Less-extreme heat waves can still stunt insect growth

April 24, 2021

Your tomato plants might appreciate this insect taking a hit, but what does it mean for the environment? (Wikimedia/Daniel Schwen)

Repeated exposure to sublethal heat waves can severely impede the development of the relatively hardy tobacco hornworm larvae, according to a new study that adds to a growing body of literature describing how the insect kingdom is faring as the world warms.

Where most research on insect response to extreme weather has focused on brief but intense heat waves or floods, the authors of this paper wanted to measure the impact of longer heat waves at more realistic temperatures, which are far more frequent than their hotter counterparts. Their article was published April 6 in the Journal of Experimental Biology.

"People have been paying more attention to these sort of repeated sublethal events where one exposure doesn't have a lot of harm necessarily, but when you experience it repeatedly, in fact, it does," said lead author Joel Kingsolver, a Kenan distinguished professor of biology at the University of North Carolina, Chapel Hill. "It can reduce your survival, or your growth rate, or your development — or all those sorts of things."

The tobacco hornworm is an enormous caterpillar that weighs up to 15 grams — more than a hummingbird. It was an ideal model insect for this study because as it is a relatively heat-resistant insect, whatever happens to one during prolonged heat waves is likely to happen in other, less heat-tolerant species, as well — if not more so, according to Kingsolver. 

The researchers chose to examine tobacco hornworm larvae because insects spend a much larger period of their short lives in a larval stage than in any other period, Kingsolver added.

In the study, he and his colleagues defined prolonged heat waves as any stretch of three days where temperatures reached anywhere between 40 and 44 degrees Celsius. They divided hornworms into groups that experienced prolonged heat waves: three groups experienced one three-day heat wave, another three experienced two three-day heat waves and one other group baked for three three-day heat waves. A control group experienced none. 

To carry out their study, the researchers kept the experimental hornworm groups in a background chamber set to mimic daily fluctuations in temperature, peaking at 35 degrees Celsius. At different stages, they moved these groups into a heated environmental chamber that simulated heat waves, removing them after three days. 

Compared with the control group, hornworms exposed to three three-day heat waves were about 1 gram smaller — a 20% reduction in size. On the other hand, they found that single heat waves promoted hornworm growth.

"A single heat wave can actually speed up development, and growing faster can be a good thing," said Kingsolver. "It means you're not hanging out in the field, exposed to predators and parasites and other sorts of things."

On the other hand, hornworms exposed to two heat waves did not die at a greater rate than the control group. In fact, they even developed into moths slightly faster: about 22 days to the control's 24.

"We were actually surprised that single heat waves didn't have more impact," Kingsolver said in an interview with The Academic Times. "I mean, three days where it gets 42 degrees centigrade, around 104 degrees Fahrenheit, or something like that? That's hot."

Because tobacco hornworms depend on external energy to regulate their body heat, the same principle that accelerated growth also promoted higher mortality among those that saw longer heat waves. According to Kingsolver, high temperatures speed up hornworm metabolism. But when temperatures get too hot for too long, they can overload the hornworms with oxidative stress, which can either impede their growth or kill them.

These findings may have significant implications for agriculture, as the tobacco hornworm is a common pest of tobacco and tomato plants. Another recent paper co-authored by Kingsolver shows that parasites that mitigate hornworm populations are not as heat-tolerant as the hornworms themselves. Heat waves that do not harm hornworms seem to kill their parasites; one of these is Cotesia congregata, a small wasp species that lays its eggs inside the bodies of live hornworms. For future research, Kingsolver plans to continue investigating how heat waves disrupt these host-parasite interactions.

"If parasites can't tolerate heat as well as the things that they attack, then all those parasites could disappear," Kingsolver said. "We don't know if that's generally true, but now we have a number of cases where that is true for insects. Parasitoids are the most common biocontrol agent for insect agricultural pests, so wiping out parasitoids is a bad thing for controlling pests."

The study, "Responses of Manduca sexta larvae to heat waves," published April 6 in the Journal of Experimental Biology, was authored by Joel G. Kingsolver, M. Elizabeth Moore, Kate E. Augustine and Christina A. Hill, University of North Carolina at Chapel Hill. 

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