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 ectoparasitoid 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, Pteromalus 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.

The structure of cynipid oak galls: patterns in the evolution of an extended phenotype

“…Although generalist parasitoids commonly in£ict mortalities of 40^100% on oak gallwasps (Washburn & Cornell 1981;Askew 1984;Scho« nrogge et al 1995;Plantard et al 1996), it remains unclear to what extent particular gall traits a¡ect parasitoid attack rates. Of the characters shared by di¡erent Andricus clades, two (high gall hardness and large gall diameter) have been shown to impede attack by certain parasitoid species in cynipid galls (Askew 1965;Washburn & Cornell 1979) and other insect gall-inducer systems (Weis et al 1985;Price & Clancy 1986;Craig et al 1990). Although defensive functions have been suggested for the other traits conserved within Andricus clades (sticky outer surfaces, an air space between the larval chamber and the outer wall, and surface spines (Askew 1984)), their adaptive signi¢cance has yet to be demonstrated.…”

Response by Specialist and Generalist Natural Enemies to Agroecosystem Diversification: A Selective Review

“…The enemies hypothesis considers the importance of plant factors primarily through the effects of varied nectar sources and microhabitat complexity and quality in attracting and retaining those enemies that can use alternate prey species (Root 1973, Cromartie 1981, Risch 1981. These factors may indeed be important to the foraging success of many enemies, especially of generalist enemies (see reviews by Price et al [1980], Vinson [1981], and Altieri & Letourneau [1982]). Furthermore, these factors may lead to patterns of higher predation and parasitism in some diverse cultures (e.g., Altieri et al [1981] and Letourneau & Altieri [1983]).…”

Los Chalcidoidea (Hymenoptera) asociados con agallas de Cinípidos (Hymenoptera, Cynipidae) en la Comunidad de Madrid

“…En efecto, la entomofauna asociada a las agallas de cinípidos es compleja y diversa hasta el punto que pueden ser consideradas «puntos calientes» ecológicos. En este sentido, albergan una fauna asociada integrada fundamentalmente por tres niveles tróficos: inquilinos, parasitoides y sucesores, con gran cantidad de especies interelacionadas entre sí en complejas redes tróficas (Askew, 1961(Askew, , 1975a(Askew, , 1984Csóka et al, 2005;Hawkins & Goeden, 1984;Hayward & Stone, 2005;Memmott & Godfray, 1992;Price & Clancy, 1986;Redfern & Askew, 1992;Shorthouse, 1973;Wiebes-Rijks & Shorthouse, 1992). De una simple agalla de la generación ágama de Andricus kollari pueden depender más de 20 especies de artrópodos y las agallas con múltiples cámaras larvales de Biorhiza pallida o Andricus quercusradicis pueden albergar más de un centenar de insectos de múltiples especies (Nieves-Aldrey, 2001;Csóka et al, 2005).…”