RED IMPORTED FIRE ANT IN NORTH CAROLINA
By: Charles Apperson and Michael Waldvogel, Extension Entomology
Insect Note - ENT/rsc-35
North Carolina’s red imported fire ant infestation continues to expand, partially as a result of recent mild winters but more recently due to increased residential and industrial development and subsequent introductions of fire ants in infested sod and nursery stock. Although fire ant stings are not fatal for most people, they are painful. The mounds that the ants build can interfere with the operation of machinery in agricultural fields. It is not practical to eradicate these ants, but their populations can be controlled, and the chance of contact with people can be minimized. This publication discusses the red imported fire ant and suggests suitable control methods.
The red imported fire ant, Solenopsis invicta, a native of southern Brazil, currently inhabits eleven southern states, as well as isolated areas in New Mexico and California. In North Carolina, imported fire ant is found in 71 of 100 counties including isolated areas in western NC. These areas (shown in red on this map). These areas are currently under quarantine by the U.S. Department of Agriculture (USDA) and the North Carolina Department of Agriculture & Consumer Services (NCDA&CS), regulating the export of certain items that might carry ant infestations to other parts of the country.
From an agricultural perspective, red imported fire ants are nuisances primarily because they annoy field workers and because their mounds may damage harvesting equipment. Livestock injury and crop damage are usually minor. Fire ants have a much greater impact on the orna-mental plant, sod, and landscaping industries because of problems associated with shipping infested plant material into uninfested areas of the country (see the section entitled "Quarantine Assistance”). Mounds discovered in previously uninfested areas of North Carolina are frequently traced to landscaping performed at commercial and residential developments. For the general public, two aspects of red imported fire ant infestations are particularly annoying: the unsightly mounds formed in lawns and yards and the painful stings received when mounds are disturbed. Within 24 hours after a person is stung, a pustule-like sore forms at each sting site (as shown here), which usually itches intensively. Scratching the pustule may rupture the skin, leading to secondary infection and scarring. A small proportion of people stung is highly allergic to fire ant stings and requires immediate medical attention. As red imported fire ants spread into more populated areas of the state, more people are likely to be stung. Encounters with fire ants can be expected not only outdoors but indoors as well. In other southern states foraging ants have invaded private residences and buildings such as offices, hospitals, and nursing homes. In these situations, fire ant control is more critical and potentially more difficult because of concerns related to both the ants and the indoor use of chemical insecticides.
Photograph by Daniel Wojcik., USDA-ARS
Adult red imported fire ants are reddish to dark brown and occur in five forms: (1) minor workers, about 1/8 inch long; (2) major workers, about 1/4 inch long; (3) winged males and (4) females, each about 1/3 inch long; and (5) queens, about 1/3 inch long. Fire ant mounds vary in size but are usually in direct proportion to the size of the colony. For example, a mound that is 2 feet in diameter and 18 inches high may contain about 100,000 workers, several hundred winged adults, and one queen. If you break open an active fire ant mound, you typically find the "brood" - whitish rice grain-like larvae and pupae. These immature ants will eventually develop into workers or winged adults. Mounds constructed in clay soils are usually symmetrical and dome-shaped; mounds built in sandy soils tend to be irregularly shaped. It is often difficult to distinguish the red imported fire ant from the tropical fire ant and the southern fire ant, which are also found in North Carolina. For positive identification, take a specimen to your county Cooperative Extension Center.
During the spring and summer, winged males and fe-males leave the mound and mate in the air. After mating, females become queens and may fly as far as 10 miles from the parent colony. However, most queens descend to the ground within much shorter distances. Only a very small percentage of queens survive after landing. Most queens are killed by foraging ants, especially other fire ants. If a queen survives, she sheds her wings, burrows into the ground, and lays eggs to begin a new colony. In the late fall, many small colonies of fire ants will appear. Many of the colonies will not survive the winter unless the weather is mild.
Fire ants prefer oily and greasy foods. They also feed on many other insects and, from that standpoint, could be considered beneficial. To find food, workers forage around their mound often in underground tunnels that radiate from the mound. If the mound is disturbed, ants swarm out and sting the intruder.
Fire ant mounds are typically found in
Because fire ants cannot be eradicated over wide areas, the goal should be to manage the ants with a combination of chemical and non-chemical control tactics in order to eliminate fire ants in areas where they pose the most immediate hazard to people, pets and livestock, and to reduce infestations to "acceptable" levels. Options for control depend on the setting (e.g., agricultural fields, pastures, home lawns, schools, etc.).
There are two basic approaches to chemical control of fire ants. An insecticide can be applied to individual mounds or it can be broadcast over a wide area infested with fire ant colonies. Individual mound treatments are usually more environmentally and ecologically acceptable because they use less insecticide and limit areas treated as compared to broadcast treatments, and they are likely to have less impact on non-target insects. 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. Always follow the label directions when applying any fire ant insecticide.
mounds may be treated with a liquid or dust insecticide formulation or with an insecticidal
Ant baits also can be used to treat individual mounds. These baits are essentially a mixture of an insecticide and a food that is attractive to fire ants. Worker ants carry particles of the bait back to the mound and feed them to the "brood" (larvae or immature ants) and the queen. Even when the insecticide kills the queen, workers may be active inside the mound for several weeks before the colony finally disappears. Baits are somewhat slow acting but easier to apply than mound drenches. Therefore, they are best used in situations where many mounds must be treated, or when water for mixing mound drenches is difficult to obtain, or when the risk of human or non-target animal contact is low and there is no urgent need to eliminate the infestation. The active ingredients in ant baits are rapidly degraded by high temperature, high humidity, and intense sunlight. The baits can be rendered ineffective in a few hours by these conditions. Follow this procedure when using baits.
