Monarch butterfly (Danaus plexippus) migratory populations in North America have declined over the last 20 years. Milkweed (Asclepias spp.) is the primary larval host plant for monarchs. A key conservation strategy for monarchs has been to create habitat, with a focus on milkweed seeds and plugs. However, not all milkweed may offer the same benefits for monarch conservation. Milkweed taxa may vary in a number of ways, including amount of nutrients, protective toxins, and latex production, which may make some taxa more suitable hosts for the monarch. While some comparisons of milkweed suitability have been made for taxa in the midwestern and eastern United States, very little is known about milkweed in the southwestern United States. Although understudied, the Southwest is an important part of the migration route. Declining populations have led to a focus on conservation efforts and habitat creation in this region, making it even more critical that we understand how monarchs are using milkweed taxa. We tested female egg-laying preference and larval performance in captivity on two species of milkweed commonly available for habitat projects in the Southwest: Arizona milkweed (Asclepias angustifolia) and pineneedle milkweed (Asclepias linaria). These species also have a natural range overlap. We tested four measures of larval performance: survival, duration, mass, and adult forewing length.
Results/Conclusions
Monarchs were more likely to lay eggs on A. angustifolia than A. linaria. A mean of 94.3% eggs were laid on A. angustifolia, while a mean of 5.6% eggs were laid on A. linaria over the course of the experiment. Larvae had a higher survival rate and grew faster when raised on A. angustifolia as opposed to A. linaria. The mean proportion of larvae surviving was 0.79 on A. angustifolia and 0.4 on A. linaria. Additionally, adults from larvae raised on A. angustifolia were larger (mean forewing length = 45.1 mm) than those raised on A. linaria (mean forewing length = 41.8 mm). Field data also showed that monarch caterpillars were more likely to be found on A. angustifolia. We recommend A. angustifolia for habitat conservation projects in the Southwest, within the natural range of A. angustifolia, and specifically where the range of A. angustifolia overlaps with A. linaria. We also recommend monarch preference and performance be tested on additional western milkweed species. Ongoing research continues to evaluate how these preferences may change under shifting climate regimes in the future.