How to Manage Pests

UC Pest Management Guidelines

Avocado

Ants

Scientific names:
Argentine ant: Linepithema humile
Native gray ant: Formica aerata
Southern fire ant: Solenopsis xyloni

(Reviewed 1/07, updated 6/10)

In this Guideline:


DESCRIPTION OF THE PESTS   (household ant key)

Most ants (family Formicidae) are wingless workers (sterile females). Workers search for food outside the nest, dig tunnels, and care for the tiny, pale, grublike ant larvae in the nest. Adult ants can also be winged males that die soon after mating or reproductive females (queens) that lay tiny elliptical eggs in underground nests. Queens and males are usually observed only during their brief mating season when they develop wings and swarm outside of the nest.

Ants have a narrow constriction between the thorax and abdomen. Their antennae are distinctly elbowed. Winged ants have hind wings that are much shorter than the forewings. It can be very helpful to identify the species present as ant biology and management often differ among species. An illustrated key is available.

The most prevalent species is the Argentine ant, which travels in characteristic trails with numerous individuals. Workers are about 0.13 inch (3 mm) long, uniformly deep brown to light black and do not sting and rarely bite. The Argentine ant has one petiole node (hump) between the thorax and the abdomen.

Native gray ants, also called field ants, are larger than the other ants. Native gray ants are up to 0.3 inch (7.5 mm) long and have one petiole node. Gray ants nest in topsoil or under rocks and debris. Individuals move in an irregular jerky manner and generally do not travel in trails or sting.

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. It has two nodes (humps) between the thorax and the abdomen. Workers size is variable range from 0.1 to 0.018 inch (2.5–4.5 mm) long. Southern fire ants nest beneath loose mounds or craters and do not aggregate in colonies as large as those of the Argentine ant. Southern fire ants may swarm over the ground and may sting when disturbed. They forage mostly in the morning and early evening and usually do not travel in conspicuous trails.

Be especially alert for the highly aggressive red imported fire ant (Solenopsis invicta =S. wagneri). Red imported fire ants run up any objects they encounter and have a venomous sting, which can seriously injure people. Red imported fire ants can be recognized in part by their size, which varies greatly among workers. Large and small ants, 0.08 to 0.25 inches (2–6 mm) long occur together in the same clump or trail. Except for southern fire ants, which also range in size, workers outside the nest are about the same size for all other ants likely to be found in California groves. Report suspected red imported fire ant infestations to agricultural officials. Contacts include telephoning 1-888-4FIREANT toll free and the http://www.fireant.ca.gov Web site.

DAMAGE

Ants are important natural enemies of many insect pests and provide benefits such as improving soil. However, ants sometimes chew crop twigs and tender bark, damage irrigation tubing, or annoy workers. In avocado, ants are pests primarily because they disrupt biological control of other pests. Ants are primarily a problem in young avocado trees where mealybugs and other honeydew-producers are occasional pests. Ants protect these food sources from natural enemies, causing phloem-sucking insects to become more abundant. When honeydew-producers are present, ants also increase populations of armored scales and some other pests that do not excrete honeydew. Ants are general predators that attack most any other predator or parasite they encounter, regardless of what host that natural enemy is seeking.

MANAGEMENT

Periodically inspect for ants and bark damage under trunk wraps of young trees. Check for ants on trees of any age if honeydew-producing insects are a problem. If ants have swollen, almost translucent abdomens, this can indicate they are honeydew‑collecting species.

Ants do not have effective natural enemies, except for competition with other ants. Cultivation controls ants, but creates dust and disturbing soil near trees damages roots. Insecticide mixed with bait is the preferred chemical control. Baits are slow acting, but effective over the long-term because they take advantage of ants' food-sharing behavior. Ants spread insecticide bait throughout the colony, including to nest-bound immatures and queens underground.

The best time to bait is late winter to early spring when ant numbers are relatively low. Bait effectiveness varies with ant species, availability of alternative food, active ingredient, type of bait, and the time of year. To determine which bait to use, offer a small quantity of each of several baits and observe which is preferred by the ants.

Solid baits are applied for fire ants. Argentine ant and other honeydew-feeding species are most effectively controlled by liquid baits, which must be applied in registered bait stations. Check for the registration and availability of new liquid baits and baits stations to control honeydew-feeding ants. Apply an effective bait in spots near nests or on trails. Spot treating takes advantage of ants' trailing behavior, which leads nest mates to locations where food is concentrated. Spot treatment minimizes toxicity to non-pest ant species, which compete with pest ants and help to limit their populations. Broadcasting baits or widespread spraying with insecticide is expensive, and may not reach many ants within nests underground.

Common name Amount to use R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. STICKY POLYBUTENE MATERIALS#
  (Tanglefoot) Label rates
  COMMENTS: For use on all varieties. Use polybutene-based products only. Do not apply sticky materials directly on the trunk; use a 6- to 18-inch wrap under the sticky material to protect the tree from sunburn. Exercise caution in applying multiple applications (more than 3 or 4); watch for symptoms of bark cracking. Apply the sticky band high enough to avoid sprinklers, dust, and direct sunlight. Reactivate periodically by rubbing with a stick to remove dust. Skirt- and canopy-prune trees so that ants have access only via the trunk. Check to ensure that low hanging branches, sticks, weeds, etc., are not allowing ants access to trees.
 
B. BORIC ACID#
  (Gourmet) Label rates 0
  MODE OF ACTION GROUP NUMBER1: —
  COMMENTS: Available only for organically grown fruit under a Special Local Need (SLN) registration. Liquid boric acid formulation with sweet bait for use only in approved bait stations that meet EPA ChemSAC criteria. For use against honeydew-feeding ant species, including Argentine ant and native gray ant.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.
# Acceptable for use on organically grown produce.
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). For additional information, see their Web site at http://www.irac-online.org/.
Not applicable.

IMPORTANT LINKS

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Avocado
UC ANR Publication 3436

Invertebrates

B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
J. G. Morse, Entomology, UC Riverside
M. S. Hoddle, Entomology, UC Riverside
Acknowledgment for contributions to Invertebrates:
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County
M. Blua, Entomology, UC Riverside
P. Oevering, UC Cooperative Extension, Ventura County
D. Machlitt, Consulting Entomology Services, Moorpark, CA
T. Roberts, Integrated Consulting Entomology, Ventura, CA
B. B. Westerdahl, Nematology, UC Davis

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