Mosquitoes Take Breeding with Just a Pinch of Salt

Scientists have discovered that too much or too little sodium can be detrimental to the breeding success of mosquitoes and other insects. Christopher J. Paradise, a Penn State postdoctoral researcher in the Department of Biology, and William A. Dunson, professor emeritus of biology, describe their discovery, which they made while conducting a study on the effect of atmospheric deposition of sulfates, nitrates, and acids on insects breeding in small pools of water, in a recent issue of the journal Annals of the Entomological Society of America. They performed their study in the Pennsylvania State Gamelands near Sandy Ridge in Centre County.

Paradise and Dunson examined populations of insects living in pockets of water with different ion concentrations in natural and simulated treeholes. Treeholes often form from stumps left behind after a forest has been completely cut down: when new trunks sprout from a tree stump, the center rots, and water collects in the resulting hole. Treeholes are common in such secondary-growth forests, occurring at the rate of up to 25 to 30 per acre. They also are found in neighborhood streets and backyards. "Treehole communities," as researchers refer to them, consist of aquatic insects such as the eastern treehole mosquito, biting midges, and marsh beetles, among others.

The researchers used simulated treeholes to study the amounts and effects of sodium on insects. Their simulated treeholes are insulated plastic containers covered with netting and filled with fixed amounts of simulated treehole water, dried treehole sediment, and oak leaves. All the simulated treeholes, which were exposed to the same temperature and rain conditions, were composed of the same nutrients and water volume, with various controlled levels of sodium.

Paradise and Dunson's research shows a consistent relationship between low sodium levels and high insect population densities in natural treeholes, suggesting that the amount of sodium is depleted by high numbers of insect larvae, or alternatively, that females may choose to lay their eggs in waters lower in sodium.

"The habitat of larval aquatic insects is determined by where the female lays her eggs and the levels of essential nutrients there can affect individual and population growth rates," Paradise explains. The researchers found that insects sometimes prefer to lay eggs in treeholes with lower sodium levels or higher water levels, and that the amount of sodium in a treehole is dependent on the amounts of leaf litter and water it has, the treehole orientation, the angle of its opening, the size of the opening, and how easily leaf litter and water can drop in. "Female mosquitoes prefer dark and rough tree walls, horizontal openings, and the presence of organic decay by-products," Paradise says. They also found, conversely, that mosquitoes actually grow larger in simulated treeholes with higher sodium levels. "This apparently contradictory finding can result from the need for mosquitoes to absorb sodium during their development and explains how a decrease in sodium concentration can occur if enough mosquitoes are present," Dunson explains.

An additional study performed by Penn State undergraduate biology honors student Michael J. Baltzley, under the direction of Paradise and Dunson, found that the density of the mosquito population, as well as the sodium levels in the treehole, will interact to affect an insect's breeding success. In small treeholes with the highest naturally-occurring sodium levels and low population densities, the mosquitoes grow faster, mature quicker, emerge larger, and escape the treehole before it dries during the summer. In treeholes with very low levels of sodium and high population densities, on the other hand, all the insects grow poorly. The researchers concluded that there is an ideal range for sodium concentration for insect populations to thrive: too much or too little sodium has negative consequences for developing insects.

So is it a good idea to dump lots of salt in your backyard treeholes? "No," Paradise says. "While measuring sodium levels is a good method for predicting the density of a given population of mosquitoes, it is not a good method for control because every single treehole in the area would need to be treated with excess sodium." "Instead," he suggests, "it's best if people protect themselves from mosquitoes by relatively safe and easy methods like covering up with clothes or burning citronella candles."

This research was funded by the Environmental Protection Agency.

Contacts:

Christopher Paradise, 814-865-2461, cqp2@psu.edu

Barbara K. Kennedy, 814-863-8748, science@psu.edu (Penn State science public information) .