Ant infestations in almonds are a serious concern for growers

(Reprinted with permission of Ag Alert, a publication of the California Farm Bureau. This article is based on a presentation made at the PMA field day held May 2 in Modesto)

By Christine Souza
Assistant Editor

Several ant species have taken over almond orchards to feed on the nuts, which means less money for California almond growers. To prevent such infestations, University of California researchers are studying ways growers can avoid this problem altogether.
 
"Several of us have been working in almonds up and down the valley to try to come up with programs for managing ants in the orchard. Of course ants have been a problem in orchards for probably the last 20 years or more, particularly as we got more into drip irrigation or low volume irrigation," said Rich Coviello, UC Cooperative Extension entomology advisor in Fresno County. "We have really created this situation where we have habitat very suitable for ant colonization and ant build-up, especially where you have such things as cover crops providing a forage or nutrition source for ant colonies."This time of year, Coviello is talking to almond growers about how to control ants in the orchard and outlined two ant species found in California almond orchards.

Rich Coviello, center, demonstrates ant control techniques.

"There is quite a range of damage the ants are going to be causing to the nuts, all the way from a little bit of feeding to where they hollow out the entire nut. This is the sort of damage we are concerned with because it is going to go through and count against your grade," Coviello said.

Growers will find a number of ants in the orchards, therefore Coviello said, so it is important to be able to distinguish the ant species to determine the size of the pest problem. Basically, the easiest way to do this is first realize that ants have multiple sizes, such as the major workers which are one-quarter of an inch or more long, and the smaller workers will be from one-sixteenth to one-eighth of an inch in length. 

Two damaging ant species commonly found across include the Southern fire ant, Solenopsis xyloni and the pavement ant, Tetramerium caespitum. These are known as two-node ants since they have two bumps on the place between the thorax and the remainder of the abdomen. The Southern fire ant is red and black and has a painful sting that causes visible swelling. It has a wider distribution than the pavement ant and causes more damage. The pavement ant is black and using a hand lens, it can be distinguished that they have longitudinal furrows on the head. Activity of these ant pests peaks in the morning and again before sunset.  

The longer nuts remain on the ground, the greater the damage. The damage potential of ants is less in weed-free orchards and those without cover crops. Damage is also lower on varieties that have nuts with a tight shell seal or with narrow shell splits.

The bottom line is, baits work very well when treating these ants, Coviello said, based on 2000 trials tested in Fresno, Kern and Merced counties, which showed that baits are taken back to the nest and weaken the entire colony. Since baits are primarily insect growth regulators, they take several weeks to work. Clinch and Esteem treatments work equivalently to Lorsban and must go on probably four-to-seven weeks before harvest, Coviello added.

With Clinch, there is some mortality of the foragers that pick it up. Distance and Esteem have no affect on foragers, so there is no real reduction in numbers of ants until those foragers basically die off from natural causes and then there is a lack of reproduction from the queen and the colony. Distance or Esteem should be applied at least six weeks before harvest.

"We probably wouldn't recommend the baits go on later than about eight or nine weeks because we have in earlier years, seen some resurgence later on. It will hold the ants down for 10-to-12 weeks, then we start seeing them come back again," Coviello said. "We think this is due to the fact that there are secondary queens in the colony that are really not being fed so they are not picking up enough of the bait to become sterilized. So later on they are recruited to begin reproduction, so that is what is happening."

In the 2000 trial, there was no observed benefit to doubling up the bait application whether it was a May application followed by a pre-harvest application or even one application right after the other within a few days or a week. There was also no benefit to following up the bait with a Lorsban application.

For organic growers, Coviello said, he has not seen very good results with boric acid. The biggest reason is he is unsure as to what the concentration of boric acid should be in the solution to repel the ants. More study still needs to happen in the areas of weed cover as it relates to bait applications and shell split width as it relates to nut damage, he said.