Diadromus pulchellus, a monophagous parasitoid of the leek moth, Acrolepiopsis assectella, a specialist of plants in the Allium genus, was raised on four series of chrysalids coming from four diets: leek leaves (LL), an artificial diet with (D + L) or without (D - L) leek powder, and an artificial diet plus 1% methyl-cysteinesulfoxide, a precursor of characteristic sulfur volatiles (D + P). Various behavioral and physiological responses by the parent parasitoids and the progeny were studied. No differences were observed in behavior among the four series. The higher precursor concentration observed in LL and in D + P than in D + L and D - L was responsible for a lower emergence ratio and higher egg production in LL and D + P. The consequences of these variations are discussed in relation to narrowness of the tritrophic system. The fact that sulfur volatiles favorable to behavior arise from nonvolatile sulfur precursors, having negative effects on physiology, can also contribute to the maintenance of the system. none