Research shows the effects of low temperatures on insects

Monarch butterfly on flower. Credit: Jeremy McNeil

Climate change has wreaked havoc on the life cycles of many species, and now a pair of Western students are shedding light on how it affects the survival of two prestigious insects.

Working under the direction of biology professor Jeremy McNeil, master’s student Campbell Mackay investigates how feeding in different species of milkweed affects the potential for the migration of fall-born Monarch butterflies to winter regions in Mexico.

In the past few years, lingering summer temperatures It has led some autumn butterflies to mate rather than migrate, and their offspring – if they survive – emerge too late for their own good.

Meanwhile, masters student Kaelen Mackay (unrelated) shows how rising temperatures can wipe out true armyworm, a crop pest that farmers desperately want to get rid of.

Research shows the effects of low temperatures on insects

Armyworm crawls away over the plant. Credit: Wikimedia Commons

First, army worms. These insects winter in the southern United States and migrate to Canada and the northern United States in the summer. Adult butterflies lay their eggs on weeds and grains, and when the eggs hatch, the larvae begin to feed on the plant. They chew large holes in the leaves and can consume entire fields.

The Achilles heel is that they do not do well in temperatures above 30 degrees Celsius. In hot weather, larvae and moths can move in search of cooler locations, but in the pupal stage between the larva and the moth the insect is vulnerable. It lacks legs or wings, and has no way out of the heat. “Virgins are kind of stuck,” said Kaelen McKay.

With the number of days reaching 30°C or more in southwestern Ontario, Mackay questioned the effect of prolonged heat on the reproductive ability of pupal stage moths.

In the lab, some pupae were exposed to 30 °C for 48 hours at four different periods during their transformation. Then she compared the ability of the emerging mites to reproduce with that of mites that did not tolerate long periods of up to 30 degrees Celsius. Not only did the mites from the heat-treated pupae produce fewer eggs, but those eggs were less likely to be fertile.

Mackay also dissected the females to count the number of sperm inside them. They are packages containing sperm and nutrients that the male transmits to the female during mating. The casings remain in the female’s body.

In this way, Mackay was able to determine how often the females had mated, and found that those exposed to high temperatures mate at a lower rate. She hopes the next step in her research will reveal the mechanism behind this less frequent mating. She and McNeill suspect it has something to do with how heat affects the pheromones released by both females and males.

“The females release pheromones to attract a mate, and upon arrival the males produce a scent that the female uses to decide whether or not to accept him,” McNeill said.

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

“These guys are not invasive. They are really bad when they come in large numbers. They are eaten by birds and spiders so their absence may have a ripple 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-popular insect endangered. One contributing factor could be long-lasting summer temperatures, which could signal the butterfly that it is time to breed rather than migrate to Mexico for the winter.

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

For the very few normal adults that appear in late October or early November, it’s too late to get south before the cold kills them. They are dead end residents.

However, temperature is not the only factor: the quality of the host plant also affects the development of monarchs. While its larvae feed only on milkweed, there are several different types of the plant in Ontario and they vary in quality.

Campbell Mackay investigates how the feeding of each of the three common species of milkweed affects the size of the kings’ inhabited populations in any given year. He suspects that at least one species of milkweed may slow the growth of monarchs and cause butterflies to appear later in the year.

He hopes that his findings, expected at the end of the year, will help gardeners choose species milkweed Planting. “[And] If we can understand what happens to the king, we may better understand what happens to other migratory insect species.”


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