Falling Temperatures do not Necessarily Mean Fewer Insects

By Richard Levine

With record-breaking cold temperatures in much of the United States recently, newspaper headlines have suggested that the freezing weather this winter could mean fewer insects next spring. For example:

- The New York Times: “Celebrating Deep Freeze, Insect Experts See a Chance to Kill Off Invasive Species”

- National Public Radio: “The Upside Of The Bitter Cold: It Kills Bugs That Kill Trees”

- Minnesota Public Radio News: “Extreme cold may wipe out high percentage emerald ash borer larvae”

However, to borrow a quote from Mark Twain, reports of insect deaths have been greatly exaggerated. While it’s true that insects will die if exposed to very cold temperatures for prolonged periods of time, many are able to survive, depending on the insect and the circumstances. Fortunately, the journalists who wrote the articles above balanced the optimistic headlines with more realistic views in the actual text after doing their research and talking to entomologists.

It’s Déjà vu All Over Again …

This is nothing new, of course. In fact, two years ago we faced a very similar situation when an extraordinarily mild winter gave rise to headlines about how the warmer temperatures would mean greater insect populations come spring. Mosquitoes for example would thrive, the thinking went, because of the lack of freezing temperatures.

However, leaders of the Entomological Society of America said “Don’t Bug Out Over Warmer Weather” in a press release, explaining that lots of other factors affect insect populations besides temperatures.

“States like Alaska and Minnesota are famous for their brutally cold winters, and yet they are also known to have extremely active mosquito populations during the summer,” said 2012 ESA President Grayson Brown, who explained that mosquitoes are even more affected by the amount of rain during the spring, since they need water to lay their eggs.

ESA’s 2012 Vice President Robert Wiedenmann said that in some cases the warm winter could even cause harm. “Some insects that emerge earlier than normal because of warm temperatures may not find the appropriate food sources available and could starve,” he said. “Likewise, mild winters may favor the predatory or parasitic insects that help keep pests in check, and result in fewer pests. Insect ecology is affected by a number of factors and is not solely dependent on winter or spring temperatures.”

Long Story Short: It’s Complicated …

Which bring us to our current situation. While it’s true that extremely cold temperatures for prolonged periods of time can decrease insect populations, other factors are at play as well.

The Emerald Ash Borer (EAB) is a good example, since it has received so much press recently. Ironically, the recent cold spell could actually end up helping the EAB in certain areas because the freezing temperatures might harm EAB predators.

A few years ago, scientists in Michigan and other states began releasing tiny parasitoid wasps that help control the EAB by laying eggs into or on the EAB larvae.

“In general, parasitoids are more susceptible to stressors (e.g., pesticides, cold temperatures) than their hosts,” said Dr. Jian Duan, one of the scientists who has been rearing and releasing the wasps. “If this year’s cold temperature kills overwintering EAB larvae, it will surely kill the associated parasitoids — even more so than EAB.”

“Prolonged very cold temperatures can definitely kill off both EAB and the parasitoids, and the parasitoids appear to be less cold-hardy than the EAB themselves,” said Dr. Jonathan Lelito, another USDA researcher.

Dr. Lelito went on to explain that even in extremely cold regions like northern Minnesota and parts of Canada, where a significant portion of EABs may have died because of the cold, the effect will not extirpate the species completely.

“Even with 50% mortality, the populations will recover in a few years or so and the infestation will continue on,” he said. “But biological control is a long game. Occasional setbacks will occur, and the populations of both hosts and parasitoids will tend to oscillate through time anyway. The long-term goal is the establishment of a balance, and severe weather events are just a step in the long march, so to speak.”

The same holds true for other insects. Once again: It’s complicated.

——————–

Richard Levine is Communications Program Manager at the Entomological Society of America and editor of the Entomology Today Blog.

Comments

  1. There’s a lot of nuance that goes into understanding the cold tolerance of the EAB parasitoids, especially when trying to compare their cold tolerance to EAB itself. One species looks to be about as cold tolerant as EAB, and another appears to be in the same boat so far based on preliminary data. One species does appear less cold hardy than EAB though. It’s definitely a complicated story:

    http://www.sciencedirect.com/science/article/pii/S1049964413001965

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