by Joan McIntyre, Extension Master Gardener
Mosquitoes are a perennial summer pest, and the temptation to resort to commercial spraying to deal with this scourge can be strong. A key consideration to keep in mind is that all insecticides used to target adult mosquitoes are nonselective and will kill all insects that come in contact with the chemicals, including pollinators and other beneficial insects. Insects are essential to pollinating our crops, aiding in decomposition of dead plants and animals, returning nutrients to the soil, and keeping pest insects in check. Multiple studies, however, have documented a steep decline in insects over the last several decades, driven in part by widespread use of pesticides.
Common Insecticides for Mosquito Control
Pyrethroids are the most common class of insecticides used to control mosquitoes (and other insect pests) and can be recognized under the list of active ingredients by their common names almost always ending in -thrin or -ate, such as permethrin or esfenvalerate. These insecticides have been derived from the chemical composition of a natural insecticide known as pyrethrum or pyrethrins found in chrysanthemum flowers. Pyrethroids have been synthesized to become more toxic to insects and to last longer in the environment. Organophosphates (malathion, naled) are another class of insecticides used to control mosquitoes and other insects.
Insecticide effectiveness in controlling mosquitoes is limited and short-lived, requiring repeated treatments every three weeks or so. One study found that spraying had almost no impact in controlling Culex spp mosquitoes—one of the two most common genera of mosquitoes in Virginia—as these mosquitoes spend most of their time in the tree canopy. Since mosquitoes have ranges of 500 yards to 2 miles, they can quickly repopulate sprayed areas unless breeding sources are eliminated. At the same time, the residual effects of these pesticides can last several weeks and will continue to harm pollinators and other insects visiting and feeding on the treated plants.
The market is awash with products claiming to control mosquitoes. Some may be misting devices that release pesticides such as permethrin that will harm any insects coming in contact with the chemical. Other products claim to repel or attract and trap mosquitoes but are unlikely to have been rigorously tested. Most appear to be ineffective, and some such as CO2 traps may actually attract more mosquitoes than they trap. Certain products such as bug zappers can be more harmful to other insects than to mosquitoes. The key rule is “buyer beware”—read the list of active ingredients and research independent testing of such products.
Do Natural Alternatives Work?
Many commercial pesticide applicators offer treatments with natural essential oils to eliminate or act as a repellent barrier to mosquitoes. The United States Environmental Protection Agency considers essential oils such as garlic, citronella, cedarwood, and thyme oils as minimum risk pesticides and does not require testing for effectiveness before they go on the market. There is no research on these products that shows they are effective in controlling mosquitoes in the United States.
While many of these essential oils are known to have repellent and even insecticidal properties, available research suggests that their effectiveness even as a repellent is questionable. Most essential oils are volatile when exposed to air and thus evaporate quickly. Consumer Reports’ evaluations of mosquito repellents have found that those made from various essential oils are largely ineffective after about an hour or so compared to those using DEET, picaridin or oil of lemon eucalyptus (the only essential oil with proven effectiveness as a repellent). Customers should ask to see the label of products touted as natural and not harmful to pollinators to verify that the active ingredients do not include a pesticide in addition to any essential oils.
Best Strategies for Controlling Mosquitoes
Mosquitoes are best controlled at the larval stage, not as adults. Female mosquitoes bite for a blood meal before laying eggs in standing water. These eggs hatch in 2-3 days, and adults emerge in 7-10 days. The most effective control is to dump the standing water at least every 7 days—bird baths, potted plant saucers, leaky hoses, toys left outside, kiddie pools, clogged gutters, corrugated drain pipes, upturned lids, pet water bowls. Even a tablespoon of water left for 7 days is enough for mosquitoes. Get the kids involved, and encourage your neighbors to do the same.
Where you can’t dump the water, you can use Bti (Bacillus thuringiensis israelensis), the active ingredient in Mosquito Dunks and Mosquito Bits, which kills mosquito larvae without harming birds, beneficial insects, or other wildlife. You can also consider alternatives to English ivy and other dense ground covers where mosquitoes rest and breed during the day. Other strategies for enjoying your yard include using a fan on a patio or deck to keep mosquitoes at bay, wearing long pants and a long-sleeved shirt, or using a mosquito repellent with DEET, picaridin, oil of lemon eucalyptus, or IR3535.
If You Have to Spray
If you have decided that you cannot enjoy your yard without spraying for mosquitoes, here are some guidelines for hiring a pesticide applicator to minimize the harm to other pollinators:
- Use only a pest management company that has a Virginia Department of Agriculture and Consumer Services (VDACS) license.
- Ask for an integrated pest management (IPM) plan that includes strategies to identify and manage breeding sites as well as pesticide application.
- Compare several proposals and look for reputation for quality of service as well as price.
- Examine labels for the products to be used to know what pesticides are included and to verify claims that only essential oils or other “natural” products are being used.
- Insist that pesticides be applied at dawn and dusk when pollinators are less likely to be active and not be applied to food crops or flowering plants.
- Specify ultra-low volume pesticides during the coolest part of the day to limit drift and that applicators not spray on windy days or before rain is expected.
If you have concerns about pesticide misuse, you can contact the Virginia Department of Agricultural and Consumer Services at (804) 371-6560. This office takes reports of pesticide application misuse very seriously.
Remember—there is no insecticide targeting adult mosquitoes that will not harm pollinators or other beneficial insects.
- Protecting Yourself From Mosquitoes … Without Harming Pollinators
- Mosquito Control Begins with a Home Walk-Around
- Controlling Mosquitoes & Ticks in Your Yard without Pesticides (online public education class)
- What is a pyrethroid insecticide? Texas A&M Agrilife Extension
- Many Mosquito Controls Only Hammer Buyer’s Pocketbook, Texas A&M Agrilife Extension
- Essential Oils for Mosquito Control, Maryland Grows Blog, University of Maryland
- Active Ingredients Eligible for Minimum Risk Pesticide Use, College of Agriculture and Life Sciences, Cornell University
- Insect Repellent Buying Guide, Consumer Reports, May 03, 2021