Fire Ants in Pastures
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Red imported fire ants, Solenopsis invicta, workers are approximately ¼ in long, red-brown with shiny, dark abdomens. Fire ants are our only ants that have two bumps between the thorax and abdomen. Winged females are larger, measuring approximately 1⁄3 inch in length.
Fire ants in established pastures can have a serious impact on both livestock and companion animal (horse) operations by causing animal and personnel injury and equipment damage. Losses can also be incurred through decreased hay production, feed and forage loss and reduced animal feeding.
Fire ant colonies consist of three adult types: winged males, reproductive (females) and worker ants. Winged males and reproductive start to fly when temperatures are between 70-95°F (spring and fall) and mate in flight. Winged males die shortly after and successfully mated females will search for a place to establish a new colony, shed their wings and start digging chambers in the soil for their eggs. New queens lay between 12-25 eggs that hatch within 7-10 days- established queens can lay as many as 800 eggs per day. Larvae hatch from the eggs and develop for 7-10 days before pupating. Adults emerge in about two weeks. Worker ants are wingless, sterile females that protect the colony by feeding the queen, defending the nest, foraging for food and caring for the brood.
With small colonies, nests can often go undetected until after heavy rainfall when fire ants build up mounds to escape the accumulating groundwater. As the population grows, they continue to produce mounds, particularly in sunny areas or next to sidewalks, driveways and other areas where soil temperatures tend to be higher.
Research at Texas A&M University suggests that drenching mounds with a large volume ( > 2 gal) of hot water (> 90°F) or severe mechanical disruption can significantly reduce fire ant activity, however, satellite mounds can form within a few feet of the original mound within a few days. These methods can have useful, but temporary impact on fire ant colonies in areas where a chemical application is not possible.
Fire ant activity in livestock pastures can cause significant injury to livestock in mid-summer because the ants may forage for food and moisture in the same areas where livestock are grazing and/or giving birth. If possible, schedule fertility programs to avoid birthing during the heat of the summer which can minimize fire ant-animal contact. Alternatively, provide livestock with a designated birthing area or pasture that has previously been treated with fire ant insecticides and has been checked for signs of active fire ant mounds.
Home remedies such as applying instant grits, molasses, aspartame or club soda to ant mounds do not work. Pouring chlorine, ammonia, gasoline or diesel fuel on mounds can contaminate the soil and groundwater and is dangerous.
There are a number of beneficial organisms in the environment that can have an impact on fire ant populations if environmental conditions are conducive. Microorganisms such as microsporidia can infect immature and adult fire ants, causing shorter life spans and, ultimately, colony decline over several months. The insect-parasitic fungus Beauveria bassiana, toxic to white grubs and chinch bugs, produces spores that attach to ant exoskeleton, germinate and grow inside and outside the ant. However, B. bassiana is much more effective when it comes in direct contact with individual ants rather than applied to the soil surface.
Phorid flies are small, hump-backed flies that can parasitize adult fire ants. Adult flies hover above ant mounds, waiting to come in contact with a foraging worker ant. Once this occurs, females will lay an egg in the ant behind the head. When the egg hatches, the larva feeds inside the ant for approximately three weeks before the ant’s head detaches from its body and dies. Although phorid flies only decrease the colony population by less than 3%, fire ant workers escape to the underground when phorid flies are detected and are less likely to forage and build mounds.
Chemical applications are most effective when fire ants become active in late spring and early fall when air temperatures are between 70-95°F. Make sure to confirm fire ants are actively foraging prior to application by placing a potato chip or a slice of hot dog (pic) on the pasture surface near the mound (not on the mound) and allow it to sit for 5-10 minutes. There are two basic approaches to chemical control of fire ants. An insecticide can be applied to individual mounds (drench, bait) or it can be broadcast (granules, bait) over a larger area infested with fire ant colonies. Regardless of the method used, the objective is to kill not only the workers but also the queen, because she is the only ant in the colony that is capable of laying eggs.
There are few products labeled for fire ant control in pasture for companion animals (horses) not used for food or feed and even fewer available if grazing animals are used for human food consumption (see chart below). In grazed pastures, fire ant treatments will be limited to either direct mound drenches are broadcast bait treatments. Individual mound treatments are fast-acting but are more labor-intensive and generally more expensive than broadcast applications. Broadcast baits are quick, less hazardous and less expensive to apply but are slow-acting and can harm beneficial ant populations. Do not apply baits and drench at the same time. Allow 7 days between the two applications. Ants will not forage and accept bait while they are disrupted by poison. Also, pay close attention to label instructions concerning cutting / baling restrictions or the number of applications permitted in a season.
Regardless of method or product, always follow the label directions when applying any fire ant insecticide.
Specific control recommendations can be found under Fire Ant Resources on NC Turf Bugs.
The NCDA&CS Red Imported Fire Ant quarantine program restricts the movement of plant products from infested areas into non-infested areas (map). Baled hay and straw that has been stored in contact with soil requires a certificate or permit before it can be shipped outside the Imported Fire Ant area. For more information, visit http://www.ncagr.gov/plantindustry/Plant/entomology/IFA.htm
For additional information on fire ants in commercial turfgrass and home lawns, see Fire Ants in Commercial Turfgrass, Home Lawns and Landscapes.
- Fire Ant Management in Horse Operations Entomology Insect Notes. Watson, W., S. Bambara, and M. Waldvogel. 2011. NC State Extension Publications.
Management of Imported Fire Ants in Cattle Production Systems. Flanders, K. L. and B. M. Drees. 2004. Alabama Cooperative Extension System Publication ANR-1248.
- NC State Extension Plant Pathology Publications and Factsheets
- NC State Horticultural Science Publications
- North Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local N.C. Cooperative Extension center.