to be nerve insensitivity), accounted for an average of 51, 30 and 28% of cypermethrin resistance during the 1993È94, 1994È95 and 1995È96 seasons respectively.

Very high cypermethrin and fenvalerate resistance frequencies were recorded in Helicoverpa armigera (Hübner) populations in central India during the 1993–94, 1994–95 and 1995–96 cropping seasons. Synergism assays and biochemical analyses of detoxification enzyme levels indicated that mono‐oxygenases and esterases were important metabolic mechanisms mediating pyrethroid resistance. Piperonyl butoxide‐ (PBO) and profenofos‐suppressible pyrethroid resistance were correlated with enhanced levels of cytochrome P450 and general esterases respectively. Enzyme assay data indicated that high cytochrome P450 levels generally coincided with low esterase activity and vice versa. Similarly, synergist bioassays showed that PBO‐insensitive resistance was frequently associated with profenofos‐sensitive resistance and vice versa. Oxidase‐ and esterase‐mediated mechanisms evidently alternated in a reciprocal manner, with perceptible shifts in relative importance occurring during mid‐October in all three seasons and in late January in 1995. Apart from metabolic mechanisms, a synergist‐insensitive resistance mechanism (believed to be nerve insensitivity), accounted for an average of 51, 30 and 28% of cypermethrin resistance during the 1993–94, 1994–95 and 1995–96 seasons respectively. © 1997 SCI.

Infection of Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), with 'Candidatus' Liberibacter asiaticus (Las), the causal pathogen of citrus greening disease or huanglongbing, increases psyllid susceptibility to insecticides. Diaphorina citri populations in citrus occur year-round in tropical and sub-tropical habitats, and thus insecticide applications for managing this plant disease vector occur over a wide temperature range (10-40 °C). During the winter season, D. citri is occasionally exposed to periods of freezing temperatures, when temperatures fall below −6.5 °C. In this investigation, we compared insecticide susceptibility of uninfected and Las-infected D. citri at various temperatures (20-37 °C). Cold-acclimated (6 ± 1 °C) D. citri adults were less susceptible to neonicotinoid insecticides as compared with non-acclimated controls, but this trend was not observed for other insecticides tested. A positive correlation between temperature and percentage mortality caused by chlorpyriphos, imidacloprid, spinetoram, and thiamethoxam was found irrespective of infection status when evaluated at temperatures ranging between 20 and 37 °C. In contrast, a negative correlation between temperature and percentage mortality was observed for fenpropathrin for both infected and uninfected psyllids. Glutathione S-transferase levels were negatively correlated with temperature, whereas levels of cytochrome P450 and general esterase were not correlated with temperature fluctuations. These results indicate that altered insecticide susceptibility due to temperature may not be related to glutathione S-transferase, cytochrome P450, and general esterase levels. Diaphorina citri adults that carried the Las bacterium had reduced CYP4 transcript and protein levels, and ingested less than uninfected counterparts, as measured by the production of honeydew. Diaphorina citri adult feeding was greatest at 32 °C within the temperature range tested. Overall, annual temperature fluctuation does not appear to be a major factor impacting management of D. citri.