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

Environmental catastrophes, such as severe drought, can reduce host-plant q uality and/or abundance, which in turn decrease levels of herbivore populat ions. Such changes in herbivore populations affect populations of their nat ural enemies. As part of a long-term field experiment (1983-1991), galls of Eurosta solidaginis from 16 fields in central Pennsylvania were systematic ally collected from goldenrod ramets. Galls were dissected to compare the o ccurrence of E. solidaginis mortality caused by its natural enemies in 2 dr ought years (1988, 1991) with 5 pre-drought years (1983-1987) and 2 postdro ught years (1989-1990). Gall diameters were significantly smaller in both d rought years and early larval death significantly decreased E. solidaginis survivorship in the first drought year. Of the natural enemies, the parasit oid wasp Eurytoma gigantea caused significant selection for larger gall siz e 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-induce d small gall size in 1988, there was negligible selection on gall size by n atural enemies. However, populations of E. solidaginis did suffer local ext irpations at nine of the 16 fields during the first drought year and popula tion 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 abs ent. Drought therefore drastically reduced the abundance of E. solidaginis and natural enemies resulting in slow recoveries to pre-drought numbers.