Life History of Winthemia manducae (Diptera: Tachinidae), a Parasite of the Tobacco Hornworm1

Abstract

Winthemia manducae Sabrosky & DeLoach was collected from all tobacco-growing areas of North Carolina but only as far south as Florence, S. C. It was reared from pupae of Manduca sexta (Johannson) and M. quinquemaculata (Haworth). The egg and larva are described. Macrotype eggs were laid on the integument of large last-stadial host larvae, in numbers from 1 to 125 but averaged about 8 per host. Eggs noramlly did not hatch until the host prepupa molted in its cell in the soil, at which time the parasitic larvae entered the host pupa before its integument hardened. The egg stage usually lasted about 7 days, but little hatching occurred unless the host pupated. Multiple parasitism involving Apanteles congregatus (Say) prevented the egg from hatching because A. congregatus killed the host before it pupated. The larvae of W. manducae fed inside the host pupa for 7 days then left the host and pupated within 2 days, largely in the upper 3 inches of the soil. Pupation was greatly delayed, probably by bacterial action, when full-grown larvae were confined with the dead host. Adults emerged 16 days later, the males emerging first. Females began ovipositing in 4 days. Thus a generation required about 37 days in midsummer. The sex ratio of reared adults was 1:1, but males were not found in the field with the females. Larvae required 233 and pupae 500 day-degrees for development above the lower threshold of 8.3°C. A threshold between 18 and 24°C was detected below which pupation of mature larvae was greatly delayed. Only full-grown larvae were observed to overwinter and these were tolerant to experimental exposure to subfreezing temperatures. The egg appeared to be the stage most susceptible to submersion and the pupa the stage most susceptible to high temperature under field conditions.