Infection of Metarhizium anisopliae Ma6 and defense responses of host Phyllotreta striolata adults

Entomopathogenic fungus as biological control agent plays a crucial role in the integrated management of insect pests. Metarhizium anisopliae Ma6 has been identified as a highly pathogenic strain against Phyllotreta striolata (Fabricius) (Coleoptera: Chrysomelidae), one of the most economically important and dominant insect pests damaging Brassica plants. The infection of M. anisopliae Ma6 on P. striolata was observed under stereomicroscopy and scanning electron microscopy (SEM), and biochemical defense responses of P. striolata adults after infection were investigated. The changes in total amino acids and free fatty acids, and the activities of protective enzymes, including catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD), in P. striolata adults were measured. In stereomicroscopy and SEM observations, a large number of mycelia were observed on the body surface of P. striolata on the 5th day after treatment by M. anisopliae. Many conidia were germinated and covered the body of P. striolata on the 7th day after treatment. The free fatty acid, total amino acid, CAT, POD, and SOD activities all showed an increased and then decreased trend. These results suggest that entomopathogenic fungal infection triggers the defense response of hosts, which induces changes in nutrients and antioxidant enzymes in P. striolata adults. Our findings provide useful information for understanding the potential for using M. anisopliae Ma6 as a biocontrol agent.