Description of the Pest
The Argentine ant is common in most California olive-growing areas. It is about 0.13 inch (3 mm) long, uniformly deep brown to light black, and does not bite or sting. The Argentine ant has one petiole node (hump) between the thorax and the gastor (swollen part of abdomen right behind the petiole). Worker ants travel in characteristic trails on the ground and up the trunk and scaffolds. They forage during daylight hours. Ant populations peak in midsummer and early fall. Their nests are very shallow, usually within 2 inches of the soil surface.
The pavement ant is 0.13 inch (2–3 mm) long and has a dull, blackish brown body that is covered with coarse hairs. The head and thorax have many parallel furrows. Pavement ants have two nodes between the thorax and the gastor. They move in slow deliberate motion. They prefer to nest in sandy or loam soils.
The southern fire ant, also called the California or native fire ant, is light reddish brown with a black abdomen. The entire body is covered with golden hairs and has two nodes between the thorax and the gastor. Workers range in size from 0.1 to 0.18 inch (2.5–4.5 mm). They do not usually travel in conspicuous trails and will swarm over the ground when disturbed. This ant will sting when provoked. Southern fire ants build nests of loose mounds or craters near bases of trees around wetted areas and do not aggregate in colonies as large as those of the Argentine ant. They forage in the morning and early evening and are underground during hot periods.
Native gray ants, also called field ants, are larger than the other ants, measuring up to 0.3 inch (7.5 mm) and, like the Argentine ant, have one petiole node (hump). These gray ants nest in topsoil or under rocks and debris, move in an irregular jerky manner, and generally do not travel in trails or sting. Formica aerata is more common in the San Joaquin Valley whereas Formica perpilosa occurs primarily in the Coachella Valley. Native gray ants do not trail and appear solitary.
Ants can be extremely disruptive to IPM programs, especially Argentine and native gray ants. These ants feed on honeydew excreted by scales. As part of this relationship, they also protect these honeydew-producing insects from predators and parasites, thus disrupting biological control.
Ants may become a problem in olive orchards that have not been recently tilled. Manage ants when they are interfering with biological control of scales or psylla. Cultural practices and baits can be used in an integrated program.
Tilling the soil for weed control will disturb the nesting sites of ants and help to reduce their populations.
Planting a cover crop of common vetch (Vicia sativa) can help to keep gray field ants (Formica sp.) off the trees. Common vetch has an abundance of nectaries that attract the ants away from the honeydew-producing insects. In grape vineyard studies, common vetch was planted in a 80:20 mixture with 20% Merced rye so that it could establish in late fall and winter in order to attract the ants during spring and early summer. The addition of rye to the mixture helps to provide structure and support in the cover crop for the vetch.
Organically Acceptable Methods
Organically acceptable management tools are cultural control methods.
Monitoring and Treatment Decisions
Monitor the orchard in spring when scale begin producing honeydew. Check the abdomen of ants descending the trunks to see if they are swollen and translucent; this helps identify them as honeydew-collecting species. Periodically inspect for ants on branches and leaves.
Baits are the preferred chemical method for ant control whenever feasible. Effective bait insecticides have slow-acting toxicants that worker ants collect and feed to other ants, including nest-building immatures and queens. For the most effective and economical control, treat when ants are active in early spring following winter rains and again in late August.
To determine which bait to use, identify your primary ant species; fire ants are predominantly protein feeders whereas most gray and black ants are sugar feeders.
Corncob Grit and Oil Baits
Solid baits utilize treated corncob grits mixed with soybean oil as the food attractant plus an insecticide. These are effective for the primarily protein-feeding fire ants. The toxicants tend to degrade in light, so apply baits early in the morning or late in the day when ants are active and will take the bait into the nest. Generally, corncob grit baits are broadcast over the acreage that needs to be treated. However, spot application of baits at the location of the ant nest is preferred over widely spreading the bait because it concentrates the food where the ants are.
Sugar-Water Based Baits
Liquid baits use a toxicant mixed in sugar water, which disguises the toxicants as well as helps attract the ants. These baits are most useful for the liquid sugar-feeding Argentine and native gray ants. Evaporation of the bait can cause the concentration of the toxicant to increase to a level in the bait that becomes repellant to ants. All liquid baits must be used in an EPA-approved bait station.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide’s properties and application timing. Always read the label of the product being used.|
|(Esteem Ant Bait 0.5%)||1.5–2 lb/acre||12||1|
|MODE-OF-ACTION GROUP NUMBER1: 7C|
|COMMENTS: A corncob grit and soy oil bait. Effective only against fire ants because they are attracted to the soy oil mixed with corncob grits bait. Apply when fire ants are most active during the season (especially early summer and fall) and when they are most active during the day (early evening and early morning when soil temperature is above 60°F). Treatments are most effective if applied 2 days after irrigation, when ant activity is at a maximum. Do not irrigate again until at least 24 hours after application. Do not apply if rainfall is anticipated with 4 to 6 hours after application. While this bait can be broadcast using properly calibrated ground equipment to assure proper dosage and uniform distribution, spot applications at the location of the ant nest are preferred. Retreatment may be desirable after 3–4 months.|
|**||Apply with enough water to provide complete coverage.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|