No abstract

Laboratory bioevaluation showed the combination assay of ingestion and topical application of crude destruxin to be efficient in enhancing its insecticidal properties. The adopted combination assay apparently simulates application of the insecticide at field level. Quantitative differences between destruxins from low- and high-virulence strains of M. anisopliae are in accordance with its presumed role in virulence.

Extracellular enzymes produced by Beauveria bassiana, are believed to play a key role in cuticle hydrolysis. Enzyme production and pathogenicity has been found to be positively correlated. Twenty-eight isolates of B. bassiana, collected from different geographical regions and host ranges were characterized by in vitro extracellular enzyme production and SDS-PAGE techniques for discerning biochemical basis for virulence among the different isolates. In vitro analysis of extracellular enzymes like protease, amylase, caseinase, chitinase and lipase was undertaken in an attempt to understand their relevance to virulence of the isolates. The different isolates of B. bassiana were evaluated for virulence to the second instar larvae of Helicoverpa armigera in laboratory bioassays. SDS-PAGE of total intracellular soluble proteins was also studied in order to understand affinities among the different isolates of B. bassiana. The relationship between enzyme production and pathogenicity and vice versa was nearly 50%. There was a 50% relationship associated with original insect host, pathogenicity and enzyme production.

Nine‐day‐old Spodoptera litura (Fab.) larvae were treated with crude destruxin (dtx) extracted from a high‐virulent (M‐19) and a low‐virulent (M‐10) isolate of the entomopathogenic fungus, Metarhizium anisopliae (Metch.), at doses that caused 30%, 50% and 90% mortality in the treated groups after 48 h. Destruxins produced a dose‐dependent decrease in the body weight of the larvae after 1, 24 and 48 h of treatment. There was a considerable hike in the activity of lipoxygenase and lipid peroxidation levels in the treated larvae with increased time of exposure to mycotoxin. The activities of total superoxide dismutase, total catalase, total peroxidase and specific ascorbate peroxidase in the larval body also registered alterations in the dtx‐treated larvae, suggesting that exposure of larvae to crude dtx induces oxidative stress which is countered by the antioxidant enzymes to an extent governed by the concentration and time of treatment, beyond which the larvae succumb to the ecofriendly biotoxin.

Pathogenecity of the well characterized entomopathogenic fungus Metarhizium anisopliae used for biocontrol of a wide range of insect pests secretes hydrolytic enzymes that degrade the host cuticle. The chitinolytic activity of high and low virulent isolates of M. anisopliae was assayed on minimal medium (MM) + colloidal chitin and MM supplemented with insect cuticles. Ex- pression pattern of four chitinase genes (chitinase (chi), chi 1, chi 2, chi 3) was profiled during pathogenic stages of the entomopathogen under in vitro and in vivo conditions. Reverse-transcription polymerase chain reaction (RT-PCR) analysis confirmed that chitinase cDNAs were expressed during the germination of fungus under nutrient-deprived conditions. RT-PCR analysis performed for the four chitinase genes on the two insect hosts Spodoptera litura and Helicoverpa armigera at six developmental stages of the pathogen displayed up-regulation in S. litura at mycosed and conidiated condition while with H. armigera there was expression only after 48 h of incubation. Differential expression of chi, chi 1 and chi 2 genes in vitro (nitrogen rich and nitrogen limiting media) and in vivo (live insect hosts S. litura and H. armigera) implicate the role of substrate differences in pathogenesis.