Intraspecific Competition in the Goldenrod Ball-Gallmaker (Eurosta solidaginis): Larval Mortality, Adult Fitness, Ovipositional and Host-Plant Response

Abstract

Intraspecific competition can be an important factor affecting population structure for herbivorous insects. Here we investigate whether high larval density has detri- mental effects for the goldenrod ball-gallmaker, Eurosta solidaginis (Diptera: Tephritidae). First, in a field study of E. solidaginis populations attacking the host-plant, Solidago altissima (Compositae) we found high numbers of ovipunctures per bud which suggested that high mortality could result from intraspecific larval competition. Second, a greenhouse experi- ment was performed to determine if larval mortality increases with numbers of larvae present in a bud of the host plant and to determine subsequent effects on emergence and fitness. Replicates of a single susceptible host-plant genotype were randomly assigned to either low or high ovipuncture-attack treatment groups. The number of eggs oviposited increased sig- nificantly with the number of ovipunctures per bud. However, by 3 wk after oviposition, there was no significant difference in the number of larvae between the two treatments. Larval mortality during this pregall stage resulted from direct larval interference, tissue necrosis (a plant resistance mechanism induced by high levels of infestation) and buds damaged by excessive ovipuncture. In multiple-galled ramets, emergence of adults from upper gall posi- tions was significantly reduced compared to the bottom gall positions, suggesting depletion of some limiting resource. Adult longevity increased significantly with adult mass, whereas longevity of adults decreased for flies that emerged from multiple-galled ramets compared to single-galled ramets. Consequently, fecundity could be affected by larval competition. Ovi- positing females decreased the rate of egg deposition at high levels of ovipuncture suggesting that buds already containing a high number of ovipunctures are less acceptable for .egg deposition. Where acceptable hosts are not easily found, females may oviposit into less pre- ferred genotypes or shift to closely related but novel host species.