Issues : Figure Sets

Figure Set 4: Fire Ant Invasion and Control by a Parasitoid

Purpose: To inform students about fire ants and promote discussion of their control.
Teaching Approach: "paired think-aloud"
Cognitive Skills: (see Bloom's Taxonomy) — interpretation, application
Student Assessment: essay about website


The red imported fire ant (Solenopsis invicta) arrived in the U.S. from Brazil around 1940. Since then it has devastated arthropods and invaded over 250 million acres in the south (Orr et al. 1995). The red fire ant also kills poultry chicks, ground-nesting birds, lizards, and snakes. The estimated damage to public, livestock, and wildlife health by fire ants is $300 million in Texas alone; an additional $200 million is spent on control. Extrapolating to other states (Florida, Georgia, Louisiana) brings fire ant damage to more than $1 billion per year (Pimentel et al. 2000).

The stinging behavior of fire ants is notorious. They sting repeatedly and cause an intense burning sensation — hence the name "fire ant". White pustules form at sting site, which can become infected, resulting in permanent scaring.

Control of the red fire ant has been a real problem. When pesticides are used, this ant reinvades treated areas more quickly than native ants. The article by Orr et al. (below) provides evidence that a parasitoid fly from Brazil could limit the ability of red fire ants to attack and out compete other ants.

Humans have caused the dramatic spread of imported fire ants. Natural dispersion is by mating flights, colony movement, or rafting during flood events. The movement of ants coincided with the housing boom after World War II, largely due to the transfer of grass sod and woody ornamental plants for landscaping. In 1958, federal regulations limited the movement of nursery stock and grass sod, but by then imported fire ants had moved into eight southern states.

The following is from the University of Minnesota website (see references): "One of the identifying characteristics of a fire ant colony is the earthen nest or mound. The mound is a conically-shaped dome of excavated soil that has a hard, rain-resistant crust. The mound averages 0.40 m in diameter and 0.25 m in height. In heavier soils, a mound can exceed 1.0 m in height and 1.5 m in diameter. There are usually no external openings in the mound; tunnels approximately 25-50 mm below the surface radiate from the mound allowing foraging workers ready egress and ingress. The purpose of the mound is three-fold: 1) to be a flight platform for nuptial flights; 2) to raise the colony above the water table in saturated ground and; 3) to act as a passive solar collector to supply warmth to the colony during the cold winter months. Although mound size and shape differs to some extent based on soil type, during the dry hot days of late summer and early fall, new mounds are not formed and older mounds are not maintained. While mounds are important to a colony, they are not essential for colony survival. Given a dark, protected site with sufficient moisture and an adequate supply of food, fire ants will nest in a wide variety of sites (e.g. rotten logs, walls of buildings, under sidewalks and roads, in automobiles, in dried cow manure).

Fire ants are omnivorous, feeding on almost any plant or animal material; although insects seem to be their preferred food. In rural habitats, fire ants have a major impact on ground nesting animals from insects to reptiles to birds to mammals. The arrival of imported fire ants into an ecosystem wrecks havoc on the local ecological community. Studies (Allen et al. 1995) have shown that a minimum two-fold reduction occurs among populations of field mice, oviparous snakes, turtles and other vertebrates when imported fire ants are allowed to establish colonies within a given area. In some instances, the depredation by fire ants has completely eliminated some species from an ecosystem (Porter & Savignano 1990). The reduction or elimination of a species or group of species from a system has repercussions throughout the local food web. Not only do imported fire ants reduce animal populations, they also feed on plants. Fire ants attack young saplings and seedlings. They destroy buds and developing fruits and have been shown to feed on the seeds of 139 species of native wildflowers and grasses (Lockley unpubl.). Secondarily, fire ants "nurse" numerous homopteran pests of plants such as aphids and scale insects. Although not conclusively shown, observations indicate that their activity on the plant itself may reduce the ability of pollinators to successfully pollinate flowers.

In agriculture, fire ants have been identified as damaging fifty-seven species of cultivated plants (Adams 1986). Fire ants feed on the germinating seeds of some crops (e.g. corn, sorghum, soybeans) and the buds and developing fruits of others (e.g. citrus, okra) .

As an urban pest, imported fire ants cause many of the same problems experienced in rural areas as well as some problems unique to the urban environment. As in agriculture, imported fire ants cause significant damage to numerous plants and, as in rural habitats, fire ants can reduce the number of birds and mammals in an urban landscape. Fire ants nest within urban structures such as the walls of homes and offices. They establish colonies under sidewalks and roadways. When the site is abandoned, subsidence will cause cracks to appear and will occasionally result in the complete collapse of sections of these structures. The presence of fire ants can deter outdoor activities in yards, parks and school grounds. Home invasions can threaten small children and the elderly. House invasions are especially prevalent during periods of heavy precipitation and flooding. Fire ant colonies have been found inside automobiles, trucks and recreation vehicles (Collins et al. 1993). Traffic accidents have been caused by fire ants stinging the drivers of automobiles. Victims of highway accidents can be attacked by fire ants if they are thrown from their vehicles."


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