Superparasitism Occasionally Predisposes Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae) to Develop Gregariously in Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)¹

Experiments were designed to estimate the incidence of superparasitism of Spodoptera exigua (Hübner) and gregarious development of Cotesia marginiventris (Cresson). Dissections of host larvae revealed that superparasitism was common at all host densities (5, 10, or 20 larvae per 0.05 liter arena). At least two parasitoid eggs were present per host on day 1 after parasitoid oviposition; eggs were scarcely found on day 2 and never found on days 3, 4 or 5. More than one parasitoid first instar was present per host on day 2 at all densities, although more were found in hosts of the 5 density rather than the 20 density treatment. Second instars were present in hosts on days 3, 4 and 5, and at least one was present per host on day 5. Third instars (the final instar stage) were not present inside hosts by day 5. Rearing of host larvae demonstrated that gregarious development was rare, since less than 6% of parasitized hosts yielded two third instars that spun cocoons. Neither host density nor exposure time (24 h, 48 h) had an effect on the occurrence of gregarious development. The adult emergence rate was 64% and the sex ratio was male-biased for parasitoids that developed gregariously. The adult emergence rate was 73% and the sex ratio was unbiased for those that developed solitarily. This research suggests that superparasitism of hosts by C. marginiventris can occasionally predispose parasitoid larvae to develop gregariously. Frequent superparasitism of hosts may be detrimental to the long-term propagation of C. marginiventris.