Chemicals mediate many ecological interactions. Penicillium expansum liberates metabolites that are toxic to Drosophila larvae. Earlier damage induces the mould to produce more of and different kinds of these chemical defences. But the exact chemistry differs between mould strains before and after induction. We show that mould metabolites after induction have marked effects on the behaviour and mortality of Drosophila larvae and also on the growth and mortality of co-occuring yeasts. Thus, the metabolites of moulds, particularly induced moulds, have wide ecological footprints in ecosystems.