Monarch butterfly egg-laying behaviour: implications for restoration
November 16, 2018
To a monarch butterfly
You who go through the day
like a winged tiger
burning as you fly
tell me what supernatural life
is painted on your wings
so that after this life
I may see you in my night
-Homero Aridjis (Translated by George McWhirter)
The monarch butterfly (Ascelpias syriaca) is loved by many across North America, so much so as to inspire poems such as the one above by Mexican poet and monarch activist, Homero Aridjis. In addition to their beauty and cultural significance, monarchs hold an important ecological role as pollinators. The eastern North American population however, is in serious decline with a loss of 95% in only the past 20 years. Concern that these monarchs may disappear, taking with them both their ability to inspire people and to act as pollinators, is unfortunately well warranted.
As with other species around the globe that are decreasing, habitat loss is a likely offender. In particular, milkweed plants, which are obligate host plants for monarch larvae, have been disappearing due to the use of glyphosate herbicides on agricultural fields to kill weeds. This issue has been exacerbated over the years with a rise in soy and corn crops that are genetically engineered to be resistant to the herbicide while other plants, such as milkweed, are not.
Pitman et al. (2018) launched a study examining female monarch egg-laying preferences in terms of milkweed habitat. One of their main goals was that results from their research could help inform milkweed restoration planning. Their study took place over two summers (2015 and 2016) in southwestern Ontario and included the north shore of Lake Erie, a major migration pathway for monarchs on their way to and from their overwintering grounds.
The researchers counted the number of monarch eggs on more than 30,000 milkweed stems found in eight agricultural crop fields (growing herbicide-treated corn or soy), nine non-agricultural, and nine roadside areas and measured the size and density of milkweed patches. They also quantified important threats to monarchs such as the abundance of invertebrate predators and parasitoids in the milkweed patches and the occurrence of a single-celled protozoan parasite. In terms of predation, early life stages are particularly vulnerable having an estimated 88 – 98% mortality rate.
Overall, agricultural lands with small, low density milkweed patches had the highest egg densities and may present the best option to attract egg-laying female monarchs. There was no evidence that predator and parasitoid abundance and occurrence of the protozoan parasite were particularly associated with this type of habitat. In order to restore milkweed habitat on agricultural lands, it would be important to work with landowners such as through existing pollinator or ecosystem services incentive programs (e.g., Alternative Land Use Services; ALUS).
The authors also suggested that non-agricultural areas may be the second most suitable landscape since egg densities were second highest. Milkweed restoration along roadsides was the least desirable approach as the authors found the lowest egg densities in this landscape. Roadsides can present hazards to butterflies due to vehicle collisions, exposure to heavy metals emitted from cars, road salt runoff, and regular mowing of roadside vegetation.
Read full article here (journal subscription required):
Pitman, GM, Flockhart DTT, Norris DR. 2018. Patterns and causes of oviposition in monarch butterflies: Implications for milkweed restoration. Biological Conservation. 217:54-65.