Influence of Helicoverpa zea (Lepidoptera: Noctuidae) age during the last instar on rates of parasitization by the larval-pupal parasitoid, Archytas marmoratus (Diptera: Tachinidae)

Effects of host age during each of the 5 d of the last host instar were determined by exposing larvae of Helicoverpa (= Heliothis) zea (Boddie) to newly larviposited maggots of the larval-pupal parasitoid Archytas marmoratus (Townsend). Based on determinations made in host pupae, lower percentages of older hosts were parasitized and lower percentages of maggots from older hosts developed to the second instar. Weights of maggots that parasitized older hosts were lower in 1-d-old host pupae. For older hosts, mean length of the larval stage of A. marmoratus after host pupation was longer; weights of young A. marmoratus larvae revealed that maggots developed very slowly until just before the host pupated. Surviving maggots developing from older hosts used the longer developmental times to overcome the weight differentials, and weights of A. marmoratus pupae were the same regardless of host age and parasitoid weight at the beginning of the host's pupal stage. These and other data indicate that complex behavior allows the A. marmoratus maggot to evade host defenses and grow to a size that enables the parasitoid to avoid encapsulation when it finally enters the host hemocele.