Interactions Among Three Trophic Levels: Gall size and Parasitoid Attack

Abstract

The diameter of galls formed on shoots of arroyo willow, Salix lasiolepis, and initiated by the tenthredinid sawfly Euura lasiolepis strongly influenced the pattern of attack by the ectopar- asitoid Pteromalus sp. Gall diameter is regulated by the plant; significant differences in mean gall diameter persisted between adjacent clones for 5 yr of study, and a genotypic influence is suggested because cuttings grown in a common environment maintained the relative gall size characters of the parental clones. Mean gall diameters per clone ranged from 3.5 to 7.5 mm. For a parasitoid with a mean ovipositor length of 1.89 mm (which must reach a host in the gall center from the gall exterior), many potential hosts in the larger galls could not be attacked. Attack by Pteromalus was concentrated in the first two galls on a shoot, and these were, on the average, the smallest galls on shoots. To avoid the effect of synchrony of attack phenology with production of small galls, the effect of gall diameter was examined only on galls in positions 1 and 2 on stems. Attack was still concentrated on the smaller galls, so we rejected the Random Attack Hypothesis and accepted the Gall Diameter Hypothesis. Winter precipitation affected gall diameter only after the driest winter in the winters 1978-1979 to 1982-1983, and a significant negative correlation between winter precipitation and percent interval parasitism was evident in only one of four clones tested for this effect. Gall diameter was strongly controlled by the ovipositing female, with minor, if any, influence by larval feeding. Therefore, Pter- omalus attack could not have reduced gall diameter by more than 1.9% or 0.11 mm on average. Pteromalus attack on smaller galls was not the result of retarded gall growth subsequent to attack; the ectoparasitoid females were selecting smaller galls. The long-term relationship between mean gall diameter per clone and percent interval parasitism over 31 clone years from 1979 to 1983 was consistent with the Gall Diameter Hypothesis: a significant negative relationship was evident, accounting for 35% of the variance.