Effects of Metarhizium anisopliae on the histochemistry and enzymatic activities of Helicoverpa armigera (Hübner, 1809)

Metarhizium anisopliae, is widely utilized as a biocontrol agent in agriculture due to its diverse mechanisms of action and adaptability to various field conditions. However, the immune responses of the target insects significantly influence M. anisopliae virulence against insect pests. Among these pests, the tomato fruit borer, Helicoverpa armigera, stands out as a polyphagous and economically significant species for its resistance to numerous insecticides. Understanding the interaction between M. anisopliae and H. armigera immune defense is therefore critical for developing effective biocontrol strategies. In this study, five isolates of M. anisopliae were evaluated against the second and fourth instars of H. armigera using leaf dip-bioassay under controlled laboratory conditions. The isolate ICAR SBI Ma 08 was the most effective against both instars, with lowest LC₅₀ (2.56 × 10⁶ & 3.56 × 10⁸) and LT₅₀ values (4.36 and 6.36 days). The order of efficacy of other strains was ICAR SBI Ma 01 > ICAR SBI Ma 69 > ICAR SBI Ma 04 > ICAR SBI Ma 172. This isolate effectively evaded the immune response of H. armigera by inhibiting the activities of detoxifying enzymes such as esterase, mixed-function oxidase, acetylcholinesterase, and polyphenol oxidase. Conversely, following infection with this potent strain, larvae exhibited increased activities of glutathione S-transferase, catalase, aryl-acylamidase, and peroxidase. An increase in spore concentration had a positive impact on M. anisopliae pathogenicity. Histopathological studies showed that ICAR SBI Ma 08 successfully breached the first epidermal layer, with mycelia reaching the promeristem within 24 h post-infection. M. anisopliae ICAR SBI Ma 08 exhibited the highest efficacy against the second instar with a reduced LT₅₀, and its modulating enzymatic activity suggested that this isolate could be effectively deployed against the first and second instars of H. armigera. Developing an appropriate formulation and ascertaining its efficacy at the field level will confirm ICAR SBI Ma08's potential as a mycoinsecticide.