Side effects of immune response of Colorado potato beetle, Leptinotarsa decemlineata against the entomopathogenic nematode, Steinernema carpocapsae infection

Abstract

Entomopathogenic nematodes (EPNs) are lethal pathogens of agricultural insect pests. Little is
known about their sublethal effects on the insect hosts. The lethal effects of Steinernema carpocapsae
on fourth instar larvae of Colorado potato beetle (CPB), Leptinotarsa decemlineata were detected
using soil infection and direct injection of the nematode into the hemocel. LC20 and LC80 values of 7.8
(3.0 - 13.4) infective juveniles (IJs) and 126.7 (91-206.7) IJs were obtained for the soil application
method and 10.2 (8.7 - 11.4) IJs and 22.7 (19.73 - 28.0) IJs for direct injection, respecitvely. Sublethal
effects of S. carpocapsae on last instar larvae and subsequent surviving adults and phenoloxidase
(PO) activity in hemolymph of nematode-injected last instar larvae were investigated. Sublethal effects
included adult cuticular discoloration, deformation of the wings, legs and antenna and decreased
fertilized egg production in females. Considering cuticular discoloration in most treated insects, it is
hypothesized that production of PO in the insect larvae infected with an entomopathogenic nematode,
S. carpocapsae might have costs for surviving adult insects. PO specific activity in CPB against S.
carpocapsae generally increased up to 48 h post injection. Here in, the sublethal effects are discussed
as a potential tread-offs of PO production in nematode-injected insects.