Effects of host age on parasitism and progeny allocation in Trichogrammatidae

The impact of host age on the number of hosts killed, survival of progeny, progeny allocation, and sex allocation was examined for several Trichogramm atidae (Hymenoptera) species in laboratory choice tests. Individual female parasitoids were provided with young, medium-aged and old eggs of one of th ree lepidopterous host species: Trichoplusia ni (Hubner) (Noctuidae), Pieri s rapae (L.) (Pieridae), or Plutella xylostella (L.) (Plutellidae). Trichog rammatid species behaved as gregarious parasitoids with the first two host species, and as solitary parasitoids with eggs of the smaller latter one. T hey mostly preferred young eggs of T. ni, but did not discriminate among P. rapae eggs of different ages, and often preferred young or medium-aged P. xylostella eggs over old eggs. Survival of progeny did not vary constantly with host age, although it was often very low in P. rapae eggs of any age. Clutch size frequently decreased with host age in both T. ni and P. rapae. Offspring sex ratio did not change with age of T. ni and P. rapae eggs, and rarely did so in P. xylostella eggs. In regard to host age, the results wi th T. ni are the ones which are the most in agreement with optimal foraging theoretical predictions, as clutch size was the highest in preferred young er eggs.