Survey for milkweed (Asclepias spp.) and monarch butterflies (Danaus plexippus) in Lemhi County, Idaho

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Abstract:

The Asclepias genus (milkweed) is the exclusive larval host plant of the monarch butterfly.
Thorough research on the eastern population of monarch butterflies has identified loss of
milkweed as a major driving factor in the observed declines of monarchs at overwintering sites
in Mexico. Much less is known about the conservation status of the western monarch population,
however, loss of milkweed is implicated in similar population declines observed at California
overwintering sites. Data are needed at multiple scales to identify locations and quality of
monarch breeding sites in order to understand existing or emerging threats and effectively plan
conservation strategies. We conducted an exploratory survey during spring and summer 2014 to
document milkweed locations, monarch breeding activity, and general milkweed habitat
characteristics in Lemhi County, Idaho. We documented 156 occurrences of showy milkweed (A.
speciosa) ranging in elevation from 933 to 1,879 m and described milkweed phenology.
Milkweed was found in areas with moderate degrees of disturbance, including roadsides,
agricultural areas, and public lands. We observed various monarch life stages at 15% of
milkweed occurrences and described breeding phenology in relation to milkweed phenophases.
We estimated the monarch breeding season in Lemhi County to be 1 June to 15 August and
calculated sufficient degree days (778 C) to support development of two generations of monarch
butterflies. The vast majority (90%) of milkweed sites in our study area were variably impacted
by anthropogenic activities, including roadside management, agricultural practices, and
permitted activities on public lands, suggesting a widespread lack of public awareness of the
obligate relationship between milkweeds and monarchs and the declining status of western
monarchs. We offer management recommendations to address local and broad-scale
conservation needs of monarch butterflies, including expanded communication and information
sharing with land managers and the general public.

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Shelf Note: Electronic Files - Zoology: INVERTEBRATES