Managing Orchard Ants – time to scout for activity!

Pavement and Fire ants can cause significant damage to the almond crop. Feeding throughout the year on seeds of grasses and broadleaves, ants are only problematic once the almonds are shaken to the ground. Ants can completely hollow out the nut, leaving only the pellicle behind. Damage increases the longer the almonds are on the ground.

Within the San Joaquin Valley, the southern fire ant has a wider distribution and usually causes more damage than the pavement ant. Pavement ants are found in the northern San Joaquin Valley and into the Sacramento Valley. Within Merced County, we have both the Southern Fire Ant and the Pavement Ant. Recently, Merced County has had infestations of the Red Imported Fire Ant (RIFA), a third problematic ant for almond growers, requiring quarantine actions to control this invasive pest. Please see Guide to Identifying Orchard Ants to help identify ants.
Ants tend to be problematic on drip or sprinkler irrigated orchards. Within flood irrigated blocks, ants are limited to tree berms. Moist, cool temperatures increase above ground activity of ants; hot temperatures drive the ants deeper into the colony. Varieties with a tight shell seal are more resistant to ant damage, or with shell splits less than 0.03 inches wide. Shell seal can vary greatly year to year depending on crop load, nut size, and horticultural practices.
The orchard should be surveyed for ant colonies 2-3 days after an irrigation in the months of April – May in order to determine if a treatment is needed. Once the temperatures increase, problematic ant surface activity decreases, giving an inaccurate count of the population. In scouting for ants to determine if treatment is necessary, choose five survey areas per orchard, each about 1000 sq. ft., including the soil area from mid-alley to mid-alley beneath trees. Count the number of active colonies in each area, sampling five different areas of the orchard. Total all the ant colonies to get the number in a 5000 square foot area and compare it to the table below which gives an indication of the amount of damage you can expect at harvest. A survey sheet can be found here, and also can be found at the UC IPM website.
Several strategies can be employed to help control ant populations and prevent the corresponding damage. Letting the almonds dry longer on the tree and rapidly removing them from the field after harvest will prevent damage, but not always possible due to other harvest activities, nut moisture content, and navel orange worm pressure. In these cases, baits and conventional sprays can be used to help reduce the populations to a reasonable level. Baits are slow acting insect growth regulators; they need to be applied several weeks before harvest. They are selective for ants and highly effective, weakening or killing the entire colony. Baits should not be used within 24 hours after an irrigation or 48 hours before an irrigation. Baits absorb moisture, which reduces its attractiveness to the ant. Do not keep baits year-to-year as they will go rancid. Conventional sprays of chlorpyrifos can also be applied to control ant populations at harvest, but will only kill foraging ants. Spraying two weeks prior to harvest is the recommended timing for these types of applications.

Please see the UC IPM Website for updated pest management strategies and materials. An entry later this week will help in identifying orchard ants.

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