For the past two decades, the overwintering population of monarch butterflies has been declining, with milkweed (Asclepias sp) loss in the breeding grounds a major contributing factor. It is estimated that the current 1.3 billion stems of milkweed in the North Central U.S. produce 3.2 hectares of overwintering monarchs. With a conservation goal of 6 hectares, up to an additional 1.6 billion stems of milkweed need to be established. Survival of lepidopteran larvae is known to be low because of predators, pathogens, parasitoids, and competition (Zalucki et al. 2002). Nail et al. (2015) estimated ≤ 10% of monarch larvae survive to 5th instar. Larval movement among host plants is one mechanism to reduce immature mortality. Monarch larvae “move extensively” in late instars (Zalucki & Rochester 2004). Frequency of movement between milkweed plants is unknown but is an essential parameter for habitat restoration planning. Under greenhouse conditions, without top-down pressure, single 1st instar larvae were placed in cages with two, three, or four single-stemmed common milkweed (Asclepias syriaca) plants. Larval instar and location (specific plant or cage) were recorded twice daily until pupation. Number of stems utilized was estimated.
Results/Conclusions
The number of plants had no effect on developmental endpoints. In all treatments, larvae moved an average of four times. Initial movement was observed in the fourth instar, but the majority of movement occurred in the fifth (p = 0.001). Larvae utilized two or three milkweed plants through pupation. Larvae spent more time on plants when provided four stems than when provided two stems (p = 0.0078), suggesting a random walk behavior. Regardless of treatment, all larvae abandoned their natal plant despite the absence of predators or parasitoids or insufficient milkweed biomass. These observations suggest that there is selection pressure for larvae that move. Habitat restorations should contain patches of at least 3 closely-spaced milkweed stems to support larval development. Future studies will estimate the amount of plant biomass consumed prior to plant abandonment and explore the effect of larval abundance/competition on movement frequency. Estimates of distance traveled by 5th instar larvae in a field-setting will be obtained using radio telemetry.