Fitness-related offspring sex allocation of Anastatus disparis, a gypsy moth egg parasitoid, on different-sized host species

Anastatus disparis (Ruschka) (Hymenoptera: Eupelmidae) is an egg parasitoid and considered a potential biological control agent of the gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae). Only male offspring of A.disparis emerge from single eggs of L.dispar in the laboratory, and A.disparis exhibits low parasitism on L.dispar in the field. We therefore selected several lepidopteran species with various body sizes to evaluate the optimal egg size for hosting A.disparis. In addition, we explored whether the nutritional content of a single L.dispar egg influences the sex of A.disparis offspring and why female offspring can be reared from L.dispar eggs in the field. The results indicated that host egg size decisively influenced the body size and sex ratio of the parasitoid offspring. Therefore, larger hosts, especially the largest eggs of Antheraea pernyi Guerin-Meneville (Saturniidae), might increase the fitness of A.disparis females. Lymantria dispar eggs concealed in the larger egg shell of A.pernyi produced female A.disparis, suggesting that adult A.disparis should prefer hosts with larger bodies and that the nutritional content of L.dispar eggs did not play a decisive role in the sex allocation of A.disparis. The results also indicated that the egg mass and the fur cover of L.dispar egg masses might be the key factors inducing female A.disparis to lay female offspring in L.dispar eggs.