The distribution of risk and density-dependent mortality in the galls of Eurosta solidaginis, the goldenrod gall fly

ABSTRACT.

• 1 This paper explores the net effect of a suite of mortality factors on a sedentary prey, the larvae of the goldenrod gall fly, Eurosta solidaginis Fitch (Diptera: Tephritidae).
• 2 Mortality is caused by unknown factors early in larval development, two species of parasitoid wasp (Hymenoptera: Eurytomidae), an inquiline beetle larva (Coleoptera: Mordellidae), and during the winter months downy woodpeckers Picoides pubescens (L.).
• 3 Distribution of mortality among galls relative to prey (gall) distribution was measured and discussed with respect to the distribution of relative risk of predation.
• 4 Galls are by and large contagiously distributed among quadrats, and mortality is distributed in a comparable pattern to that of galls.
• 5 The pattern of mortality on Eurosta larvae is neither density-dependent nor aggregated independently of gall distribution. Persistence in the system is probably a result of a combination of other factors such as adult mortality and early larval death which may have intergenerational density-dependent effects, and the linkage of locally unstable sub-populations via migration.