Mosquito Memory

Mosquitoes that can spread disease might be able to learn to avoid feeding on horses until the smell of pesticides has subsided.

mosquito Culex tarsalis
Mosquitoes that can spread disease might be able to learn to avoid feeding on horses until the smell of pesticides has subsided. Courtesy Joseph Berger, Bugwood

Horse owners and veterinarians are often as irritated by mosquitoes as the horses they care for. Despite dousing horses in pesticides, the effects of those chemicals seems to wear off in just a short time. Humans try to follow the recommendations to wear light clothing and forego scented lotions and creams. Still, the “buggy” harassment continues.

A recent study investigated behavioral changes in mosquitoes exposed to pesticides and how this affects efficacy of chemical control [Sougoufara, S.; Yorkston-Dives, H.; Aklee, N.M.; et al. Standardized bioassays reveal that mosquitoes learn to avoid compounds used in chemical vector control after a single sub-lethal exposure. Nature Scientific Reports 2022, 12:2206; https://doi.org/10.1038/s41598-022-05754-2].

Finding a blood meal is essential to mosquito survival and reproduction. Unfortunately for people and horses, “vector-borne diseases comprise 17% of the global burden of communicable diseases.” The mosquito is considered the deadliest “animal” in the world due to the havoc it wreaks by serving as a vector for lethal infectious diseases.

Horses in the United States are at risk for mosquito-borne Eastern equine encephalitis (EEE), Western equine encephalitis (WEE) and West Nile virus (WNV). Vaccines are very effective at curtailing the risk of infection, yet horses are still harassed by large numbers of mosquitos that come out to feed at dusk and dawn.

Along with insect-deterring mesh blankets and leg coverings, pesticides are used regularly to deter insect irritation of horses. Yet, only a small number of pesticides are available that pose low risk of toxicity to animals and humans. That results in mosquito resistance through multiple pathways—gene mutation that affects protein binding sites for insecticides, metabolic changes of detoxifying enzymes, and physical modifications that reduce insecticide penetration into the insect cuticle.

Behavioral Modification of Mosquitoes

Another resistance mechanism occurs through behavioral modification—mosquitoes learn to associate olfactory or visual stimulus as a noxious risk. This study evaluated learned avoidance behavior by female mosquitoes to the pesticide containing pyrethroids, carbamates and organophosphates.

In the study, female mosquitoes avoided blood feeding for more than 12 hours after exposure to a sublethal dose of pesticide. Some died as a result of not feeding. However, many sought out other areas for feeding and nesting that did not contain the smell of pesticides, thereby effectively enabling mosquitoes to persist and reproduce.

The authors concluded: Two species of female mosquitoes studied—Aedes and Culex—“are able to learn to avoid the smell of and their exposure to insecticides in order to maximize their survival.”

Such associative learning is a form of behavioral resistance that reduces the efficacy of current chemical vector control. Even though they are tiny and seemingly “brainless,” it turns out that mosquitoes have evolved multiple and highly effective mechanisms to ensure their survival.

It is possible that as toxicity wanes after the initial spraying of insecticide on a horse, mosquitoes are exposed to a sub-lethal dose hours later—that is when they might learn to avoid landing and feeding until the odor concentration subsides hours after a horse has been treated.

Take-Home Message

Educating horse owners about the fact that mosquitoes might learn to “wait” after horses have been treated with pesticides or might avoid landing and feeding until insecticide odor subsides could help them understand they must reinforce chemical control to a level that is more likely to result in effective insect eradication.

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