In 2018, populations of the monarch butterfly (Danaus plexippus) in the West declined ~95%, putting them in quasi-extinction mode. This study attempts to better understand the life cycle of monarchs and their host plants, milkweed (Asclepias spp.), to aid in monarch conservation. It has been suggested that monarchs select host plant species with lower toxicity, which negatively impacts the survival rate of larvae. To test this, I observed monarch behavior in relation to two species of milkweed both native to Utah; showy milkweed (A. speciosa), which has high physical defenses, and swamp milkweed (A. incarnata), which has high chemical defenses. As showy milkweed has lower chemical defenses, I hypothesized that monarchs would select this species for oviposition over swamp milkweed and that this would result in lower larval survival. To determine a species host preference, I counted eggs and caterpillars found on swamp and showy milkweed at four sites. An estimation of larval survival was made by counting the number of caterpillars found in their fifth instar and assuming that they made it to adulthood. I also observed milkweed phenological stage, apparent health, and presence of pests (aphids, blue milkweed beetles, and red milkweed beetles) in order to better understand which variables may play a role in monarch host selection.
Results/Conclusions
Monarch butterflies showed a significant preference for showy milkweed with 9.67% of milkweed selected vs 5.83% in swamp (x2=7.9012, df=1, p-value=0.0049). Offspring had significantly higher survivorship when reared on swamp milkweed with 7.04% larval survival vs 0.95% in showy (x2=4.7706, df=1, p-value=0.0289). There was no significant preference shown in regards to milkweed phenological stage or health, however in both species and at all four sites monarchs selected milkweed individuals in above average health and in a vegetative stage most frequently. The presence of blue and red milkweed beetles did not seem to impact monarch host preference or milkweed health. Aphids were found on both species of milkweed at an equal rate, however swamp milkweed with aphids had significantly reduced health (H=10.421, df=1, p-value=0.001) while the same was not true for showy milkweed (H=2.092, df=1, p-value=0.148). Eggs/caterpillars were found on plants with aphids significantly more when observing swamp milkweed (x2=8.0024, df=1, p-value=0.0047). These results indicate that monarch butterflies are making maladaptive choices for oviposition host species which is negatively impacting offspring survival. Presence of aphids does not seem to impact selection, but might increase larval survival rates.