Understanding the relationship between invasive plants and herbivores in their introduced range is an important aspect of conservation biology. The small cabbage white butterfly (Pieris rapae) and garlic mustard (Alliaria petiolata) share a common evolutionary past in Europe. Some researchers suggest that Pieris cannot complete its life cycle on garlic mustard in North America. This finding is surprising because Pieris has fewer competitors on garlic mustard in North America compared to in Europe. We explored mechanistic explanations for the geographic shifts in this plant/herbivore relationship with two lab experiments. First, we compared the oviposition preferences of Pieris on garlic mustard collected from wild local populations and commercially-grown brassica species [i.e., mustard greens (Brassica juncea), collard greens (B. oleracea), and kale (B. oleracea var. acephala)].
Results/Conclusions
Overwhelmingly, adults preferred to lay eggs on mustard greens over all other species (52% of all eggs; One-Way ANOVA, F= 49.21, P < 0.0001, df = 95; Tukey-Kramer post test, P < 0.05). Among the other three species, order of preference was kale (24% of eggs) > garlic mustard (14% of eggs) > collard greens (9% of eggs). In a pilot experiment, larvae grew fast on commercially-grown mustard greens but survivorship to adult approached zero. These results lead us to our second experiment. We tested fitness differences in terms of growth, survival, and days to pupation of larvae raised on one of the following: garlic mustard, commercially-grown mustard greens, and greenhouse-grown mustard greens. The garlic mustard was collected from local wild populations; the commercially-grown mustard greens were purchased in the grocery store; and we grew the greenhouse-grown mustard greens without any herbicides. Growth was highest in the two mustard green treatments. Survival was not different for Pieris raised on garlic mustard and greenhouse-grown mustard greens, but was significantly lower for Pieris raised on commercial-grown mustard greens suggesting that commercial-grown mustard greens have an effective herbivore deterrent. Our results suggest that Pieris may not feed extensively on garlic mustard in North America because (1) of ecological constraints not physiological constraints, and (2) alternative host plants that confer higher fitness are readily available. Even though garlic mustard is in the New World with an enemy from home, it still may be benefiting from enemy release because of shifts in Pieris host choice.
