Drought Influences Monarch Host Plant Selection

Monarch butterflies (Danaus plexippus) rely on cues from larval host-plants, milkweeds (Asclepias) to decide where to lay their eggs, a choice that matters because newly hatched caterpillars have limited ability to move between plants. Because water availability can alter plant traits, we asked whether water limitation changes how female monarchs select narrowleaf milkweed (Asclepias fascicularis) for oviposition (egg-laying).

In choice trials, female monarchs laid about 40% more eggs on reduced-water plants than on well-watered plants. We expected this preference might be explained by changes in plant secondary chemistry, the specialized chemical compounds plants produce for functions such as defense and signaling. In particular, we were interested in flavonol glycosides, which are a group of compounds used by monarchs as oviposition cues. Instead, the measured plant secondary metabolites did not differ between water treatments. The females could distinguish between the plants, but the trait driving that preference remains unclear.

These results show that plant growing conditions can influence monarch host-plant selection even when the underlying cue is not obvious. As water limitation becomes increasingly common across the western United States, understanding how environmental conditions alter interactions between monarchs and milkweed may be important for habitat restoration and conservation.

This work, which was done with undergraduate assistant Konnor Kost and my PhD advisor Beth Pringle, was published in the Journal of Insect Science in 2023 as “Plant water limitation and its impact on the oviposition preferences of the monarch butterfly.”