Comparative analysis of immune responses in Colorado potato beetle larvae during development of mycoses caused by Metarhizium robertsii, M. brunneum, and M. pemphigi

Abstract

A comparative investigation of humoral and cellular immune response in larvae of Colorado potato beetle Leptinotarsa decemlineata was conducted under development of mycoses caused by entomopatho- genic fungi Metarhizium robertsii, M. brunneum and M. pemphigi. The larvae were found highly suscep- tible to M. robertsii, M. brunneum and less susceptible to M. pemphigi. The susceptibility to the fungi was not correlated with the rate of conidia germination in epicuticular extracts of larvae. A non-specific for Colorado beetle pathogen M. pemphigi did not cause significant changes in the immune response and did not result in colonization of larvae hemocoel. Infection with M. robertsi and M. brunneum led to an increase in total hemocyte count at the initial stages of mycoses (day 2) followed by a sharp decrease on day 3. The strongest decrease was observed for the immunocompetent cells - plasmatocytes and granu- locytes. Enhanced phenoloxidase activity in hemolymph and cuticle was found on days 2 and 3 after in- fection. These changes in immune reactions correlated with the level of virulence of the strains. Thus, the immune response in Colorado potato beetle larvae is an important factor determining differences in the development of mycoses caused by different Metarhizium species.