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Khapra Beetle Can’t Beat the Heat

khapra beetle - Trogoderma granarium

The khapra beetle (Trogoderma granarium) does outsize damage to stored grains and is a top target as a potential invasive species at ports and border crossings. Researchers in Canada have found the threshold temperature that will kill the beetle at all life stages, even diapause. (Image credit: Pest and Diseases Image Library, Bugwood.org)

By Ed Ricciuti

Canadian scientists are literally turning up the heat on a tiny, brownish beetle that, only a couple of millimeters long, causes outsize damage to cereals, grains, and other commodities in storage. Thought to be from India, the khapra beetle (Trogoderma granarium) has spread over much of southern Asia and Africa and is cropping up elsewhere, and researchers are working to improve methods to keep it from invading new territory.

Ed Ricciuti

Ed Ricciuti

Controlling insect damage to cereal crops to keep them marketable does not stop with the harvest. It continues in storage, where grain is vulnerable to pests such as the khapra beetle, which causes up to 70 percent spoilage in stored cereals that it infests.

The old standby for killing the beetle in grain, the chemical fumigant methyl bromide, has been phased out because it depletes the Earth’s ozone layer, not to mention that it is highly toxic to humans. As an alternative, scientists at the University of Lethbridge, the Canadian Food Inspection Agency, and Agriculture and Agri-Food Canada are exploring the use of temperatures high enough to kill the beetle as an environmentally safe method of control. Their research was published in December in the Journal of Economic Entomology.

Making things too hot for insect pests—by using gas, electric or steam heaters—has become an increasingly popular option for treating grain in storage and quarantine, but there is a significant challenge. As insects undergo metamorphosis, whether of the complete or incomplete variety, their life stages can be so different from one another that they might not appear to be the same insect. Think crawling caterpillar and winged butterfly. The difference goes beyond appearance and behavior, though. It can be physiological, which can mean that a management method that works well on one stage may not on others. It turns out that the khapra beetle is highly resistant to heat, and insecticides, while in diapause.

During diapause, an insect is dormant. Its development is in suspended animation or frozen in time, so to speak, and it is less susceptible to unfavorable environmental conditions. Diapause is one of the ways insects escape threats to survival, such as extremes in moisture and temperature, crowding, and isolation, which are among the external cues that trigger it.

Since stored grain can contain all stages of a targeted insect, sufficient heat must be applied to kill it at any point in its life cycle, including during diapause. Most insects enter diapause according to a predetermined pattern, but some khapra beetles can initiate what is called “facultative diapause”—diapause on demand, essentially—to avoid unfavorable conditions. It happens during their fifth and final instar and can last for up to three years.

The researchers were faced with a puzzle of sorts. Heat can trigger diapause but, at a certain point, it can also kill beetles undergoing diapause. The goal was to find that point. To pinpoint temperatures that would kill the beetle in diapause, as well as the rest of its stages, the researchers triggered diapause in an experimental colony of khapra beetles by overcrowding them, then subjected them to a range of high temperatures.

“If we control the most resistant stage, all other stages should be controlled,” says lead author Diana Wilches, a technician with the Canadian Food Inspection Agency. “Kicking them out of diapause, then hitting with heat would be ideal, but we don’t understand their diapause that well. We have a study working on diapause termination,” she says.

The research reported in the paper indicates that exposing the beetles to a temperature of 60 degrees Celsius (140 degrees Fahrenheit) does the trick. Minimum time of exposure was figured at two hours, with two to 12 hours suggested for disinfestation during quarantine. All stages of the life cycle—diapausing and non-diapausing larvae, pupae, eggs and adults—are vulnerable to the treatment, according to the study.

In the United States, the khapra beetle has not gained a permanent foothold, though the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service is vigilant, listing the beetle among its most-wanted invasive “hungry pests.” Since the 1950s, infestations have been reported from several states but were quickly quashed. Strict quarantine is enforced to prevent infestations, because 67 percent of the continental United States has a climate suitable for the species.

Researchers are continuing to explore various aspects of heat treatment, including upon the quality of the grain itself. Some grains, such as wheat, are much more resistant to heat than others. Other research has shown that high temperatures over a short period can kill insects without harming grain. As scientists learn more, producers should be able to tweak the process to maintain its effectiveness.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

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