This is a guest post by Frank Lab postdoc Michael JustInsects, like people, prefer warmer weather. For people, warmer temperatures might mean more trips to the park or beach. When it’s colder we can add a layer or adjust the thermostat in our homes to be comfortable. Insects have some tricks of their own to prevent getting too hot or too cold, but, like a sweater in a blizzard, these tricks only work up to a point. These tricks contribute to what scientists call the thermal tolerance of an insect species.
What is thermal tolerance? Basically, it’s the range of temperatures in which an insect is able to survive; not too hot or too cold, but just right. This tolerance differs among insects and we don’t know the tolerance for most insects. But when we do know the thermal tolerance of an insect (figured out through experiments), we can use this information in our predictions about where it might go as the climate changes and also how to better manage insects that are or might be pests.
For many of the insects we consider pests of our backyard and street trees, cold winters are probably what prevents them from spreading further north from where they are right now. That’s because they are less likely to survive low winter temperatures. Recently, our winter temperatures have become warmer. As a result, some pests have been moving north and staying put. They are able to survive in these warmer temperatures because they fall within their thermal tolerance.
Gloomy scales are a red maple pest found from New York to Florida, but are most abundant in the middle of that range in places like Raleigh, NC. However, red maples are present from Canada to Florida, so why aren’t gloomy scales found on all of these trees? What makes Raleigh just right (not too hot, not too cold)? Could it be that the more northern winters are too cold and outside of their thermal tolerance? To examine this, we conducted a series of laboratory experiments to estimate the thermal tolerance of gloomy scales (both hot and cold tolerance).
We tested scales collected in Newark, DE, Raleigh, NC and Gainesville, FL, which approximately represents the northern, middle, and southern parts of the gloomy scale range. We found that scales were most tolerant of temperatures from Raleigh, NC, a city where they are most abundant. We also found that scales from Newark, our northernmost city, could tolerate the coldest experimental temperatures (-4.7°C; the average January low temperature for Newark) for twice as long as the scales from Gainesville.
What about warming temperatures in the south? In general, warmer temperatures are better for insects. Previous work in the lab has shown that gloomy scales are more abundant in warmer areas. But temperatures can be too warm for insects too. For example, in this study, we found the fewest scales in our southernmost city, Gainesville. So, we do not think that additional heat will benefit gloomy scales on street trees in southern cities. However, warming temperatures in forests at these southern latitudes may allow gloomy scales to become pests in forests, which are cooler than cities at the same latitude (we discuss the possibility of city tree pests becoming forest tree pests in a recent paper).
So, while gloomy scale may be able to expand into new northern cities with additional warming, they might also retreat from too-warm cities in the south if the temperatures don’t align with their thermal tolerance.
Read the full paper here:
Just, M.G., Frank, S.D., 2020. Thermal tolerance of gloomy scale (Hemiptera: Diaspididae) in the eastern United States. Environmental Entomology. 49, 104–114. https://doi.org/10.1093/ee/nvz154.