Pest control baits include one or more active ingredients – the pesticide, along with inactive ingredients to form the bait. One of these inactive ingredients is the attractant; that is, the food or other substance on which the specific insect, rodent, or other pest likes to feed.

For years the standard attractant in cockroach baits was glucose, a sweet, sugary substance that that cockroaches loved.


But it was gradually noticed that the baits weren't doing their job. The cockroaches weren't being killed because they were no longer attracted to the bait; they had, in fact, become averse to the bait, avoiding it at all costs. They would no longer feed on it, so cockroach infestations were continuing to survive despite best efforts of homeowners or pest management professionals to eliminate them.

 

Why Did the Cockroaches Stop Eating the Bait?

Cockroaches are survivors. They are one of the oldest groups of insects on Earth, having survived for hundreds of millions of years through all the Earth's changes that caused many other living creatures to become extinct. In the same way that they learned and evolved to withstand conditions that were the death knell for others, so too had this bait aversion become an evolutionary survival trait. In a study published May 24, 2013, in the journal Science, North Carolina State University (NC State) entomologists explain why.

The sweet-tooth that "normal" German cockroaches had had for glucose become bitter.

That is, according to the study, glucose now set off bitter receptors in roach taste buds, causing the roaches to avoid foods that bring on this taste-bud reaction. Because the aversion has a genetic basis, it carries to the generation's offspring, so that more and more of the insects began avoiding any baits that contained glucose.

“We don’t know if glucose actually tastes bitter to glucose-averse roaches, but we do know that glucose triggers the bitter receptor neurons that would be triggered by caffeine or other bitter compounds,” says Dr. Coby Schal, the Blanton J. Whitmire Distinguished Professor of Entomology at NC State and the corresponding author of the paper. “That causes the glucose-averse roach to close its mouth and run away from glucose in tests.”

In fact, glucose-averse roaches that were forced to taste glucose refused to ingest the sugar, similar to a child who spits out her bitter-tasting food, the report said. Interestingly, however, the cockroaches had not become averse to all sugary substances—the same cockroaches that were glucose averse were quite willing to each fructose substances.

 

The Cockroach Cost

Although avoiding the glucose may have enabled the cockroaches to avoid pesticide baits (until the manufacturers discovered the cause and changed the formulas that is), there was a cost to those cockroaches. Because they would now not eat any glucose foods, the cockroaches faced a nutritional deficiency and stress which resulted in slowed growth.  

This study is of particular interest because numerous studies and tests have shown that pests can become resistant to the active ingredients—the toxins—in pesticides, whereas this study is proving that cockroaches, at least, will also become behaviorally resistant to foods.

(Ayako Wada-Katsumata, an NC State senior research scholar, performed most of the experiments and is the first author of the paper. She is now investigating whether roaches can learn to associate glucose with specific odors and thus use their memory to ignore baits that contain glucose. The research was supported by the National Science Foundation, the U.S. Department of Housing and Urban Development, and the Blanton J. Whitmire Endowment at NC State.)