Patterns and causes of oviposition in monarch butterflies: Implications for milkweed restoration

Abstract

Effective habitat restoration requires an understanding of species habitat preferences and the associated mechanisms driving those preferences. We examined the patterns and causes of oviposition preference in the monarch butterfly, a rapidly declining species, in southwestern Ontario at both landscape and milkweed patch spatial scales. Additionally, we measured the abundance of invertebrate predators, parasitoids and parasites across these same spatial scales. Oviposition preference was dependent on both the size of the milkweed patch and the density of milkweed within the patch, as well as landscape type. Small (<16m2), low-density (0.1–2 milkweed per m2) milkweed patches in agricultural landscape had the highest egg density compared to all types of milkweed patches in non-agricultural and roadside landscapes. Medium-sized patches had the highest predator abundance. Variation in the abundance of parasitoids, and occurrence of parasites of monarch eggs and larvae did not appear to coincide with preferred egg laying habitats. Our results suggest that investing heavily in milkweed restoration in roadside habitats should be done cautiously. Instead, a better strategy may be for managers to develop incentive programs with landowners to plant and maintain milkweeds in agricultural landscapes, which could complement other pollinator initiatives or ecosystem service programs in agricultural landscapes that focus on increasing nectar availability. Our results have important implications for restoring milkweed as an approach to counteract monarch butterflies declines.