The Biology of Heterostylum Robustum (Diptera: Bombyliidae), A Parasite of the Alkali Bee1

Abstract

Heterostylum robustum (Osten Sacken) is the principal parasite of the alkali bee ( Nomia melanderi Ckll.) in the Northwestern States. It also parasitizes other species of Nomia and at least one species of both Nomadopsis and Halictus . It ejects eggs into and near the nest mounds of its host, but does not readily discriminate between nest burrows and other depressions in the ground. The first-instar larva finds its way to a host larval cell, where it waits until the host larva is full grown before feeding on it. The parasite larva passes through four instars, progressing from a slender, active first instar through a very brief second instar, and a soft, helpless third instar, to a tough, more active fourth instar. Some larvae apparently mature on a single host, but others partially drain the fluids from a second as well. In the late summer or fall the mature larva makes an overwintering cell in the upper few inches of soil. It pupates in the late spring, and shortly before ecdysis bores to the ground surface. The adult emerges as soon as the pupa thrusts its head and thorax clear of the surface. Mating usually takes place on vegetation surrounding the host nesting site.