Article: Grace M. Pitman, DT Tyler Flockhart, and D. Ryan Norris. “Patterns and causes of oviposition in monarch butterflies: implications for milkweed restoration.” Biological Conservation vol. 217 (2018): 54-65. Doi: https://doi.org/10.1016/j.biocon.2017.10.019
Habitat Decline and the Downfall of a King
Monarch butterflies, with their vivid orange and black wings, have become a symbol of summer and a classic example of a pollinator. However, in the last 20 years, monarch populations have declined by 95% largely due to the loss of milkweed plants throughout North America. Monarch butterflies exclusively lay their eggs on milkweed and milkweed numbers in the central Midwest have declined by 40% in the last two decades, largely due to the use of herbicides.
It would seem reasonable that planting more milkweed should lead to more monarchs, right? It may be more complicated than that. There are a lot of factors that influence which milkweed plant a female monarch chooses to lay her eggs on, from the size of a milkweed patch to the surrounding landscape to predators. In order to best help monarchs replenish their population, a study conducted by Pitman and colleagues sought to discover what the “best recipe” is for restoring milkweed in the central Midwest.
Two Summers Counting Milkweed and Monarchs
In the summers of 2015 and 2016, Pitman and her research team monitored a total of 26 sites – including 86 milkweed patches – in southwest Ontario, Canada. The sites were sorted into one of three different landscape types: 1) agricultural fields with pesticides applied, 2) non-agriculture sites such as gardens and meadows, and 3) roadsides. Monarch eggs and caterpillars, as well as their predators and parasites, were counted on every milkweed plant in the study area. The size and density of milkweed patches at the sites were also measured.
And the Results Were…
Monarch egg density was related to the size and density of the milkweed patch and the landscape type. Milkweed patches were smaller, denser, and had more eggs in agricultural fields than at roadsides. In general, the lower the milkweed patch density, the higher the density of monarch eggs on those milkweeds. The abundance of predators was not influenced by landscape type, but more predators were found in medium-sized milkweed patches. Parasite abundance was affected by both year and landscape type, with roadside milkweed patches having the fewest parasites.
The overall winner was agricultural fields – with their smaller milkweed patches supporting the greatest number of eggs. In a model, it was estimated that if all the small milkweed patches in this study area were removed, the number of eggs a female monarch butterfly could lay in its lifetime would decrease by 20%.
What Does this Mean for Monarch Conservation?
According to this study, planting and managing milkweed in agriculture fields seems like the most effective approach to restore milkweed, and thus support monarch conservation. Agricultural fields supported 3.5 times more monarch eggs than private gardens and roadsides. It is still unclear why milkweed in agricultural fields appears to be the preferential habitat for egg-laying. Perhaps it is because agricultural fields are sources by nectar-rich crops and flowers or their smaller milkweed patches have fewer predators and parasites. Or perhaps it is because roadside milkweed patches are prone to stress from road salt, invasive plants, car pollutants, and even mowing. Additionally, sites like gardens may be too diverse with flowers, which may make it difficult for female butterflies to locate the milkweed.
So, should we only plant smaller and less crowded milkweed patches? The authors caution that the small milkweed patches in agricultural fields could just be due to the use of herbicides, and previous studies have suggested that large expanses of milkweed patches are good for male monarchs to find mates. However, it seems like planting milkweed in or near agricultural sites is a good approach at helping to conserve monarch butterflies.