The effects of drought on the Solidago altissima-Eurosta solidaginis-natural enemy complex: population dynamics, local extirpations, and measures of selection intensity on gall size.

摘要

Environmental catastrophes, such as severe drought, can reduce food plant quality and/or abundance, which in turn decrease levels of herbivore populations. Such changes in herbivore populations affect populations of their natural enemies. As part ofa long term field experiment (1983-1991), galls of Eurosta solidaginis from 16 fields in central Pennsylvania were systematically collected from goldenrod ramets. Galls were dissected to compare the occurrence of E. solidaginis mortality caused by its natural enemies in 2 drought years (1988 and 1991) with 5 pre-drought years (1983-87) and 2 post-drought years (1989-90). Gall diameters were significantly smaller in both drought years and early larval death significantly decreased E. solidaginis survival in the first drought year. Of the natural enemies, the parasitoid wasp Eurytoma gigantea caused significant selection for larger gall size in all pre-drought years, the 1991 drought, and both post-drought years, due to its differential attack of smaller galls. In spite of drought-induced small gall size in 1988, there was negligible selection on gall size by natural enemies. However, populations of E. solidaginis did suffer local extirpations at nine of the 16 fields during the first drought year andpopulation recoveries of the gall inducer and natural enemies varied among fields in the post-drought years. As a consequence of reduced herbivore abundance in drought and post-drought years, some natural enemy populations were absent. Drought, therefore, drastically reduced the abundance of E. solidaginis and natural enemies resulting in slow recoveries to pre-drought numbers.