Don't assume an uptick in ant activity after setting out pesticide traps means the treatment failed. New UC Riverside research shows their behavior changes as chemicals make their way through a colony.
The research from the laboratory of UCR entomology professor Dong Hwan Choe was recently published in the Multidisciplinary Digital Publishing Institute's journal Insects. It focused on the ways invasive Argentine ants, the ones most commonly found in homes and gardens in Southern California, respond to two common insecticides used in ant baits: thiamethoxam and boric acid.
While both treatments kill most of the worker ants in the colony, they do so at different rates. Thiamethoxam killed all the workers within about seven days, while boric acid killed roughly 90% within about two weeks. The ants' behavior also varied widely in the days immediately following exposure.
"During a previous study, we saw the ants behaving strangely on the first or second day after introducing a thiamethoxam bait, and it made us wonder if the behavior was related to the toxic effects of the insecticide," Choe said.
Thiamethoxam is a neonicotinoid, known to be highly effective against various types of insects. Because it is also deadly for bees, there are more restrictions for using it; it cannot be sprayed on plants during flowering season. However, using it in stationary baits reduces the likelihood it will harm insects that aren't the intended targets.
Choe and his students constructed transparent colonies in the laboratory to give them a clear view of the ants' activity both inside and outside their nesting sites.
In healthy colonies, Argentine ants spread themselves throughout the available nesting space. Most remain inside caring for the queen, young ants, and other colony members, while some workers venture outside to search for food.
After exposure to thiamethoxam, many more ants became much more active outside the nest, roaming through the foraging area before eventually dying. Because the chemical targets the nervous system, Choe suspects the extra activity may result from neurological effects.
"It's like they're all high on nicotine," Choe said. "Their nervous systems have been compromised by the insecticide."
Thiamethoxam mimics some of nicotine's effects on the insect nervous system, overstimulating it and eventually causing paralysis.
Boric acid produced nearly the opposite behavioral response. Rather than roaming outside the nest, affected ants stopped foraging altogether and gathered tightly inside the nesting space, especially near any sources of water.
Some people choose boric acid because it is less toxic for non-target insects. It affects multiple parts of the ants' bodies, particularly the lining of the midgut, which plays an important role in digestion and nutrient absorption. In some ants, boric acid is also known to damage their excretory organ, which plays central roles in water regulation and waste removal. Choe said these effects could help explain why poisoned ants congregate near water, though more research is required to prove that idea.
Understanding these differences could help pest control professionals and consumers better interpret what they see after putting out ant bait.
For example, when ants stop foraging after encountering boric acid, it might appear that a colony has been quickly eliminated. But surviving ants could later reappear. With thiamethoxam, the opposite can happen. Poisoned workers may wander into visible places like sinks, garages or windowsills for a day or two before dying, making an infestation appear to be getting worse.
The findings could also inform how manufacturers formulate ant baits. Workers share food throughout a colony, which can dilute the amount of insecticide individual ants receive. Choe said lower doses of thiamethoxam could potentially prolong the unusual roaming behavior.
Another note of caution: while there are no known cases of insecticide resistance developing in ants, they are notoriously challenging to control. As noted above, both chemicals kill most of the workers in an ant colony. However, the treatments may not always be effective in killing the queen. Therefore, a single application of bait often produces temporary relief from the pests because another invasion can occur when the queen hatches a new brood.
This project took more than two years to complete and included significant contributions from students working in Choe's lab. Beyond revealing what happens inside a poisoned ant colony, Choe hopes the findings will help people recognize that unusual activity after bait application does not necessarily mean the treatment has failed.
"Though these treatments don't work immediately, you can see by the ants' behavior that they're affected," he said.
