Ants

Ants (Formicidae) develop through four life stages : egg, larva, pupa, and adult. Most ants outside the nest are wingless, sterile females (workers). More than one dozen species of ants can potentially infest greenhouses or nurseries in California. These including those named above are distinguished in the well-illustrated Identification Key to Ant Species (PDF). It can be very helpful to identify the particular ant species present because their biology differs, as can their management.

Argentine ant workers are about 1/8 inch long and uniformly dark brown. They forage and travel in trails characteristic of many ants that move between their underground nests and irrigation lines, plants, and the surface of structures and soil. Argentine ants lack prominent hairs and have one node (hump) on the petiole, the waist-like (narrow) area between the thorax and the gaster (swollen part of abdomen behind the petiole).

The abundance of Argentine ants increases greatly during late winter and spring, peaking during midsummer and early fall. The nests where immatures and queens occur are relatively shallow and usually the entire colony is within 2 inches of the soil surface. Argentine ants feed on sweet liquids any time of year they are active. They are also attracted to and feed on protein (e.g., granules of solid ant baits) during the spring when colonies are reproducing rapidly.

Native gray ant workers are a mix of gray and reddish brown and commonly appear grayish overall. They are up to about 1/5 inch long and the largest ants commonly found around plants. Workers are relatively solitary and move in an irregular pattern. Their scattered trails contain many fewer individuals than do the trails of Argentine ants. Native gray ants nest in topsoil or under rocks and soil debris.

Southern fire ant adults have a light reddish-brown head and thorax and a blackish, hairy abdomen. Fire ants can be distinguished from most other ants by the presence of varying size workers; both relatively large (up to 1/5 inch) and small (about 1/16 inch) individuals occur together in foraging trails, unlike with Argentine ant and native gray ants where all workers are approximately the same size. Fire ants build nests with openings that are loose craters or mounds of soil, such as near the base of trees and other objects. Fire ants will bite and sting aggressively when disturbed.

Red imported fire ant adults of this species cannot readily be distinguished in the field from southern fire ant. This ant feeds on almost any plant or animal material, including chewing bark off of young trunks, which can kill the plants. When disturbed red imported fire ants will bite and sting aggressively, possibly causing allergic reactions.

Many pest ants feed on honeydew excreted by phloem-feeding insects including aphids, mealybugs, psyllids, soft scales, and whiteflies. Ants protect these other insects from their natural enemies, allowing populations to increase and become more damaging to crops. Argentine and native gray ants are the most common ant species that aggressively protect pest insects. In addition, Argentine ants and red imported fire ants can plug up irrigation sprinklers. Fire ants directly damage plants and can attack workers.

Cultural and chemical controls are used to manage pest ants.

The primary natural enemies of the ants are other species of ants. There are no known methods for effectively controlling pest ants with parasites, pathogens, or predators.

Exclude ants from benches by encircling bench legs with sticky material in a manner that keeps the material out of the way of workers. Or place each leg bottom in a shallow container filled with water, becoming an ant-exclusion moat. Cultivation reduces ant abundance, but avoid creating dust that can disrupt the effectiveness of natural enemies of other pests.

Cultural controls, including the use of sticky materials, are organically acceptable management methods.

Monitor growing areas regularly for ants, ant nests, and honeydew-producing insects at least from late winter through fall. Periodically inspect for ants and bark damage under any trunk wraps of trees.

Baits are the preferred chemical method for ant control. Effective bait insecticides have slow-acting toxicants that worker ants collect and pass to other ants during their sharing of food and colony communication chemicals. Most products attract ants using protein-based solids or sugar-based liquids, and most ant species are highly attracted to only one of these bait types. If the bait does not attract the ant species present, it provides no control.

For the most effective and economical control apply ant baits beginning in late winter to early spring when the abundance of most ant species and their above-ground activities increases. Identify the most common species of ants present to guide bait selection. Fire ants are controlled only with solid baits using grain protein and plant oil as the attractants. Argentine ant and native gray ants are primarily controlled by insecticides in sweet liquid baits. Where Argentine ants are the predominant species, both types of bait may be effective during late winter through spring.

When in doubt about the ant species and best bait choice(s), deploy two or more types of baits, observe which type(s) the ants are feed on, then more widely place out the attractive product(s) where ants are a problem. Note that ants' attraction to baits and preference for particular types can change during the growing season in part due the availability of alternative foods, such as plant seeds (protein) and insect honeydew (sugary liquid).

Corncob grit and oil baits

Solid baits generally contain corncob grits or other grains mixed with soybean oil to attractant protein-feeding fire ants. The insecticides tend to degrade in light, so apply baits early in the morning or late in the day and in shady spots beneath plant canopies when ants are active and can take the bait into the nest. These granular or pellet grain and oil baits can be broadcast over the site, but spot application at the location of the ant nests is preferred because it concentrates the food where the ants most occur.

Sugar-water baits

Sweet liquid baits attract Argentine ant and native gray ants. For liquid baits using borate (e.g., boric acid, disodium tetraborate, orthoboric acid) as the insecticide, the effective concentration is 1/2 to 1% active ingredient. Evaporation of the bait liquid can cause the concentration of the toxicant over time to increase to a concentration that repels ants. But certain types of liquid bait stations greatly reduce or largely prevent evaporation. Note that borate insecticide may only be available for use in and around structures.

Broad-spectrum, persistent insecticide sprays

Avoid spray applications for ant control; surface sprays may not reach the immature ants and queens underground, so more ants will be produced and emerge to re-infest sites. Organophosphates (e.g., malathion), phenylpyrazoles (fipronil), and pyrethroids (permethrin) historically sprayed for ants are increasingly restricted because their movement after application is contaminating surface waters. Contact the county agricultural commissioner to learn the current regulations before spraying surfaces for ant control.

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