Plants shipped and sold in nurseries and garden centers are a source of spreading infestations of red imported fire ants (Solenopsis invicta). A new study by USDA researchers shows treating root balls with the non-repellent insecticide dinotefuran is successful in eliminating infestations and preventing their re-occurrence for up to six months. (Photo by Kira Metz, USDA APHIS PPQ, Bugwood.org)

By Andrew Porterfield

Andrew Porterfield

The red imported fire ant (Solenopsis invicta) has been a pest in the U.S. since its discovery in shipping cargo sometime between 1933 and 1945, inflicting very painful bites, and creating more than $8 billion in economic damage in the country every year. It’s now found in 19 states, mostly centered on the Southeast but reported as far away as California, and it has flourished in Australia and China.

In the U.S., a Federal Imported Fire Ant Quarantine was established in 1958 to restrict movement of plants and objects that could transport the fire ant. Most researchers and officials agree that the ant’s spread is linked to shipments of nursery plants. Nursery managers have sprayed pesticides on the plant root balls to control the ants, but the use of many of these pesticides, such as chlorpyrifos, are now restricted, and the chemicals themselves are quite expensive.

A team of researchers from the U.S. Department of Agriculture’s Agricultural Research Service and Animal and Plant Health Inspection Service and Tennessee State University studied the use of non-repellent insecticides on nursery plant root balls to reduce infestations. Non-repellent pesticides would increase exposure to fire ants and allow transfer of the toxic chemicals to nestmates. The researchers’ study, published in March in the Journal of Economic Entomology, found that one non-repellent insecticide, dinotefuran, significantly reduced fire ant infestations in nursery plant root balls.

For large plants shipped and sold in nurseries and garden centers, roots are commonly wrapped in a ball shape with burlap, which can make them difficult to disinfest of any pest insects. A new study by USDA researchers shows treating root balls with the non-repellent insecticide dinotefuran is successful in eliminating infestations or red imported fire ants (Solenopsis invicta) and preventing their re-occurrence for up to six months. (Photo by Mollie Freilicher via Flickr, CC BY-NC-SA 2.0)

The researchers infested the root balls of boxwood (Buxus microphylla) plants with fire ant colonies (workers, eggs, larvae, pupae, and one queen). The team sprayed half the root balls with bifenthrin pesticide. They then sprayed four different non-repellent insecticides for comparison: dinotefuran, indoxacarb, imidacloprid, fipronil, and water as a control. They also looked at the effects of different concentrations of non-repellent pesticides and tried to determine the efficacy of residual pesticides to deter ant infestations.

Dinotefuran showed the highest level of worker reduction, at a mean of 99.99 percent reduction, followed by indoxacarb at 99.33 percent and imidacloprid at 99.49 percent. Combinations of the four non-repellent insecticides with bifenthrin resulted in considerably lower reduction levels (except for dinotefuran, at 94.29 percent). Testing the effects of reducing treatment costs using dinotefuran, the researchers tried lower concentrations of the insecticide and found more than 90 percent reductions and no difference in infestations among dinotefuran concentrations. Dinotefuran efficacy at the full labeled concentration prevented infestations for six months, while a half-rate of dinotefuran left infested root balls.

“Among the non-repellent insecticide treatments, dinotefuran with or without bifenthrin offered the most consistent quarantine level control, with 75 percent (eight) of the root balls being uninfested,” the researchers write. “The other non-repellent insecticide (imidacloprid, indoxacarb, and fipronil) treated root balls … were 0-38 percent uninfested.”

The researchers note that dinotefuran is more expensive that the two insecticides approved under the Federal Fire Ant Quarantine, chlorpyrifos and bifenthrin. Reduced rates of dinotefuran showed promising results, but “more replications are needed to definitely distinguish the effects of the dinotefuran concentrations on the numbers of uninfested and infested root balls,” they write.

Dinotefuran has its own issues, however. It is water-soluble and toxic to honey bees (Apis mellifera) and can spread through runoff, spray drift, and translocation in plants. The insecticide is currently subject to regulations and label restrictions to reduce exposure to bees. “For nurseries, dinotefuran applications only to the root balls of harvested … trees prior to bloom should reduce the risk of bee exposure,” the researchers note, adding that further research is needed to determine the best way to use such non-repellent pesticides on red imported fire ants.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Connect with him via LinkedIn or via email at [email protected].