Associational resistance, gall-fly preferences, and a stem dimorphism in Solidago altissima

In most populations of Solidago altissima, a small proportion of plants have a growth pattern in which the stem bends early in the season so that the apex is pointing downward (i.e., a “candy-cane” stem) then straightens up again later in the season. The majority of plants, however, do not show this pattern. Instead their stems remain erect from emergence through flowering and senescence. It has recently been shown that the candy-cane stems are a result of a ducking strategy that reduces the risk of attack by apex-attacking herbivores. With such a resistance advantage, it is unclear why the candy-cane morphology is always in the minority, and why the erect morphology persists at all in S. altissima populations. In this study, we tested the hypothesis that the advantage of ducking is inversely frequency dependent and thus will not be an effective resistance strategy when ducking plants are in the majority. In a series of trials, we introduced gall flies (Eurosta solidaginis) into enclosures with relatively low (15%) and relatively high (85%) frequencies of candy-cane versus erect stems. Candy-cane stems were more resistant than erect stems regardless of relative frequency; thus, the hypothesis was rejected. Overall, attack rates were lower for both stem types in the high candy-cane frequency groups than in the low candy-cane frequency groups. This frequency-dependent attack rate suggests that erect-stemmed plants gain “associational resistance” by having a majority of candy-cane neighbors, while candy-cane plants suffer from “associational susceptibility” when surrounded by erect-stemmed neighbors.