Host-plant genotype and of her herbivores influence goldenrod stem galler preference and performance

Ecologists have labored to find an explanation for the lack of a positive c orrelation between host: preference and offspring performance in herbivorou s insects. This study focuses on how one herbivore species can influence sm other herbivore species' ability to accurately assess the suitability of di fferent host-plant genotypes for larval development. In particular, we exam ined the role that an early season xylem-feeding homopteran (meadow spittle bug, Philaenus spumarius) has on the preference-performance correlation of a late-season dipteran stem galler (Eurosta solidaginis) among different go ldenrod genotypes. In a greenhouse, we released adult stem gallers into rep licate cages that contained ramets from four different goldenrod genotypes crossed with three densities of spittlebugs (0, 1, or 8 nymphs placed 2 wee ks previously on each ramet). Spittlebug feeding caused a density-dependent decline in ramet growth rates, which in turn caused a corresponding decrea se in host-plant preference by the stem gallers (number of ovipunctures per bud or proportion of ramets attacked). Goldenrod genotype and the interact ion between spittlebugs and genotypes also influenced host-plant preference by the stem galler. Goldenrod genotype had the greatest impact on stem gal ler offspring performance (gall size or survivorship). Spittlebug density a lso affected performance, but only through its interaction with goldenrod g enotype, On some genotypes, the survivorship of stem-galler larvae decrease d with increasing spittlebug density, while on other genotypes, survivorshi p remained unchanged, or actually increased, with increasing spittlebug den sity. This suggests that there was genetic variance among goldenrod genotyp es in. their norms of reaction fur their suitability as a host to the stem gallers. One possible explanation for why spittlebugs caused a significant reduction in preference, but not in performance, was that spittlebugs had v ery few long-term effects on the host plant, Flower number, flowering pheno logy, and the allocation of the ramet's biomass to different structures (be low-ground organs, stems, leaves, and flowers) were unchanged with respect to spittlebug density. The only effect of spittlebugs was a 3-4% decrease i n ramet height at the end of the growing season. We argue that the lack of a positive correlation between host-plant preference and larval performance may reflect a constraint on the discriminatory ability of female stem gall ers. The damage to goldenrods caused by spittlebugs prior to attack by the stem gallers is similar in effect to potentially innumerable other causes o f goldenrod stress (e.g., reduction in ramet growth rates). As a consequenc e, stem gallers may not be able to discern the subtle differences among str esses that identify those that will negatively affect the fitness of stem-g aller offspring. The fact that goldenrod genotypes differ in their response to stresses would only further complicate the host-selection process. We p ropose that the stem gallers may have evolved a strategy that uses simple c ues as the basis for rejecting similarly stressed plants, whether all of th ose plant genotype-stress combinations reduce performance or not.