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.