Mosquitoes Follow Heavy Rains

Doug Akers, Boone Co Extension Educator, Purdue University

Over eight inches of rain have fallen in less than a month in some areas of Indiana. Those excessive rains have resulted in severe mosquito outbreaks for some Hoosiers this month.  Of primary concern are nuisance biting mosquitoes, according to Ralph Williams, Entomologist, Purdue University. These are the types of mosquitoes that occur as the result of accumulated water from heavy rains. The Indiana State Board of Health will monitor mosquito-borne disease activity throughout the summer and will alert the public of any disease concern, Williams said.

West Nile virus has been found in Indiana in recent years. However, in areas where mosquitoes do carry the virus, very few mosquitoes (much less than 1%) are infected, according to Michael Potter, Entomologist, University of Kentucky.  Even if an infected mosquito bites you, you have less than 1% chance of severe illness. The chances of being severely ill from any one mosquito bite are extremely small, Potter added.

The two major groups of mosquitoes in Indiana, Culex sp. and Aedes (Ochlerotatus) sp., have somewhat different egg-laying habits. Culex mosquitoes lay groups of eggs on the surface of water in bird baths, tin cans, old tires, car bodies, cisterns, roof gutters and any other containers which hold water.  Aedes mosquitoes place their eggs at the base of vegetation in low lying areas that flood periodically. However, they also can deposit their eggs above the water line in artificial containers (such as tin cans, old tires, etc.) or in tree holes that hold water, Potter added. These eggs will hatch when inundated with rain water.

The larval stage begins at egg hatch. Mosquito larvae, called are "wrigglers" because of their distinctive swimming style. They can be seen when they come to the surface of the water to breathe through a distinctive tube that extends from the end of their body. The larvae feed on microorganisms in the water and grow rapidly in warm weather. Full grown larvae become pupae, often called "tumblers" because they tumble end-over-end through the water. Pupae transform into adults after a few days.
Female mosquitoes are blood feeders and may live for more than a month. They generally require a blood meal before laying eggs. Mosquitoes rely on various cues to find potential hosts on which to feed. Heat, movement, exhaled carbon dioxide, and body scent allow hungry mosquitoes to home in on their prey from long and short distances. Some mosquito species feed on humans; many feed on wild and domestic birds and mammals. A few even feed on reptiles and amphibians. Adult male mosquitoes do not bite. They live 1 to 2 weeks and feed on nectar and plant juices.

Larval Control

Use of a mosquito larvicide may be beneficial when it is impractical to eliminate a breeding site, according to Potter. Larvicides are insecticides which are used to control immature mosquitoes before they have a chance to develop into biting adults.  Trade names of some larvicides are:  Mosquito Dunks, Mosquito Quick Kill Granules, and Agnique MMF.  

Most larvicides sold to homeowners contain either the active ingredient methoprene or a toxin produced by the soil bacterium Bacillus thuringiensis israelensis (Bti). Methoprene prevents adult emergence by disrupting the development of mosquito larvae. Bti toxin attacks the digestive tract. It is lethal only to mosquito wrigglers and the larvae of some aquatic gnats and black flies. Neither methoprene nor Bti toxin is harmful to fish, waterfowl, pets, or humans when used according to label directions.

Many products and formulations containing methoprene (Altosid) and Bti (Bactimos, Vectobac) are used by mosquito abatement agencies and other professionals. Homeowners can purchase methoprene as PreStrike. It is sold as granules in shaker bottles. Less than a dozen granules are needed to prevent mosquitoes from developing in a flower pot bottom or bird bath. Less than a teaspoon of PreStrike granules is needed to treat 100 feet of rain guttering. Mosquito development will be inhibited for up to a month in ornamental ponds and similar bodies of water; longer protection is provided in sites that periodically dry out.

Various products containing Bti are available to homeowners (e.g. Mosquito Dunks or Quick Kill Mosquito Granules). Typically, one donut-shaped Mosquito Dunk is recommended per 100 square feet of water surface. The dunk breaks down slowly when wet and releases the insecticide over about a 30 day period. The Mosquito Quick Kill product is a granular formulations that begin to release the Bti toxin more quickly than the dunks, resulting in faster action. While results come more quickly, the residual life of the treatment is generally not as long as the dunk formulation. Granular formulations may be more desirable when treating smaller areas, such as flower pots or tree holes.

With a little careful observation it is fairly easy to see mosquito larvae in clear, shallow water over light colored bottoms. They are harder to see in dark, stagnant water where there is a lot of debris or vegetation. Avoid casting a shadow over the water when inspecting for mosquitoes because the larvae and pupae will dive in response to light changes. They can be captured by quickly plunging a long-handled dipper into the water.

Adult Mosquito Control

Mosquito breeding sites are not always obvious or accessible so some nearby sources will remain undetected or impractical to treat. Also, mosquitoes can fly in from some distance away, Potter said. Therefore, it may be necessary to take additional measures against adults.

Mosquitoes prefer to rest in protected sites during the day. Yards with lots of trees, shrubs, and dense vegetation or properties adjoining such areas, can more severe mosquito problems.

To help reduce intolerable levels of biting mosquitoes, insecticides can be applied to the lower limbs of shade trees, shrubs, and other shaded areas, such as under decks and along foundations. Pyrethroid insecticides are effective but will need to be reapplied periodically.

Always read and follow label directions before using any pesticide. Some homeowners may wish to enlist the services of a professional for this service.

Registered insecticides for adult mosquito control on lawns and vegetation
Insecticide (Trade Name): cyfluthrin (Bayer Advanced PowerForce Mosquito Killer) and permethrin (Ortho Mosquito B Gon, Mosquito Beater, Spectracide Mosquito Stop)

Personal Protection From Bites

While some mosquitoes are daytime biters, most are more active in the evening. Staying indoors at dusk and during evening hours will lessen the chance of being bitten. Long-sleeved shirts and pants will provide protection when outdoors but bites can still occur through thin clothing and to exposed skin. Dark colors attract mosquitoes, so wear lighter tones if you’re going to be outside.

Topically applied mosquito repellents will help to prevent bites when spending time outdoors. The most effective mosquito repellents contain the active ingredient diethyl toluamide (DEET). The higher the percentage of DEET in the product, the longer the protection lasts. Low percentage formulations are available for use with young children.

Non-DEET containing repellents (e.g. Avon Skin-So-Soft with citronella oil) may provide some relief but generally to a lesser degree and for shorter duration than DEET products. It is often desirable to apply insect repellent on outer clothing as well as the skin. Always read and follow directions on the container. Mosquito repellent should not be applied to the hands of young children, and treated skin should be washed with soap and water after returning.

For more information, please see E-52, Mosquito Management by Trained Personnel (pdf file)

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by the Purdue University Cooperative Extension Service is implied. Any person using products listed assumes full responsibility for their use in accordance with current direction of the manufacturer. Purdue University is an equal opportunity/equal access institution.

Information listed is valid only for the state of Indiana

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