Mow the Milkweed

A spined soldier bug (Podisus maculiventris) feeding on a monarch caterpillar

Nathan Haan

Scientists estimate that the number of monarch butterflies (Danaus plexippus) overwintering in Mexico each year has decreased by more than 80 percent since the late 1990s. This decline is thought to be due in part to a loss of milkweed plants that make up monarch breeding habitat in the United States and Canada. A new study indicates that cutting down milkweed stems during their growing season may provide a potential avenue for increasing monarch numbers.

Biologists Nathan Haan and Douglas Landis at Michigan State University tested whether disturbing milkweed plants through mowing affected their suitability as monarch breeding habitats. Over the course of two summers, they made over 15,000 observations of milkweed stems in twenty-three sites—primarily abandoned agricultural fields and road or power line rights-of-way—near East Lansing, MI. In mid-June, they mowed one-third of the milkweed at each site, and in mid-July, they mowed another third. The final third of the milkweed was left undisturbed.

The researchers sampled hundreds of stems each week, tracking each stem’s growth stage and whether it contained any monarch eggs, new monarch larvae, and/or spiders and insects that prey on monarch eggs and larvae. They found significantly more eggs on new growth milkweed compared to undisturbed stems, with the highest peaks occurring a few weeks after the July mowing. Furthermore, recently mowed stems showed lower predator presence. The researchers speculate that fewer predators may lead to higher survival of monarch eggs, though they did not directly test this hypothesis.

The scientists did test whether mowing was an ecological “trap” that increased egg-laying rates but decreased egg survival. They placed monarch eggs on un-mowed and recently mowed milkweed stems and monitored survival for two to four days. They found that eggs survived equally well on new growth and undisturbed stems, suggesting that mowing did not present an ecological trap, at least in their study.

Future studies may explore whether there is an optimal time for cutting milkweed, and whether cutting milkweed has potential negative effects on other organisms or affects monarch interactions with predators. Results from these studies could provide best practices for increasing monarch populations. (Biological Conservation)