April 13, 2016

By Edward M. Spevak, Ph.D.
Curator of Invertebrates

It's gardening season again and that means beautiful flowers, bountiful produce, gorgeous and hard-working bees, fluttering butterflies and the buzz of mosquitoes.

That last one is what causes many to reach for an insecticide, call the city health department or run inside and hide, and we ask, "What good are mosquitoes?"

The question brings to mind the quote by Aldo Leopold "To keep every cog and wheel is the first precaution of intelligent tinkering." But where do the "cogs and wheels" of mosquitoes fit into the "ecosystem of things" and how can we learn to live with their existence?

There are more than 3,500 species of mosquitoes found worldwide, around 175 of them in the United States and 50 in Missouri. Mosquitoes are a major component in ecosystems serving as food for a variety of aquatic invertebrates, dragonflies, fish, salamanders, frogs, lizards, birds and even other species of mosquitoes. There are also several plants, including the endangered white fringeless orchid, that depend on mosquitoes for pollination.

However, mosquitoes are known for carrying diseases. The most common and most important mosquitoes from a human disease perspective in this area are species of the genus Culex, Anopheles quadrimaculatus (marsh mosquito) and the introduced Aedes albopictus (Asian tiger mosquito). In Missouri, these species can transmit West Nile virus, St Louis encephalitis and heartworm, though the probability of contracting any of these is very small.

The carrier for the recent outbreak of the Zika virus—Aedes aegyptyi is rarer in Missouri and is a more southern species. However, it can be found in the western part of the state. According to the CDC, A. albopictus is not as capable of carrying the virus as is A. aegypti..

Ralph Waldo Emerson said that "If we walk in the woods, we must feed mosquitoes." It is this feeding that makes mosquitoes more of a nuisance pest. This nuisance is due only to the females. Both male and female adult mosquitoes feed on nectar as their primary food source, but when a female requires additional protein to produce eggs, she needs a blood meal. It is this behavior that people try to control.

However, "Mosquito Control" and "fogging" should not be synonymous in our modern world. Rachel Carson in her classic work Silent Spring pointed out the problems with a misuse and over-reliance of pesticides to control pests even though these issues had been noted over a decade earlier.

So how best do we control this nuisance? Just as in our gardens, we need to practice Integrated Pest Management (IPM) approach. Spraying insecticides or community truck fogging for adult mosquitoes should be only one small component of mosquito control. Spraying broad spectrum insecticides relies on the chemical contacting the mosquito to kill it. Culex and Anopheles mosquitoes are usually active from dusk until dawn.

However, Aedes is active during the day when beneficial insects, such as bees, are active— making day spraying problematic. One of the most important aspects of a mosquito control program should focus on the removal of water sources creating the larval habitat. Shortly, after a blood meal a female looks for a place to lay her eggs (she lays 100 to 300 eggs in a lifetime).

If conditions are right these eggs hatch in one to two days and in less than two weeks emerge as new adults. Anopheles mosquitoes lay their eggs in open standing water. Culex and Aedes will lay their eggs in more confined spaces that may hold water, e.g., rain barrels, cans, gutters, flower pots and trays, tires, bird baths and pet bowls that have not been cleaned for a while. Emptying these containers of any standing water is the most effective mosquito control as many species do not travel very far from where they emerge.

If the water cannot be removed, you can control larvae using bacterial larvacide compounds, such as Bti, that are applied directly to water where mosquito larvae are found, killing the mosquito larvae without harming other organisms. Before applying the larvacide you should see if there are larvae present to avoid wasting money and product.

Adult removal along with possible identification of species in your area for better management can be accomplished using CO2-baited light traps. There are a number of these on the market, and some include an additional chemical lure to increase their effectiveness. Many of these traps have been found to be very effective but they can also be very expensive.

Effective mosquito control depends on knowing what species you have and what their biology is to maximize your success and increase your enjoyment of the outdoors. If you are interested in working with your community to improve their mosquito control two excellent resources are available from the Xerces Society, How to Help Your Community Create an Effective Mosquito Management Plan and Ecologically Sound Mosquito Management in Wetlands (http://www.xerces.org/pesticides/).

Finally, when out in the garden you can limit your exposure to mosquitoes by wearing long sleeves, light colors and using an appropriate insect repellant. However, it should be noted that research has found that some people are just more attractive to mosquitoes than others. Just ask my wife.