The migratory population of monarch butterflies in Eastern North America has declined precipitously. Causes are likely multifactorial, but changes to agricultural disturbance regimes in the US Midwest have been strongly implicated. Since the 1990s, the vast majority of corn and soybean production in the region has shifted to the use of transgenic crops that tolerate broad-spectrum herbicides. This change effectively removed the monarch’s milkweed host plants from crop field interiors, where monarchs preferred to oviposit. Monarchs now rely instead on milkweeds in abandoned fields, reserves, and roadsides. These habitats likely contain more predators than field interiors. Furthermore, in crop fields, disturbance from mechanical cultivation and other activities may have caused milkweeds to resprout midseason, resulting in plants that were more phenologically attractive.
We tested for effects of disturbance, in the form of strategic mowing, on monarch oviposition patterns and on monarch predator abundance. We predicted that regenerating milkweed stems would be more attractive to ovipositing monarchs, and would contain less predators, providing enemy-free space for eggs and larvae. In a field experiment, we divided several milkweed patches into three sections; one was unmowed, one was mowed in mid-June, and another in mid-July. We monitored oviposition rates and predator abundance weekly.
Results/Conclusions
Monarchs laid substantially more eggs on milkweed stems in mowed areas compared to un-mowed; they laid approximately ten times as many eggs in July-mowed milkweeds compared to those that were not disturbed. Monarch predators were also suppressed on regenerating milkweed stems; after stems re-sprouted, predators were initially very rare and took 4 to 5 weeks to recolonize to pre-disturbance levels. Preliminary data also suggest survival of eggs and early-instar larvae in mowed areas is equal to or higher than in un-mowed. Adult fecundity is probably limited by time rather than the number of eggs they produce (i.e., they typically die before running out of eggs); therefore, disturbance with targeted mowing could enhance fecundity by providing phenologically attractive plants for oviposition. It may also provide enemy-free space for eggs and larvae. Future work should weigh the risks and benefits of strategic mowing (or other types of disturbance) in monarch breeding habitat in the US Midwest. Potential risks include undesirable effects on non-target species, and the possibility of an ecological trap if regenerating milkweeds attract ovipositing adults but result in reduced larval survival.