Field Tests of the Size‐Fitness Hypothesis in the Egg Parasitoid Trichogramma Pretiosum

Abstract

The hypothesis that fitness following dispersal from the natal host increases with body size was tested under field conditions in the egg parasitoid Trichogramma pretiosum. In one experiment, size distributions of males and females at emergence were compared to those of males successful in locating mates and females successful in locating hosts. Mate and host location success increased with size in smaller parasitoids but were constant with respect to size in larger parasitoids. Selection intensities were 3.5—5.4 times greater in males than females possibly reflecting the small contribution of postdispersal matings to total male fitness in this partially sibmating species. In a second experiment using field releases of genetically marked lines, the number of trap hosts parasitized by "large" females reared from Helicoverpa zea eggs and "small" females reared from Sitotroga cerealella eggs was compared. Large females parasitized more hosts than small females in one trial but the reverse was observed in a second trial. Acceptance and suitability of the trap host species were not influenced by marker line or rearing host. We hypothesize that a genotype—environment interaction based on host age, host species, and wasp genotype accounts for the heterogenous results of the second experiment. Our main conclusion is that average postdispersal fitness of adult T. pretiosum increases with size in smaller wasps and is constant with respect to size in larger wasps. Contrary to general expectation, average wasp fitness does not increase linearly with size. Most importantly, wasp size is not a reliable predictor of individual or average cohort fitness; consistent size—fitness relationships emerge only when fitness is averaged over many genotypes and environments. Systematic factors such as host age and wasp genotype can have a strong influence on wasp fitness without strongly affecting size. The implications of these results for progeny and sex allocation in Trichogramma and for the mass production of parasitoids in biological control programs are discussed.