South Dakota Bat Working Group
 Last update 4/2010

Bats & the West Nile Virus

South Dakota's Bats are small, and eat bugs. They feed upon moths, beetles, and other night flying insects, consuming up to half their body weight in insects each night.With return of the warm weather, South Dakota is likely to see the return of the mosquito-borne West Nile Virus. The following provides an overview of bats and mosquitoes here in SD.

All 12 identified bat species in South Dakota are considered generalists in their selection of insect prey, i.e., none specialize in any one particular type of insect. This should not be grounds to dismiss their contribution to insect pest control, including mosquitoes. The extent to which bats control any given insect pest is dependent upon the insect's availability and population density. Numbers of bats are greatest in areas with higher insect density. In SD, collectives of bats are typically maternity roosts. Such roosts are formed in close proximity to better food supplies as this prevents the adult females from having to fly great distances while nursing a pup that is not yet able to fly. It also is easier for juveniles to learn to hunt and feed themselves in areas of greater insect density.

SD's bats have voracious appetites and all are exclusively insectivorous. While larger bat species tend to prefer larger insect prey, such preferences come into play only when there is a choice. An abundance of a particular type of insect will result in a quick and easy meal with a minimum of foraging (important in conserving energy if you are a lactating bat or a juvenile trying to put on weight to get you through your first winter). I have seen a ranch adjacent to a creek with a large maternity roost of Myotis lucifugus (Little Brown Bat) near Buffalo, SD, whose owners swore they had not seen any mosquitoes while surrounding ranchers complained of it being a bad year for mosquitoes. (This is based upon interviews conducted during a bat presentation at the ranch.) I have documented a single bat (weighed and banded upon emergence at dusk) returning to the roost less than two hours later with an increased body weight of just over 45%. Such is not uncommon and can occur several times a night.

Several bat species prefer to form maternity roosts near water, as such locations are generally associated with greater insect density and diversity. To contend that bats do not effectively contribute to the control of mosquito populations is unjustified. Effectiveness of such control is dependent upon a variety of factors. In some circumstances occurring here in South Dakota, they are very effective. There has been no research completed in SD on the diet content of bats. Diet composition is likely to vary between habitat types for the same species. What is true in an urban area is likely to be different for rural areas. Anything that controls insectivorous pests, particularly a natural control mechanism, should not be discouraged or dismissed.

But what about rabies? The mention of this disease as a justification for not including them in a well-rounded control program is unwarranted. Bats, as with nearly all mammals, can contract the disease. Bats are not asymptomatic carriers of the virus, i.e., when they contract it, it kills them as it does with other mammal species. Other terrestrial mammals display higher incidence of the disease than do bats. Exposure to the virus is far likelier to come through contact with a dog or cat than a bat. For a bat to transmit the virus to a human, there must be physical contact with the infected animal. The virus is not transmitted through the droppings or the urine of an infected animal. Not handling bats reduces the chance of contracting the disease to nearly zero. See http://www.batcon.org/” for additional information on bats and rabies.

Bats' contributions to a healthy ecosystem far outweigh
any risks to public health with which they may be associated.

Should I put up a bat house? A lot of the information about bats and bat houses is provided by folks trying to sell you a bat house…perhaps not the most objective source of information (e.g., Put up one our bat boxes and you can kiss mosquitoes good-bye!”) Bat house research continues to refine the designs and preferred placement to optimize their use by bats. Bat houses are most successful when properly constructed (with proper placement) in an area where there is a shortage of roosts.

See Bat Conservation Internationals web site for additional info:
http://www.batcon.org/
http://ci.pierre.sd.us/Mosquito.htm