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

Abstract

South-Central US milkweeds (Asclepias spp.) are critical adult nectar and larval food resources for producing the first spring and last summer/fall generations of declining eastern migratory monarch butterflies (Danaus plexippus). MaxEnt niche models were developed for North American ranges of four important South-Central US milkweeds: Asclepias asperula ssp. capricornu, A. viridis, A. oenotheroides, and A. latifolia. Twelve models per species utilized subsets of six to eight of 95 edapho-topo-climatic variables chosen by a random subset feature selection algorithm. Milkweed weekly phenology was compared between early and late season periods of monarch activity. Novel land cover preference risk assessments were developed for milkweeds through land cover utilization-availability analyses, incorporating a novel sample bias reduction method for citizen science data before calculation of relativized electivity index (E*) land cover preference. Asclepias a. ssp. capricornu and A. viridis occurred more frequently during early season monarch activity, while A. oenotheroides and A. latifolia occurred more frequently during late season monarch activity. Milkweed utilization of roadsides varied from 6–31%. Developed-Open Space and Grassland Herbaceous land classes generally had highest benefit among milkweeds. Cultivated Crops and Shrub/Scrub had high risk. Combined milkweed high Ei* kernel density estimation surfaces resolved interior and coastal corridors of milkweed land cover preference providing functional connectivity for the monarch spring and fall migrations. A potentially critical gap in milkweed land cover benefit connectivity was identified in South Texas. Milkweed land cover preference risk assessments can be used to prioritize milkweed habitat conservation for enhancing monarch migration connectivity across the South-Central US.

Journal of Insect Conservation Survival of eggs to third instar of late-summer and fall-breeding monarch butterflies (Danaus plexippus) and queen butterflies (Danaus gilippus) in north Texas

“…Majewska While the cultivation of exotic milkweeds should be discouraged, the importance of fall monarch reproduction on native milkweeds should not be ignored. Native milkweeds, most importantly A. viridis, are widely available in the southern plains in late summer and fall (Calvert 1999;Tracy et al 2021). As such, fall reproduction is a normal aspect of the species' biology and likely represents an adaptive trade-off between the bene ts of current reproduction and those of continuing migration to Mexico.…”

“…It has been proposed that the widespread cultivation of A. curassavica might be disrupting migratory patterns of monarchs (Malcolm 2018; Clement and Crawford 2020) and increasing their exposure to parasites (Satter eld et al 2015; Malcolm 2018).However, fall reproduction is also likely to be a normal part of the phenotypic plasticity of monarchs. Native milkweeds are widely available in late summer and fall in the southern U.S (Tracy et al 2021(Calvert 1999).…”

“…It would be useful to replicate the Georgia study using A. viridis because, unlike the other native species mentioned above, the cardenolide content and the cardenolide polarity pro les of A. viridis and A. curassavica are very similar (Rasmann and Agrawal 2011). Furthermore, A. viridis is abundant in Texas and Oklahoma in the fall (Baum and Sharber 2012; Dee and Baum 2019;Tracy et al 2021) and at the landscape scale much more abundant than A. curassavica(Calvert 1999). Our study site contained only A. viridis and it seems unlikely that exposure to A. curassavica was an important factor in stimulating reproduction on our study site.…”