Host-Preference Studies with the Egg Parasite Trichogramma semifumatum (Hymenoptera: Trichogrammatidae)1,2

Abstract

Investigations on the host preference in the hymenopterous egg parasite Trichogramma semifumatum (Perkins), using a laboratory culture maintained on Trichoplusia ni (Hübner) eggs for 4 months (over 12 generations) showed various preference levels for the eggs of 7 species of Lepidoptera. Size, relative penetrability of chorion, and age of hosts' eggs appear to be the most important factors affecting host preference. Parasite oviposition in smaller, older, or otherwise less suitable eggs always resulted in a higher female :male ratio. The great survival value of a higher female:male ratio under unfavorable conditions in nature is indicated and the mechanism is discussed. The culturing of parasites on Sitotroga cerealella (Olivier) eggs for 3 generations and testing them for host preference on T. ni and S. cerealella eggs weakened but did not significantly change the host preference shown earlier for T. ni eggs. Parasites from a culture that had been maintained on S. cercalella eggs for 3 years (over 100 generations) showed a reversal of the host preference by exhibiting a marked preference for S. cerealella eggs over those of T. ni, confirming the Hopkins' host-selection principle in this entomophagous insect.