Ant management in gardens and landscapes
Ants (Formicidae) are Hymenoptera closely related to bees and wasps. Of about 200 ant species known in California, only a few are commonly pests. Most species are not bothersome and rarely or never are pests. Ants can improve soil conditions and some are useful predators of pest insects.
Although sometimes confused, distinguishing ants and termites is not difficult. Ants have
- A narrow constriction between the abdomen and thorax.
- Antennae that are distinctly elbowed.
- Hind wings that are much shorter than forewings, if wings are present.
- A broad waist, where the abdomen and thorax meet.
- Antennae that are not elbowed.
- Wings of equal length, when present.
It can be very helpful to identify the particular ant species present because their biology differs, as can their management. Ant baits (attractant and insecticide combined) vary in composition. Those for outdoor use attract ants using protein-based solids or sugar-based liquids. Many 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.
To identify ants to species
- Collect them in a noncrushing manner. For example, knock them from plant parts onto paper or scoop them from flat surfaces with stiff paper. Seal the infested paper in an ant proof container and freeze this at least a few minutes. Thaw and examine the dead ants as below. Learn how to recognize colonies of the red imported fire ant (RIFA) from a distance; do NOT attempt to collect ants suspected of being these. Even myrmecologists (ant experts) sometimes regret collecting RIFA!
- Consult the appropriate ant key as below and observe ants under magnification.
- Compare features of pest ants to illustrations in the keys.
- Locate the narrow (front) portion of the abdomen (petiole), which is the first one or two abdominal segments. Count the number of nodes (projections) on the petiole; there are one, two, or apparently none. None is because the node is hidden as with odorous house ant, Tapinoma sessile.
- Look for the other, distinguishing characteristics including body contour, hairs and spines, and the number and relative lengths and widths of antennal segments.
To identify common outdoor species in California consult publications such as the ant key from Integrated Pest Management for Citrus. For ants found inside, consult Key to Identifying Common Household Ants. You can take freshly killed ants to the county agricultural commissioner or your local University of California Cooperative Extension office for identification.
Ants develop through four life stages. Most individuals are wingless workers (sterile females) that care for the pale, grublike immatures (larvae and pupae) in the nest, defend the colony from natural enemies, dig tunnels, and forage for food outside the nest.
Each colony has one or more queens (fertile females) that spend most of their time in the nest laying eggs. Males occur only during the brief mating season when males and queens develop wings and may mate in flights or swarms outside of the nest. Most species of ants nest underground or beneath buildings or rocks or other objects.
As workers returning with food meet those departing their nest, they exchange a tiny droplet (food sharing, or trophallaxis) containing communication chemicals and nutrients. Slow-acting insecticide baits take advantage of this behavior. Foraging ants collect and carry the insecticide and spread it throughout the entire colony during food sharing, thereby poisoning ants underground and queens in their nest.
Ants become pests in gardens and landscapes when they feed on sugary honeydew excreted by other insects that suck plant sap (phloem). Ants tend these aphids, mealybugs, soft scales, whiteflies and other honeydew-producing insects and harass or kill parasites and predators that might otherwise control honeydew producers. Ants can also disrupt the biological control of pests that do not excrete honeydew, such as armored scales and mites.
Ants may bite or sting or both when contacted or disturbed by domesticated or wild animals or people. Some species invade homes and other buildings seeking food, shelter, and water. Carpenter ants, Camponotus spp., tunnel in wood. Those that chew bark or plant parts include pavement ant, Tetramorium caespitum; red imported fire ant, Solenopsis invicta; and southern fire ant, Solenopsis xyloni.
It generally is not feasible to totally eliminate ants from an outdoor area. Focus management efforts where ants are direct pests, such as when they are tending honeydew-producing insects that are bothersome or damaging plants. When ants are abundant on plants near structures, they are more likely to come indoors.
Where ants or honeydew-producing insects have been problems, beginning in late winter or early spring inspect the trunks of those plants and the surface of soil nearby for trails of ants. During cool weather, look for ants during the warm times of day. During hot weather, monitor for ants during the cooler times of day.
Late winter and spring are the best times to monitor for ants because they are less abundant and easier to manage. When numerous ants of a pest species are found, you may need to trace their trails to determine where the ants are feeding, such as on leaves, succulent stems, and twigs infested with honeydew-excreting insects.
Use a combination of methods to manage ants:
- Apply an effective, enclosed, insecticide bait to poison ant colonies.
- Apply sticky material (e.g., Tanglefoot) or other effective barriers to bark, encircling trunks to exclude ants.
