Nectar plant abundance and richness are not primary drivers of monarch ovipositing patterns at a local scale

Background/Question/Methods

Choosing habitats with sufficient floral resources is essential for monarch butterflies (Danaus plexippus) because they require milkweed to lay their eggs and need to fuel their search for milkweed by feeding on a variety of nectar-rich forbs. It is becoming more difficult for monarchs to find adequate resources in their breeding range due to declines in milkweed and nectar forbs from land development and pesticide use.  Our research objective is to determine whether monarchs spatially separate or combine ovipositing and foraging activities within a habitat.  It is unknown whether monarchs prefer to oviposit on milkweed located near nectar forbs or prefer to oviposit and forage in separate habitats.  We examined whether monarchs laid eggs on milkweed more often when there were few or no nectar resources nearby (in grass monocultures) or when there were milkweed plants surrounded by abundant nectar resources (in prairies).  We planted two common species of native milkweed (Asclepias syriaca and Asclepias incarnata) in large, adjacent switchgrass fields and prairies at four locations in southwestern Michigan. We surveyed milkweed for monarch eggs and larvae daily in the summer of 2012 to examine the influence of habitat type or milkweed host species on number of monarch eggs oviposited.

Results/Conclusions

We observed three distinct generations of monarchs in the summer of 2012.  In the first generation, 35 total eggs were observed at three of four locations, in the second, 8 eggs were observed at one location, and in the third generation, 36 total eggs were observed at all four locations.  No significant differences in number of eggs observed were found between habitat type (switchgrass versus prairie) or milkweed species (A. syriaca versus A. incarnata) within each generation nor with all generations combined.  However, at each location there was often variation in monarch ovipositing between host species and habitat type.  After the first generation, a severe drought affected Michigan and the monarch's upper Midwest breeding range, with no rain falling in over a month, and there were high rates of milkweed herbivory by grasshoppers and mammals.  The drought and herbivory effects varied between our study locations in their impact on milkweed and nectar forb health and survival, possibly due to differences in soil type, moisture retention, and other site-specific factors.  Still, both before and after drought and herbivory, we found that presence of nectar plants did not drive differences in monarch ovipositing between switchgrass and prairie habitats.