Mosquitoes transmit various diseases such as malaria and dengue fever. About 241 million cases of malaria occurred worldwide in 2020, with a few million more cases in 2021. Almost half of the world’s population lives in regions where it is possible to contract the dengue virus. Insects also destroy a third of agriculture.
New research by scientists at the University of California, Riverside, has potential for insect control through volatile repellents that could be applied to surfaces such as window sills, cabin eaves, house entrances, backyards, outdoor produce storage areas, entrances to cattle sheds, etc. alongside crops.
Researchers focused on ammonia, a basic volatile compound found in insect environments. In low concentrations, such as in human sweat, ammonia is an attractant for mosquitoes and other insects. However, in high concentrations, for example in household cleaners, ammonia is no longer attractive to insects. Researchers studied what happens to the olfactory (smell) and gustatory (taste) systems of fruit flies and mosquitoes in the presence of ammonia.
Ananda Sankar Ray.
“We found that the olfactory neurons appear to have a burst of activity and then go quiet for a while,” said Anandasankar Ray, professor of molecular, cellular and systems biology who led the study, which appears in the journal iScience. “During the resting phase, neurons cannot detect odorants, which means insects cannot effectively find human skin odor.”
When Ray’s team tested the taste systems of fruit flies and mosquitoes (Aedes aegypti), they found a similar response.
“In terms of taste, we found that ammonia and ‘amines’ — derivatives of ammonia that make up many synthetic odorants — don’t produce the flash activity that we see in the olfactory system,” Ray said. “But they show the inhibition that we found in the olfactory system. We were able to show that ammonia silences the sugar and salt reaction in insects.”
After most insects find a place to land via their olfactory system, their taste system executes. Mosquitoes use the labella — sensory probes that help find a good spot to bite — on their legs to taste food. Fine hairs called sensilla on the labella of fruit flies allow the flies to taste potential food without eating it.
According to Ray, the discovery could be used to create effective insect repellents in the future.
“While compounds like ammonia, which have a high pH and are basic, can’t be used on the skin because of their caustic properties, they can be used in other ways,” he said. “Many stinging insects fly into the houses from outside. In most parts of the world, insects bite people and pets indoors and often at night. For example, if walls where insects land and wait contain a high pH material, mosquitoes would be affected. When a high pH compound, such as An amine, for example, spread around entrances to houses and animal pens could similarly repel mosquitoes.”
The study was supported by grants from the National Institute on Deafness and Other Communication Disorders, a member of the National Institutes of Health.
Ray was joined in the study by Jonathan Trevorrow Clark, Anindya Ganguly, Jadrian Ejercito, Matthew Luy and Anupama Dahanukar.
The research work is entitled “Chemosensory detection of aversive concentrations of ammonia and basic volatile amines in insects”.
It’s the humidity
In a separate article published in the journal Scientific Reports, Ray’s lab examined moisture-induced behavioral modification in the Asian citrus leaf flea, or ACP, which transmits citrus leaf disease, and mosquitoes. Although insects can sense moisture, little research has been done on disrupting their moisture-sensitive neurons.
“We found that amine odorants inhibit the moisture response,” Ray said. “We identified neurons in the ACP that sense moisture and found that certain amines might inhibit their moisture perception. We then showed that this was conserved in fruit flies. This is probably the first time researchers have shown that moisture sensing can be inhibited by odorants.”
The researchers then tested moisture sensing in pregnant mosquitoes (Aedes aegypti and Anopheles coluzzi), which are attracted to bodies of water to lay eggs. In the laboratory, two cups of water were presented to blood-fed mosquitoes ready to lay eggs. A cup contained a small vial with an odor that inhibits moisture perception. The researchers found that the mosquitoes avoided laying eggs there, preferring the untreated cup instead.
“This suggests that it is possible to block the moisture-sensitive neurons in insects by using a volatile chemical and lower the level of oviposition,” Ray said.
He also explained that ACPs appear to avoid high humidity. To test their moisture reading, his lab used a Y-shaped tube with high moisture in both arms. The ACP countries initially did not favor either arm. But when researchers inserted a moisture-blocking odorant into an arm, ACPs began to prefer it because they could no longer feel the moisture.
“This means that by blocking the insects’ ability to sense water with a volatile odorant, we can manipulate their moisture-sensing pathway and alter their behavior in predictable ways,” Ray said. “In the future, it may be possible to manipulate amines to prevent insect oviposition in certain areas.”
The research has implications for regions where mosquitoes spread disease. After a blood meal, mosquitoes look for water in which to lay their eggs. A single female mosquito can lay up to 300 eggs in a single night.
“Because of this extremely high reproductive potential, we see an explosion of mosquitoes from spring through summer,” Ray said. “Where you have water with mosquito larvae, it is extremely difficult to control the mosquito population. That is why in tropical countries it is impossible to kill all mosquitoes. Even if there are a few mosquitoes left, they multiply very quickly.”
The study was supported by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health.
Ray, the founder of two startups, Sensorygen Inc. and Remote Epigenetics Inc., was brought on board by UCR’s Coutinho‐Abreu and Trevorrow Clark.
The title of the research paper is “Pentylamine Inhibits Moisture Sensing in Insect Vectors of Human and Plant-borne Pathogens”.
Credit for the cover image showing a mosquito and eggs: doug4537/Getty images.
