TORONTO -- Efforts aimed at reversing a steep decline in the monarch butterfly population by planting traditionally unloved milkweed need to take into account the size and location of the planted patches, new Canadian research indicates.
The study, out of the University of Guelph, finds that restoring milkweed along roadways is less effective than doing so on agricultural lands. Milkweed is critical to the survival of the monarch population because it's believed to be the only plant the butterflies feed on as caterpillars.
"Roadside patches, which received half the number of eggs compared to agricultural landscapes, may potentially pose a number of threats to monarchs because of vehicle collision and accumulation of noxious chemicals," the study's authors write.
"A better strategy may be for managers to develop incentive programs with landowners to plant and maintain milkweeds in agricultural landscapes."
The research, published in the journal Biological Conservation, looked at what monarch butterflies prefer when it comes to laying eggs on milkweed.
Milkweed plants on farm land, according to the two-year study carried out primarily by now graduated master's student Grace Pitman, have more than three times the amount of monarch eggs than those growing in urban gardens and roadsides. That might be because female monarchs can find the plants more easily in agricultural fields.
"Based on these findings, it will be important to develop programs with landowners and other pollinator initiatives or ecosystem service programs to actively restore milkweed in agricultural landscapes," the authors write. "Ideal areas for planting milkweed patches are crop margins, field corners, and other marginalized cropland within close proximity to crop fields."
At the same time, small patches in gardens and urban parks can still be useful for adult monarchs to lay eggs and find nectar for themselves, the paper states.
Most monarchs migrate thousands of kilometres along various routes through the United States between Mexico and Canada, where they are mostly found in southern Ontario, southwestern Quebec, and the most southerly parts of Manitoba and British Columbia. They are considered an indicator species of the overall health of the environment.
However, monarch populations are estimated to have plunged by 90 per cent or more across North America over the past two decades. Part of the decline has been blamed on the attempted eradication of the otherwise prolific milkweed, which has traditionally been viewed, or officially listed, as a noxious weed, even poisonous to livestock.
For several years now, conservationists have been pushing milkweed restoration as a way to counter the monarch decline, arguing the plant can easily be controlled in an agricultural setting. Roadsides, which are plentiful in the monarch range, have featured prominently in the restoration push.
"It looks like it's not the best place to attract females to lay eggs," study co-author Ryan Norris, an associate professor in the department of integrative biology, said from Guelph, Ont., on Wednesday. "Whether females don't like roadside habitats, or they do like them but are getting killed, is something that needs to be looked at further."
Small, low-density milkweed patches of under 16 square metres in agricultural landscape appear to be most useful, in part because they tend to be home to fewer predators that feast on monarch eggs or larvae than medium-sized patches, the study finds. Large patches may be important for male monarchs to search for mates, but they might interfere with females trying to lay eggs.
One big unknown, Norris said, is whether more eggs translate into more adult monarchs and what impact pesticides might have on survival rates.
"That's a trickier thing to measure," Norris said. "It's much harder than going around counting eggs."