"Two-Step Method" - You can also apply a mound drench 5-7 days after baiting to kill of remaining workers more quickly.
Broadcast treatments can be used to apply insecticides (liquids, baits, or granular insecticides) over a large infested area containing many fire ant colonies. One disadvantage of broadcast treatments is that they can also disrupt ant communities. Although most people think of ants strictly as being pests, they are also a very important parts of our ecosystem. Broadcast treatments can result in an ant community changing from one that is dominated by native ants to one dominated by imported fire ants. On the other hand, in areas with very high mound densities, broadcast applications allow large areas to be treated quickly. Areas of high public use may be protected by spring and fall broadcast applications of ant bait or a well-timed granular insecticide. One limitation on the use of granular insecticides (not granular baits) is that most of them require water (either from rain or by irrigation) to be applied shortly after the application. When rain is not expected for several days or in areas where watering may be restricted or not feasible, a granular insecticide may not be the best choice. If the area becomes reinfested with fire ants during the summer months, individual mounds can be treated with an insecticide drench or ant bait, although as noted previously control is more difficult when temperatures are high. Consult the NC Agricultural Chemicals Manual for a list of granular insecticides (and baits) that can be used against fire ants.
The key to reducing the threat
of fire ant infestations indoors is prevention, which means removing exposed
food sources that may attract these insects. In some cases, fire ants may nest indoors, e.g., inside walls or partially under concrete slab floors. In those instances you will likely see soil and other debris pushed out around expansion joints near the edge of carpeting (image at right) or around water or other utility pipes. In most situations, fire ants are simply entering the building from an outdoor nest. In those situations, the treatment objective must be to reduce the potential for accidental
stings as quickly as possible. Insecticides labeled for indoor use can be found in
NC Agricultural Chemicals Manual
. Particularly the pyrethroid insecticides (products containing chemicals such as permethrin, cyfluthrin, bifenthrin, etc.),
can be used in homes and public buildings to drive foraging ants outside
or away from high-use or critical areas, such as kitchens, recreation
rooms, patient rooms, operating rooms, or intensive care units. Select products that are specifically labeled for use indoors. Although
baits work well for many ant species that invade buildings, they are not the best choice for controlling fire ants indoors because they are more likely to draw more ants inside and potentially increasing the chance that someone will be stung.
For this reason, it is important to positively identify the ants that are invading your home before applying any control measures. Information about other ant species can be found in A Guide to House-Invading Ants and Their Control.
A good summary of fire ant control products available for use by the general public can be found in this publication from Auburn University. Options for control are more restricted for areas such as pastures that may be used for grazing horses or livestock. Consult the following publications for advice in dealing with these situations:
There are some non-chemical methods available that
can be used against fire ants; however, they may be limited in their effectiveness (or may be ineffective).
Hot Water and Mechanical DisruptionHot water (i.e., 90° F) and mechanical disruption have been used in many instances. Results of some preliminary evaluations at Texas A&M University have shown that these treatments will kill large numbers of ants; however, satellite mounds formed by surviving ants subsequently appear. Thus, these methods can have a useful, but temporary impact on fire ant colonies in areas situations where pesticides of any type are considered unacceptable. Other non-chemical mechanical devices that disrupt colonies do not have scientifically-based test data to support their effectiveness. One potential downside to using hot water is that it can damage/kill vegetation in the general vicinity.
GritsA long-standing folklore method of controlling fire ants (and other ants) has been to pour grits over the mound. The assumption in some cases has been that the ingested grit particles absorb water and cause the ants to "explode". However, fire ants (and ants in general) feed primarily on liquified foods and their digestive tracts filter out these solid particles. Results of laboratory studies at Texas A&M University have shown that foraging fire ants collect the grits, but there is no reduction in the number of ants in the colony.
Biological ControlResearch is underway to look at the use of biological control agents to control imported fire ants. These agents include parasitic flies and other ant species, as well as fungi and other microorganisms. These methods are not yet proven to be extremely effective by themselves but can help reduce fire ant populations as part of an IPM program. For more information about biological control of fire ants, visit this USDA-ARS website.
Since the major sources of new infestations outside the current fire ant range are often infested sod and nursery stock, limiting this source or ants is critical to slowing the spread of fire ants. The North Carolina Department of Agriculture & Consumer Services and the USDA-Animal and Plant Health Inspection Service (USDA-APHIS) cooperatively regulate shipments of nursery and sod farm items, such as balled nursery stock and turf, to areas outside of the quarantine zone. Under the terms of the compliance agreement, nursery operators have several ways to obtain certification and to ship nursery stock out of the quarantine zone. For information and assistance, contact the NCDA&CS.
Pest information and control recommendations presented here were developed for North Carolina and may not be appropriate for other states or regions. Any recommendations for the use of chemicals are included solely as a convenience to the reader and do not imply that insecticides are necessarily the sole or most appropriate method of control. Any mention of brand names or listing of commercial products or services in the publication does not imply endorsements by North Carolina Cooperative Extension nor discrimination against similar products or services. All recommendations for pesticide use were legal at the time of publication, but the status of pesticide registrations and use patterns are subject to change by actions of state and federal regulatory agencies. Individuals who use chemicals are responsible for using these products according to the regulations in their state and to the guidelines on the product label. Before applying any chemical, always obtain current information about its use and read the product label carefully. For assistance, contact the Cooperative Extension Center in your county.
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