Suitability of native milkweed (Asclepias) species versus cultivars for supporting monarch butterflies and bees in urban gardens.

Public interest in ecological landscaping and gardening is fueling a robust market for native plants. Most plants available to consumers through the horticulture trade are cultivated forms that have been selected for modified flowers or foliage, compactness, or other ornamental characteristics. Depending on their traits, some native plant cultivars seem to support pollinators, specialist insect folivores, and insect-based vertebrate food webs as effectively as native plant species, whereas others do not. There is particular need for information on whether native cultivars can be as effective as true or "wild-type" native species for supporting specialist native insects of conservation concern. Herein we compared the suitability of native milkweed species and their cultivars for attracting and supporting one such insect, the iconic monarch butterfly (Danaus plexippus L.), as well as native bees in urban pollinator gardens. Wild-type Asclepias incarnata L. (swamp milkweed) and Asclepias tuberosa L. (butterfly milkweed) and three additional cultivars of each that vary in stature, floral display, and foliage color were grown in a replicated common garden experiment at a public arboretum. We monitored the plants for colonization by wild monarchs, assessed their suitability for supporting monarch larvae in greenhouse trials, measured their defensive characteristics (leaf trichome density, latex, and cardenolide levels), and compared the proportionate abundance and diversity of bee families and genera visiting their blooms. Significantly more monarch eggs and larvae were found on A. incarnata than A. tuberosa in both years, but within each milkweed group, cultivars were colonized to the same extent as wild types. Despite some differences in defense allocation, all cultivars were as suitable as wild-type milkweeds in supporting monarch larval growth. Five bee families and 17 genera were represented amongst the 2,436 total bees sampled from blooms of wild-type milkweeds and their cultivars in the replicated gardens. Bee assemblages of A. incarnata were dominated by Apidae (Bombus, Xylocopa spp., and Apis mellifera), whereas A. tuberosa attracted relatively more Halictidae (especially Lasioglossum spp.) and Megachilidae. Proportionate abundance of bee families and genera was generally similar for cultivars and their respective wild types. This study suggests that, at least in small urban gardens, milkweed cultivars can be as suitable as their parental species for supporting monarch butterflies and native bees.

Invasive paper wasp turns urban pollinator gardens into ecological traps for monarch butterfly larvae.

Invasive species can be particularly disruptive when they intersect with organisms of conservation concern. Stabilizing the declining eastern migratory population of monarch butterflies (Danaus plexippus) is projected to require extensive habitat restoration across multiple land use sectors including metropolitan areas. Numerous conservation programs encourage urban citizens to plant gardens with milkweeds, the obligate larval host plants of the monarch. Here, we show that predation by Polistes dominula, an invasive paper wasp that is particularly abundant in urban settings, can turn such sites into ecological traps for monarch larvae. Polistes dominula was the predominant paper wasp seen foraging in central Kentucky pollinator gardens. In 120 observed encounters with monarch larvae on milkweeds in gardens, most second to fourth instars were killed, whereas most fifth instars escaped by thrashing or dropping. The wasps bit and carried off second instars whole, whereas third and fourth instar kills were first gutted, then processed and carried away piecemeal. Predation on sentinel larvae was much higher in urban gardens than in rural settings. The wasps exploited ornamental butterfly "hibernation boxes" in pollinator gardens as nesting habitat. Polistes dominula is an under-recognized predator that may diminish the urban sector's contributions to monarch habitat restoration.

Japanese beetles' feeding on milkweed flowers may compromise efforts to restore monarch butterfly habitat.

The eastern North American migratory population of monarch butterflies (Danaus plexippus) is in serious decline. Habitat restoration, including adding millions of host plants to compensate for loss of milkweed in US cropland, is a key part of the international conservation strategy to return this iconic butterfly to sustainable status. We report here that Popillia japonica, a polyphagous, invasive beetle, aggregates and feeds on flowers of Asclepias syriaca, the monarch's most important larval food plant, reducing fruiting and seed set by >90% and extensively damaging milkweed umbels in the field. The beetle's ongoing incursion into the monarch's key breeding grounds in the US Midwest is likely to limit pollination and outcrossing of wild and planted milkweeds, reducing their capacity to colonize new areas via seeds. Popillia japonica represents a previously undocumented threat to milkweeds that should be considered in models for monarch habitat restoration.