Research shows the effects of hotter fall temperatures on insects

Climate change has devastated the life cycles of many species, and now two western students examine how it’s affecting the survival of two high-profile insects.

Led by biology professor Jeremy McNeil, graduate student Campbell McKay is studying how feeding different spurge species might affect the likelihood of fall-born monarch butterflies migrating to their wintering grounds in Mexico.

In recent years, sustained summer temperatures have meant that some fall butterflies mate rather than migrate, and their offspring — if they survive — emerge late for their own good.

Meanwhile, graduate student Cailyn McKay (unrelated) shows how higher temperatures could kill the true armyworm, a crop pest that farmers are dying to get rid of.

First the armyworms. These insects spend the winter in the southern United States and migrate to Canada and the northern United States in the summer. The adult moths lay their eggs on grasses and grains, and when the eggs hatch, the larvae begin to feed on the plant. They eat large holes in leaves and can consume entire fields.

Their Achilles’ heel is that temperatures above 30°C are not good for them. In hot weather, larvae and moths may move in search of cooler places, but in the pupal stage between larva and moth, the insect is vulnerable. Without legs or wings, they have no way of escaping the heat. “The dolls are kind of stuck,” Cailyn McKay said.

As more and more days reached 30°C or higher in southwestern Ontario, McKay wondered what effect prolonged heat would have on the reproductive ability of moths exiting the pupal stage.

In the laboratory, she exposed some pupae to 30°C for 48 hours at four different times during their metamorphosis. She then compared the reproductive ability of the hatching moths to that of moths that had not survived longer periods of 30°C. Not only did the moths from heat-treated pupae produce fewer eggs, but those eggs were also less fertile.

McKay also dissected the females to count the spermatophores in them. These are packets of sperm and nutrients that the male transfers to the female during mating. The shells remain in the female’s body.

This allowed McKay to determine how often females had mated, and found that those exposed to high temperatures mated less frequently. She hopes that the next step in her research will uncover the mechanism behind this rarer pairing. She and McNeil suspect it has something to do with how heat affects the pheromones released by both females and males.

“Females release pheromones to attract a mate,” McNeil said, “and upon arrival males produce an odor that the female uses to decide whether or not she will accept him.”

While reducing armyworm numbers might seem like a good thing to farmers, the insects are native to Canada and are an important part of our ecosystem, McKay said.

“These guys are non-invasive. They’re only really bad when they appear in large numbers. Birds and spiders eat them, so their absence could have a knock-on effect on other species.”

The relative absence of monarch butterflies is already a cause for concern. This summer, the International Union for Conservation of Nature declared the once-ubiquitous insect endangered. One factor could be longer-lasting summer temperatures, which could signal the butterfly that it’s time to breed rather than migrate to Mexico for the winter.

Most eggs laid in the fall die as larvae or pupae, and those that survive to become adults “are often deformed and unlikely to migrate,” Campbell McKay said.

For the very few normal adults who turn up in late October or early November, it’s too late to make it south before the cold kills them. They are a dead end population.

However, temperature is not the only factor: the quality of the host plant also influences the development of the monarchs. While their larvae only feed on milkweed, Ontario has several different species of the plant that vary in quality.

Campbell McKay examines how the diet of each of the three common spurge species affects the size of the cul-de-sac population of monarchs in a given year. He suspects that at least one of the milkweed species may be slowing monarch development and causing the butterflies to emerge later in the year.

He hopes his results, expected by the end of the year, will help gardeners choose which spurge species to plant. “[And] If we can understand what happens to the monarch, we might also better understand what happens to other migratory insect species,” he said.