Nodulation Reaction to Fungal Infections in Larvae of Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) Mediated by Eicosanoids

Injecting larvae of the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), with eicosanoid biosynthesis inhibitor, phenidone (104 μg larva^-1), immediately prior to intrahemocoelic injections of the fungus Beauveria bassiana (Bals.) Vuill. (Deuteromycota: Hyphomycetes) (isolate No: HRI-215: 10 μg larva^-1 from 2 x 10⁶ blastospores ml^-1), sharply reduced (about 75 %) the nodulation reaction to the fungal challenges. Separate treatments with specific inhibitors of the major enzymes involved in eicosanoid biosynthesis, including phospholipase A2 (PLA2), cyclooxygenase, and lipoxygenase, reduced nodulation reactions to fungal infections. These findings support the view that nodule formation is a complex process involving lipoxygenase and cyclooxygenase products. The inhibitory influence of phenidone, the dual cyclooxygenase and lipoxygenase inhibitor, was apparent within each time. Phenidone-treated larvae formed about 14 nodules/insect at 0.5 h post injection (hpi), which increased to 54 at 8 hpi, whereas the ethanol-treated control larvae produced significantly more nodules at each time, from 49 nodules at 0.5 hpi to 147 at 8 hpi. The inhibitory effect of dexamethasone on nodulation was reversed by treating fungus-injected insects with the eicosanoid-precursor polyunsaturated fatty acid, arachidonic acid. These findings support our hypothesis that eicosanoids mediate insect cellular immune reactions to fungal infections in L. decemlineata.