Cotesia plutellae is a major solitary larval endoparasitoid of the diamondback moth, Plutella xylostella which is a cruciferous pest. We investigated the subsequent sex ratio of progeny, growth and development of C. plutellae when parasitization occurred at the 2nd, 3rd, and 4th host stadia. Furthermore, cues by which males enhance the opportunities to encounter potential mates were examined. Although successful parasitism of 4th instars showed a lower percentage of parasitism than those of the other stadia, no difference in sex ratio of progeny was observed at any host stadium. The ratio of females among the progeny and the parasitism rate decreased with the age of the females. The developmental duration of male wasps tended to be shorter than that of females at all stadia for which parasitization occurred, indicating protandry. The presence of host plants, i.e., a cabbage leaf, reduced the time necessary for males to successfully locate mates. Damage to cabbage due to the diamondback moth caused an increase in the number of males flying around and landing on leaves near the females, suggesting that males searching for females over a long range needed both stimulants produced by the host larvae and a female pheromone.
