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Some New Research in Ant Control

By Nick Sakovich

The most prevalent ant species found in our citrus orchards is the Argentine ant. This ant causes no direct damage to the trees themselves; i.e. they do not feed on leaves or girdle bark. Another California species, Solenopsis xyloni, the California or Southern Fire Ant, can chew on the bark of young citrus trees and cause girdling. It is strictly indirect damage that is caused by the Argentine ant. Since they feed on the carbohydrate- rich honeydew secreted by other insects such as soft scale, mealybugs, whitefly and aphids, it is in the ants best interest to protect this food source. The shear presence of a large number of ants throughout the aerial parts of the tree results in such substantial disruption of parasite and predator activities, such as egg laying and feeding, that the effectiveness of these biological control agents is greatly diminished. In addition, ants may also destroy the larvae of the predators and parasites. When California red scale, a non-honeydew producer, coexists with honeydew-producing pest species, biological control for the red scale will also be greatly diminished.

Ant populations are low during the winter. In the spring they begin building, until high populations are reached in the summer and early fall. During this warm season, the ants move their nests underneath the shade of the trees, whereas during the winter the nests are located in a more sunny location. Traditionally, a good control measure for the Argentine ant begins with skirt pruning - usually at 18-30 inches. This leaves the trunk as the only access way into the tree (check also for any weeds that may be growing up into the tree). Next, apply a soil and trunk treatment of Lorsban-4E (see DowElanco supplemental label). This will kill many of the ants, but more importantly, it leaves a chemical barrier that denies ants entry into the trees. It will also help to starve out the ant colonies. Nests around the perimeter of the orchard should also be treated.

Recently however, a number of new research trials have been focusing on some different methods of ant control.

Alarm pheromones are often released by ants when they are in danger, causing nearby ants to become agitated and disperse. Other insects sometimes produce irritating or repellent chemicals to ward off attacking ants. Many of these naturally occurring ant repellents have been identified and tested for their activity in preventing the ants from following their trails to food sources.

Drs. H.Shorey and P.Phillips recently conducted research which showed one of these natural occurring repellents, called Farnesol, to be one of the better ant-repelling compounds. When cotton twine is dipped in a combination of Farnesol and Stickem, and wrapped around the trunk of a skirt-pruned tree, ants are repelled for up to three months. With the efficacy proven, the next step is to develop a practical application technique for this material. As far as registration is concerned, Farnesol already has FDA registration as a food grade substance, so EPA and State pesticide registration should not be difficult.

Another series of experiments has also been initiated to determine the most attractive bait materials for stimulating several ant species, including the Argentine ant, to carry these baits back to the nest. Such baits have included corngrits and soybean oil, crickets and cockroaches, cornmeal, anchovies, and freeze-dried silkworm pupae. These materials are very attractive to such species as fire ants, because of their preference for fatty materials.

The Argentine ant however, being a honeydew feeder, has a strong preference for high carbohydrate liquids. High sucrose-based baits, (50% solution), were found to be the most preferred. Various concentrations of boric acid as the toxicant were also tried in combination with the high sucrose baits. It was found that the lowest concentration of boric acid, 0.25%, was as acceptable to the ants as was the sugar solution alone. Higher concentrations, 0.5 - 2%, tended to inhibit acceptance. Boric acid is an excellent toxicant for ants. However the next step is to determine whether this very low concentration (0.25%) is adequate to destroy whole colonies of the Argentine ant.