Native gray ants, found almost universally in San Joaquin Valley peach and nectarine orchards, may help farmers suppress the peach twig borer, University of California scientists report in the November/December issue of California Agriculture magazine.
"Farmers tend to think of ants exclusively as pests," said UC Berkeley biological control specialist Kent Daane, who is based at the Kearney Agricultural Center near Parlier. "And in most cases they are. But our findings may change some farmers' opinions about the native gray ant."
The native gray ant's new-found status as a beneficial insect does not necessarily transfer to other crops. In citrus, prunes, almonds and other crops it can be a serious pest because it will eat the nut meat and protect aphids and scales from natural enemies. But the article's co-authors, Daane and former UC Berkeley graduate student Jeffrey Dlott, found that it is the most important predator of peach twig borer larvae in peaches and nectarines.
Peach twig borer, in its damaging larval stage, is a tiny worm that feeds on peach, nectarine, almond, plum and prune shoots or fruit . The pest has about 30 natural enemies, which by themselves generally do not reduce peach twig borer populations enough to avoid the damage that costs farmers money. However, native gray ant and other PTB predators may work in concert with other control methods -- such as Bacillus thuringiensis sprays at bloom or synthetic pesticides.
The quarter-inch-long native gray ant is found in California and parts of Nevada and Oregon. Its nests are commonly hidden under the soil or in the decaying wood of tree trunks and roots. It exits the nest through cracks in the soil and forages individually for food on the ground or in the trees.
For these reasons, native gray ants may be less apparent than other ant species that have nests with very noticeable ant hills, such as fire ants, or that forage in large numbers along well-defined ant trails, such as Argentine ants. Since fire ants and Argentine ants rarely feed on PTB larvae, they do not help peach and nectarine farmers suppress the pest.
Gray ant colonies have a queen that produces eggs, and workers that tend the queen, care for lavae and forage for food. One food they seek out is the high-protein PTB larvae. Although it would appear simple to use this natural tendency to reduce PTB populations, Daane said it's difficult to manipulate native gray ant populations in orchards.
"We were surprised to find the lack of any relationship between orchard floor management practices and native gray ant numbers," Daane said.
Cover crops or resident vegetation can provide alternative food sources, such as other prey, seeds or nectar. But native gray ant populations were found in all kinds of ecosystems, including clean cultivation. Daane said cover cropping or disking may still benefit native gray ants by reducing the number of southern fire ants, which prefer dry, undisturbed soils.
"Because the native gray ant and southern fire ant compete for foraging territory, any cultural practice that decreases one species may allow the other species to increase in numbers," Daane said.
The only cultural practice that dramatically lowered native gray ant abundance was in-season application of organophosphate, carbamate or pyrethroid insecticides. And even in these cases, the native gray ant populations bounced back quickly.