Study System
Goldenrod (Solidago altissima ) is a clonal patchily distributed old-field plant that is common in eastern North America. A common goldenrod herbivore is Eurosta solidaginis —a univoltine fly whose larvae spur the development of a spherical stem gall (Uhler 1951).Eurosta emerges as an adult in late May in Southern Ontario, mates, and oviposits gall-inducing larvae into the apical meristem of fresh goldenrod shoots in the following ~2 weeks. Gall size is a complex function of fly genotype, plant genotype, and environmental conditions, but adaptive shifts in gall size corresponding to patterns of selection have been observed (Craig et al. 2007, Start and Gilbert 2016).
Selection and resultant adaptive evolution stems from attack by enemies of Eurosta , including Mordellistena beetle larvae, two parasitoid wasps (Eurytoma spp. ), and birds (downy woodpeckers and chickadees). Eurytoma gigantea and birds also attack galls of different sizes, preferring to forage on small and large galls respectively (Abrahamson et al. 1989). As a result, selection on gall size tends to favor intermediate forms (stabilizing selection) but can also have a directional component favoring the size favored by the rarer enemy. These victim-enemy interactions vary considerably through time (Weis and Kapelinski 1994), and across space (Craig et al. 2007, Start et al. 2018, Start 2019), including at very small spatial scales (Start and Gilbert 2016, Start et al. 2019).
Beyond preferences for different sized galls, the enemies ofEurosta also differ in phenology. Beetle larvae and parasitoid wasps attack galls in early summer while the gall is still growing and goldenrod tissue is still alive, whereas bird attack predominantly occurs over winter after the plant tissue has died but while the larvae remains alive inside the gall. During the late fall, it is common for some dead goldenrod stems (but that nevertheless have livingEurosta in them) to be knocked over in windstorms. Because birds prefer to attack galls higher off the ground (Start 2018a), galls on knocked-over stems will likely experience lower bird predation, causing selection to be more positive (favor larger galls). In the rare event that windstorms knock down all stems and galls on the landscape, then we would expect bird predation to be homogenous and low, and selection to be consistent among populations and positive.