Abstract
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.
Highlights
The eastern migratory population of the monarch, Danaus plexippus L., probably the best known butterfly in the world, has declined in abundance by > 90% in the last two decades[1] and is considered at risk of extirpation[2,3]
Popillia japonica Newman, commonly known as the Japanese beetle [JB], is an invasive, polyphagous scarab that was first discovered in Riverton, New Jersey, USA, near Philadelphia, in 191618
We verify the extent of JB aggregation on milkweed and damage to umbels in wild stands of milkweed, clarify which stage of bloom and floral parts the JB prefers to feed upon, and assess the impact of JB florivory on fruit and seed set of A. syriaca umbels in the field
Summary
The eastern migratory population of the monarch, Danaus plexippus L., probably the best known butterfly in the world, has declined in abundance by > 90% in the last two decades[1] and is considered at risk of extirpation[2,3]. The monarch has become an international conservation icon with power to mobilize scientists, organizations, and the public into actions to help restore its populations, and shape environmental policy[4,5,6,7]. Conservation of this specialist herbivore requires understanding the threats affecting its annual abundance, one of which is loss of milkweed (Asclepias species), the essential larval host plants, in the monarch's summer breeding grounds in the Midwestern United States[8,9,10,11,12]. We verify the extent of JB aggregation on milkweed and damage to umbels in wild stands of milkweed, clarify which stage of bloom and floral parts the JB prefers to feed upon, and assess the impact of JB florivory on fruit and seed set of A. syriaca umbels in the field
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