Walter J. Bentley, Regional IPM Entomologist
UC Kearney Agricultural Research and Extension Center
Ant damage to almonds continues to be a difficult problem for farmers to manage. In all areas of the state populations of the three species of ants which cause damage appear to be increasing. From Fresno County going south to Kern County the California fire ant (also called the southern fire ant), Solenopsis xyloni, is the damaging species. Another, less common species, is the thief ant, Solenopsis molesta, which is slightly smaller than Xyloni. In Merced, Stanislaus, and San Joaquin Counties both the California fire ant and the pavement ant Tetramorium caespitum can be found causing damage. In the Sacramento Valley the pavement ant is the predominant species.
Although almond farmers will know, from past history of damage, whether or not they have a problem species in the orchard, there are other species that are considered more beneficial than harmful. These include the bicolored pyramid ant, Doriomyrma bicolorand the native gray ant, Formica aerata. This latter species has been found to actively feed on peach twig borer in unsprayed peach orchards. The best way to separate the damaging species from those not causing damage is by examining the thin waist between the last pair of legs and the abdomen (stomach). Both the pavement and the California fire ant possess two bumps or nodes while the beneficial species have only one node. Also, the damaging species will often be found with weeds growing within the nest and are easily excited by disturbing the soil near their nests. Finally, the pavement, California fire ant, and thief ant will aggressively bite and sting. The California fire ant sting, in particular, is quite painful.
While control information has applied to all three damaging species, monitoring methods and thresholds where only developed for the California fire ant. This species is considered the most damaging. We do know that feeding on almonds occurs when the nuts are on the ground, after being shaken from the tree. In only a few instances will ants climb into trees to feed on nuts. This usually occurs when limbs touch the ground giving ants have easy access to hullsplit nuts.
The primary factors that influence damage include the population of ants in the orchard and the length of time the nuts are left on the ground to dry. The longer the nuts are on the ground, the more damage you can expect. And, as you might guess, the greater the population in the orchard the more potential for damage. Years ago work was done on treatment thresholds for the California fire ant and the damage potential for 3 different population densities is shown in Figure 1. This information is also available in the Almond Pest Management Manual and The Pest Management Guidelines for Almond. In general colony counts of 3 or more per 1000 square feet can lead to damage at harvest, if conditions are right. This is a big if, especially for the California fire ant, because high temperature (95 degrees Fahrenheit or higher) will restrict their feeding. If such high temperatures exist while the nuts are drying on the ground, damage from the California fire ant will be minimal. This is true even for very high populations. If daily highs are less than 90 degrees F., damage can be extensive. The same information is not known for the pavement ant although it appears to behave similarly.
There are some cultural management techniques that can be used to keep ant damage to a minimum but be cautious, particularly if navel orangeworm is also a problem. Where ants are the primary pest, leave the almonds on the tree for as much drying as possible. This will allow you to pick the nuts from the ground without a delay to dry. You can try to schedule the shaking of heavily infested blocks late in the season, again to keep the nuts on the tree for as long as possible. Beware of however if you have a navel orangeworm problem. The delayed shaking will increase worm damage.
Another point to remember is that only the soft shell varieties, such as Nonpareil, are damaged by ants. Hard shells such as Mission, Butte, Padre and, to some extent Carmel, are not fed upon. Don’t bother to treat for ants if you have a hard shell orchard.
Ant baits have been very effective in reducing damage, when applied at the correct time. These include such products as Esteem©, Clinch© and Extinguish©. Each requires a specific time interval, prior to shaking, to be applied. In applying baits, be sure to mow any cover prior to the application. For further information go to the IPM Guidelines for Almond at http://www.ipm.ucdavis.edu/PMG.