Host Age and Parasitism by Trichogramma minutum (Hymenoptera: Trichogrammatidae)2

In the laboratory, Trichogramma minutum Riley killed nearly 100% of the eggs of the cabbage looper, Trichoplusia ni (H¼bner), that were less than 11 hours old at the time of stinging, about 15% when the eggs were 27 hours old, and almost 95% when the eggs were ready for eclosion. At all ages of host eggs, the percent in which parasite eggs were laid was about the same as the percent killed, but some hosts of intermediate age hatched despite the presence of parasite eggs. The percent host eggs turning black before parasite emergence was consistently lower than the percent killed, especially with older hosts, and 24-hour-old eggs had an increased probability of being parasitized when they were attacked more than once. Failure of parasites to oviposit and death of parasite eggs were caused by some physical or chemical properly common to both living hosts and hosts killed by gamma rays. Females found about equal numbers of 1-. 25-, or 49-hour-old eggs but did not find as many 69-hour-old eggs with the black head capsule showing. At all ages of host eggs, the percent eggs accepted forstinging was about the same.