Monarch Butterfly Distribution and Breeding Ecology in Idaho and Washington

Abstract

Studies of monarch butterflies (Danaus plexippus) and their milkweed (Asclepias spp.) host plants in North America have focused primarily on monarch populations ranging east of the Rocky Mountains. We report the first systematic assessment of monarch butterfly and milkweed populations in the western states of Idaho and Washington, states at the northern tier of western monarch breeding range. Results of our 2-year study (2016)(2017) offer new insights into monarch breeding habitat distribution, characteristics, and threat factors in our 2 states. We documented milkweeds and breeding monarchs in all 16 climate divisions in our study area. Milkweed and breeding monarch phenologies were examined with evidence supporting 2, and possibly 3 monarch generations produced in Idaho and Washington. Key monarch breeding habitats were moist-soil sites within matrices of grasslands, wetlands, deciduous forest, and shrub-steppe supporting large, contiguous, and high-density milkweed stands. Co-occurrence of showy milkweed (A. speciosa) and swamp milkweed (A. incarnata) was an important indicator of productive monarch breeding habitat in Idaho. Nectar plants were generally limited in quantity and richness across the study area, particularly in late summer, and included frequently-used non-native, invasive species. Primary threats at milkweed sites were invasive plant species, herbicide application, and mowing, followed by secondary threats of recreational disturbance, livestock grazing, insecticide application, loss of floodplain function, and wildfire. We provide management recommendations and research needs to address ongoing stressors and knowledge gaps in Idaho and Washington with the goal of conserving monarchs and their habitats in the West.

Distribution, phenology, and land cover preference risk assessments of four South-Central US milkweeds (Asclepias spp.) important for monarch butterflies

“…The highest land cover preference risk was found with Cultivated Crops for all four milkweed species. Waterbury et al (2019) similarly found cultivated croplands had low utilization for northwestern milkweeds. We found a few A. oenotheroides in Cultivated Crops, and A. latifolia occurred in cultivated crops in cotton elds of the Texas panhandle in the 1960's to 1970's before the increased use of glyphosate resistant cotton (S. Feagley, Soil and Crop Sciences Department, TAMU, retired, personal communication).…”

“…6). Waterbury et al (2019) found highest utilization of northwestern milkweeds for grassland/herbaceous cover. Asclepias latifolia favors grasslands, plains and prairies, trails, rocky canyon slopes, railways, and roadsides (Woodson 1954;Hart et al 2000;Nabhan et al 2015).…”

“…Phenology relative to monarch activity Brower et al (2018) found that early season monarch spring migratory activity in north central Florida coincided well with the vegetative phenology of the local milkweed monarch larval host, A. humistrata (pinewoods milkweed). Waterbury et al (2019) found that phenology of Idaho and Washington A. speciosa was also well synchronized with early spring monarch arrival and oviposition. Similarly, we found that both early and late season monarch activity was generally well coordinated with early and late season peaks in milkweed vegetative phenology of the four studied milkweeds in the South-Central US.…”

“…Ascelpias syriaca has signi cantly declined over the last century in conjunction with agricultural intensi cation (Boyle et al 2019), including recent increased use of glyphosate herbicides among corn and soybean crops (Pleasants 2017;Stenoien et al 2018). Waterbury et al (2019) found that A. speciosa, A. incarnata, and other Northwestern US milkweeds rarely utilized land covers of cultivated crops, developed, pasture/hay, and mixed and evergreen forest, and major habitat threats included invasive plants, herbicide use, and mowing. In comparison to data on A. syriaca in the Midwest (Hartzler 2010;Pleasants 2017;Kaul and Wilsey 2019) and A. speciosa in the Northwest, information is lacking on the utilization and risk of different LULC types for South-Central US milkweeds.…”

Further Insights on the Migration Biology of Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) from the Pacific Northwest

“…Until recently, unlike the eastern population, relatively little was known about many aspects of the biology and ecology of western monarchs [ 6 ]. However, research during the past five years has provided information on habitats, host plants, breeding [ 7 , 8 , 9 , 10 ], and migration [ 11 , 12 , 13 , 14 ]. Research has also helped document the continuing decline of western monarch populations [ 4 , 5 , 15 ].…”