Problem ants in almond orchards are nothing new, but their increasing numbers and damage to almonds have elevated the critters to the Ten Most Wanted List.
That ants have become a top-of-mind concern with growers was evident at the Blue Diamond® Growers annual meeting last December. The subject of ants was a top vote getter for a cultural practices seminar at the annual meeting. Rich Coviello and Mark Freeman of UC Extension, leading authorities on ants, presented their findings from recent field tests.
"You can have a big population of problem ants in your orchard and not even know it," Coviello announced. He said the he has toured orchards with their owners and pointed out ant colonies that they didn't even know existed, because the nests were hidden under a cover crop.
Factors Affecting Buildup
Why are ant numbers increasing? The migration to low-volume irrigation is a leading reason for the buildup, Coviello and Freeman agreed. This is not a criticism of low-volume methods, they said, just an alert that some additional control measures must now be taken.
Low-volume irrigation attracts the two leading ant pests_the southern fire ant and the pavement ant_ becaused these two species like to locate their nests near moisture, but not in it. The wet-dry soil pattern created by drip and microsprinkler emitters is an ideal enviornment for them, the experts explained. Other ant species, including field and pyramid ants, locate in dry areas away from irrigation lines.
Other factors favoring an ant population buildup, said Coviello, include:
- The swtich to IPM controls for navel orange worm (NOW). Hull Split sprays helped control problem ants.
- Drought conditions in the late 1980s and early 1990s. Ants like warm, dry weather.
- Warm winters in recent years, which increased over-wintering survival.
Almond Eating Ants
Southern Fire Ant
This is the primary ant pest in the southern and central San Joaquin Valley, the experts noted. A native to California and the Southwest, it is closely related to the imported fire ant that has infested the souteastern U.S.
- Appearance: The southern fire ant is the only one that comes in two sizes_petite and oversize. Most of the workers that you'll see running around are small, but a few will be Shaq-size. All have black or dark-colored rear ends, with the rest of the ant reddish-brown and covered with golden hairs. If you have a lens, you'll see two small nodes between the large abdomen in the rear and the thorax where the legs are attached.
- Behavior: These almond-eaters come with an attitude. If you stomp on the ground near their burrow, they'll swarm out of the nest looking for a fight. These are the only ants you'll find in your orchard that will do that.
- Location: Californians to the core, fire ants love their version of the beach: sun, sand and water. Look for them in sunny spots, in sandy soil, near the wetted areas of low-volume irrigation. In spring they locate their colonies on the south side of trees close to the trunk for warmth, moving out into the open as the season progresses.
This species infests the northern third of the San Joaquin Valley and the almond-producing regions of the Sacramento Valley.
- Appearance: All black or blackish-brown, small, and worried-looking these ants are distinguished by a deeply furrowed brow, which is clearly evident with a lens.
- Behavior: These laid-back fellows move at a slow, deliberate pace, and do not react much to disturbances.
- Location: They build their mounds near moisture, in sunlight.
Ants to Leave Alone
Except for the two bad actors descibed above, ants can be helpful as insect predators, so it's a good idea to avoid harming the following species that may be homesteading your orchard.
Red Harvester: This big fellow is red all over and has two nodes on his abdomen. They build big, low nests (12 inches across) in open, sunny, dry areas. They spread their diggings in a fan shape pattern from the entrance, which is on one edge of the nest. They are active most of the day, but do not trail, prefering to forage for seeds (but not almonds) alone.
Field Ant: Another big ant, this one is brown or gray, has one node, and builds its nest at the base of trees or near weeds. The nests are tough to spot. When built on a berm, the holes are small without any debris around them, but on the orchard floor the holes can be large with dirt scattered randomly about. These predators eat other insects and are active all day long. They move quickly in a jerky fashion.
Pyramid Ant: These ants have the misfortune to look a lot like a fire ant: small, red and black. But they have only one node and sport a pyramid-shape bump on the bases of their thoraxes. But unlike fire ants they build neat, volcano-shaped nests in open, dry, sunny areas. When distrubed, they move rapidly in jerky, random fashion around their nests, but do not swarm like the fire ant does. Pyramid ants feed on honeydew and other insects and ignore almonds.
Kernels: Both species attack almonds that are lying on the ground, entering an open shell to gnaw on the kernel or gaining entry through NOW damage. Ants do not burrow into a hull, they need an opening to make their entry. Consequently, they affect loose suture almond varieties the most and are less of a problem on tight-seal varieties.
Trees: Coviello noted, however, that ants have been known to crawl up into trees and damage nuts hanging on the limbs. Some have been observed feeding on the sap of young trees by building up a soil mound inside the paper wrappers put in place to protect saplings from sun damage. Ants attack the tender bark from their mound, eating the sap that oozes from the wounds. If extensive, such damage can weaken and even kill young trees.
Roots: While there isn't much data on it, it is believed that ant colonies located close to tree roots will sometimes attack the roots. A tree weakened by other causes could suffer serious damage from such attacks.
Weed Seed Eaters: Coviello noted that tests have shown that there is considerably more damage to almonds in an orchard with native vegetation as a cover crop than in a weed-free orchard they like the seeds. It is also believed that the cooling effect of a cover crop can provide more opportunity for ants to forage in hot weather.
Cool Customers: Fire and pavement ants prefer a sunny but cool environment, one that ranges between 70 and 95 degrees F. They don't forage when the temperature drops below 60 degrees F. or rises above 98 degrees F. This explains why a midday walk through an orchard in summer turns up few if any problem ants.
Coviello and Freeman acknowledged that the best advice for minimizing ant damage runs contrary to advice for minimizing NOW damage. That is, "get nuts off the ground as soon as possible."
The next best advice is on which chemicals to apply and when. The experts emphasized that there is no use in attempting to eradicate harmful ants, but their numbers can be controlled. Lorsban is the only chemical currently approved for ants that provides a significant degree of control. It kills ants by contact, residual contact, and fuming.
Another type of chemical an insect growth regulator named Logic was found to be very effective for long-term control, although it requires about eight weeks to show significant results. Logic has not yet been registered in California.
Application of Lorsban: Coviello and Freeman advised starting control measures in early spring late February or early March. This will help prevent rapid population increases later on. Treatment at that time is also more effective than later on because the nests are shallow. The pupae are no more than 1/4 to 1/2 inch below the surface, while after the weather warms, the ants burrow down five feet or more making control more difficult and potentially less effective.
Even with a good kill in the spring, follow up applications will more than likely be needed, because ant queens are prolific. You can wipe out more than half of the workers and the queen will keep on pumping out eggs, rebuilding her colony to full strength in a month, Freeman explained. Moreover, even a major kill of resident ants won't guarantee none will be around at harvest. In one of Freeman's tests, his spring application practically eliminated fire ants for the next two months, but new ones arrived to take their place. So be vigilant and keep them under control all spring and summer.
If it appears you still have a problem population as harvest nears, spray again just before harvest, Freeman advised. This application may carry you through harvest and minimize crop damage.
Application rates: For best results, the experts advised using the high concentration rate of Lorsban (8 pints per acre in 25 or more gallons of water) noted on the label.
Dow-Elanco advises not applying Lorsban where weed growth would disrupt uniform coverage of the orchard floor. They recommend that you mow or chemical mow first, then apply Lorsban.
For best results, the company suggests applying Lorsban when the outside temperature is below 90 degrees F.