Effects of insect cuticular fatty acids on in vitro growth and pathogenicity of the entomopathogenic fungus Conidiobolus coronatus

Eighteen fatty acids identified in the cuticle of three insect species representing differing susceptibilities to C. coronatus infection, were tested for effects on the in vitro growth and pathogenicity of the parasitic fungus. At all applied concentrations (0.1–0.0001% w/v) growth was inhibited by C16:0, C16:1, C18:0, C18:1, C18:2, C18:3, C20:0 and C20:1. At high concentrations spore germination was inhibited by C7:0, C8:0, C9:0, C10:0, C12:0, C18:2 and C18:3 and hyphal growth was merely retarded by C5:0, C6:0, C6:2, C14:0, C16:0, C16:1, C18:0, C18:1, C20:0 and C20:1. The presence of C15:0 at the 0.1% concentration stimulated growth of C. coronatus. Sporulation was inhibited by all concentrations of C16:0 and C18–20 fatty acids. Low concentrations of C5:0, C6:0, C6:2 and C7:0 enhanced sporulation. Fatty acids C5–12 as well as C18:3, C20:0 and C20:1 decreased the ability of fungal colonies to infect G. mellonella while C16:1 elevated it thus suggesting that C16:1 may stimulate production of enzymes involved in the host invasion. Toxicity of metabolites released into incubation medium decreased with varying degrees in the presence of C6:0, C6:2, C7:0, C9:0, C12:0, C16:1, C18:2, C18:3, C20:0 and C20:1; other fatty acids had no effect. Further work is needed to analyse the effects of exogenous fatty acids on the C. coronatus enzymes implicated in fungal pathogenicity as well as on the production of insecticidal metabolites.