- Keep irrigation water, leaf litter and mulch, and vegetation at least several inches back from foundations and walls of buildings where ants are not desired inside.
- Manage the aphids, mealybugs, psyllids, scales, whiteflies, or other insects secreting the honeydew on which ants are feeding. If ripening fruit is the source of ant attraction, pick, prune, or knock that from plants and if unwanted dispose of it.
- Prune shrubs and trees back away from structures to eliminate bridges to buildings.
- Remove plants too near to structures if they consistently host ants or honeydew producers and ants coming indoors has been a problem.
- Seal crevices and gaps in buildings through which insects enter.
Insecticide sprays kill only the foraging ants; more ants will emerge from their nests. Ant sprays commonly contain pyrethroids (e.g., bifenthrin, permethrin) or other insecticides that can kill a wide variety of insects. Urban uses of these broad-spectrum insecticides are contaminating surface waters and killing aquatic organisms.
Insecticide baits. Insecticide mixed with an attractant is the preferred chemical method for ant control. Insecticide baits when properly used selectively kill ants and their entire colony without contaminating the environment with pesticide. To become spread to ants underground, baits MUST be slow acting. Although baits require users to be patient, they can provide more effective and longer control of ants than sprays. Control with insecticide baits may take more than one week.
For the most economical (least pesticide needed) and effective ant control, apply bait beginning in late winter or early spring when ant populations are lowest. Place bait near ant nests and trails, at the basal stem or trunk of infested plants, and along foundations outside walls where ants have been observed inside.
Insecticide bait effectiveness varies with active ingredient (which insecticide), ant species, availability of alternative food, and the type of bait (e.g., protein versus sugar). Ants' food preferences can change seasonally. For example, protein baits can attract the common Argentine ant, Linepithema humile, in spring when colonies are producing young; sweet liquid baits attract the Argentine ant year-round. Users may benefit from applying more than one bait type and observing which bait(s) ants feed on.
Bait types. Abamectin and borates are example insecticides used in ant baits. Baits include pre-packaged liquids or solids that are applied enclosed in small containers (bait stations). Other baits are granules that are broadcast outdoors.
Liquid bait comprised of sugar water (the attractant) and the insecticide boric acid or borax (sodium tetraborate decahydrate) can be applied in refillable, reusable bait stations. Properly mixed these borates are effective against Argentine ant and other sugar-feeding ants. Effective borate and sucrose (kitchen sugar) baits are comprised of 1/2 to 1% boric acid and 10 to 25% sucrose in water. For example, dissolve 1/2 teaspoon of powdered boric acid and 9 teaspoons of granulated sugar in 1 cup hot water to create a solution of approximately 1% boric acid and 19% sucrose.
Slight changes in products can dramatically reduce their effectiveness, such as too high a concentration of borate or preservatives in the product. Boric acid concentrations higher than 1% as in some commercial products are LESS effective than products with 0.5 to 1% boric acid because higher borate concentrations become repellant to ants.
Sticky barriers. Apply a several-inch wide band of sticky material around (encircling) woody trunks to prevent ants and other flightless insects from climbing to leaves and twigs. Examine plants' entire outer surface of leaves and stems (canopy) for branches that touch other plants or structures. Prune to eliminate these ant bridges. Alternatively, also sticky band woody trunks of adjacent (touching) plants and prune off bridges on the outside of the plant group.
Sticky material is more effective when applied above the reach of irrigation sprinkling. Avoid application to horizontal surfaces where birds may roost. Check sticky material at least every 1 to 2 weeks and stir with a stick; the material may become clogged with debris and dead ants that provide a bridge to cross.
Do NOT apply sticky material directly on
- bark that receives direct sunlight, which heats the material
- green or succulent stems
- thin-barked or young woody plants
- woody plants pruned the current or last season
Plant damage (phytotoxicity) may occur under such circumstances. Instead, wrap the trunk with duct tape or a strip of fabric tree wrap and apply the sticky material on that. Remove and replace the old tape or wrap periodically and apply new wrap and sticky material to avoid girdling (growth-restricting) injury to trunks.
For more information, consult Pest Notes: Ants, Toxicity and Repellency of Borate-Sucrose Water Baits to Argentine Ants , Urban Pest Management of Ants in California, and for particular species Pest Notes: Carpenter Ants and Pest Notes: Red Imported Fire Ant.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Adult Argentine ant, Linepithema humile, tending soft scales.
Ants tending aphids and feeding on their honeydew.
Bark chewed by fire ants, girdling and killing the tree.
Adult California field ant or gray ant, Formica aerata, killing a caterpillar.
Refillable, reusable station with sugar bait and borate insecticide properly mixed.