FIELD-TESTS OF THE SIZE-FITNESS HYPOTHESIS IN THE EGG PARASITOID TRICHOGRAMMA-PRETIOSUM

The hypothesis that fitness following dispersal from the natal host in creases with body size was tested under field conditions in the egg pa rasitoid Trichogramma pretiosum. In one experiment, size distributions of males and females at emergence were compared to those of males suc cessful in locating mates and females successful in locating hosts. Ma te and host location success increased with size in smaller parasitoid s but were constant with respect to size in larger parasitoids. Select ion intensities were 3.5-5.4 times greater in males than females, poss ibly reflecting the small contribution of postdispersal to total male fitness in this partially sibmating species. In a second experiment us ing field releases of genetically marked lines, the number of trap hos ts parasitized by ''large'' females reared from Helicoverpa zea eggs a nd ''small'' females reared from Sitotroga cerealella eggs was compare d. Large females parasitized more hosts than small females in one tria l bur the reverse was observed in a second trial. Acceptance and suita bility of the trap host species were not influenced by marker line or rearing host. We hypothesize that a genotype-environment interaction b ased on host age, host species, and wasp genotype accounts for the het erogenous results of the second experiment. Our main conclusion is tha t average postdispersal fitness of adult T. pretiosum increases with s ize in smaller wasps and is constant with respect to size in larger wa sps. Contrary to general expectation, average wasp fitness does not in crease linearly with size. Most importantly, wasp size is not a reliab le predictor of individual or average cohort fitness; consistent size- fitness relationships emerge only when fitness is averaged over many g enotypes and environments. Systematic factors such as host age and was p genotype can have a strong influence on wasp fitness without strongl y affecting size. The implications of these results for progeny and se x allocation in Trichogramma and for the mass production of parasitoid s in biological control programs ale discussed.