Growers or pest control advisors who have observed significant ant activity on orchard floors should be aware of the probability of crop damage if ant numbers are not reduced prior to shaking.

Surveys for ant colonies conducted before high temperatures slowed ant surface activity can show their distribution within an orchard. Cooler June temperatures increased orchard floor activity for some ant species.

The Almond Doctor, David Doll, writes that crop damage occurs as ant populations increase. These ants can consume or chew on kernels, leaving empty pellicles. Longer drying time on the ground increases the likelihood of damage.

Varieties with a tight shell seal, or with shell splits less than 0.03 inches wide, are more resistant to ant damage. Shell seal can vary greatly from year to year depending on crop load, nut size and horticultural practices.

Ant Species Vary by Growing Region

Different growing regions in the Central Valley favor certain ant species. The southern fire ant is found in most growing areas and can cause more damage than the pavement ant, which is more likely to be found in the northern San Joaquin Valley and into the Sacramento Valley. Merced County has also had infestations of red imported fire ants.

Identifying Pest Ants

To save time and avoid unnecessary, expensive ant treatments, it is important to determine whether the ants in an orchard are pest ants.

California has approximately 245 native and 25 introduced ant species. Very few species are an economic threat. The majority of ant damage is caused by three species:

• Southern fire ant
• Thief ant
• Pavement ant

Non-pest ants that often occur in almond orchards include:

• Native gray ant
• Pyramid ant
• California harvester ant

Foraging activity and behavior, nest location and ant mound structure are useful for identification. UC IPM Guidelines note that foraging activity of southern fire ants and pavement ants slows when daytime temperatures are high. Gray ants and California harvester ants remain active during high temperatures. There are also differences in mound shapes and nesting areas.

Significant Damage

Pavement ants and southern fire ants can cause significant damage to almonds while they dry on the ground. Damage increases the longer the almonds remain on the ground.

Flood-irrigated orchards limit ant activity to tree berms, while drip- or sprinkler-irrigated orchards allow ant activity across orchard floors. Moist, cool conditions increase aboveground ant activity. Hot temperatures drive ants deeper into their colonies.

If spring monitoring revealed high populations of pest ants, the Almond Board of California offers several strategies to prevent crop damage.

Insecticide Sprays

Insecticide sprays can be used to help reduce ant populations to a reasonable level.

Conventional chlorpyrifos sprays can also be applied to control ant populations at harvest, but they will only kill foraging ants. Spraying two weeks prior to harvest is the recommended timing for these types of applications.

Ant Baits

Baits are another management option. They are slow-acting insect growth regulators and must be applied several weeks before harvest. Baits are selective for ants and can be effective in weakening or killing an entire colony.

Baits should not be used within 24 hours after irrigation or 48 hours before irrigation because they absorb moisture, reducing their attractiveness to ants. Baits can also go rancid if stored for long periods.

UC IPM Guidelines identify two primary categories of effective ant bait:

• Insect growth regulator baits
• Stomach poison baits

Baits use soybean oil as an attractant and toxicant carrier and specifically attract protein-feeding ants.

When deciding on a management strategy, consider the lag time between bait application and population reduction. Insect growth regulator baits work more slowly, taking up to eight weeks to show significant results.

In contrast, the stomach toxicants abamectin and metaflumizone require only one to three weeks to significantly reduce foraging activity.

Harvest Management

Another strategy is to allow almonds to dry longer on the tree and then remove them rapidly from the field after harvest. Reducing the amount of time almonds remain on the orchard floor can help prevent ant damage.