Codling moths (Cydia pomonella) are a serious pest that attack walnut trees (Juglans spp.), which grow in U.S. Department of Agriculture plant hardiness zones 3 through 9. The 1/2- to 3/4-inch long gray moths typically lay their eggs in the spring on the upper leaf surface or fruit of the tree. Once the eggs hatch, the larvae burrow into the nuts, causing the infected walnuts to either fall from the tree or stay attached but become inedible. You can control these damaging pests with various methods.
Hang codling moth traps containing the lure included in the trap kit from the walnut tree. Use two traps for a standard size tree and one trap for dwarf or smaller trees. Replace the codling moth lure inside the trap eight weeks later. These traps capture male moths before they have a chance to mate with female moths.
Cover walnuts with a maggot barrier that expands as the fruit grows. These nylon stocking-like bags prevent the larva of the codling moth from boring into the walnuts.
Release Trichogramma platneri -- which is a parasitic wasp that attacks the codling moth eggs -- in your garden when sunset temperatures are at least 62 degrees and codling moths are regularly caught in moth traps. Parasitic wasps are available for purchase at garden supple centers.
Mix 1 to 1 1/2 teaspoons of a biological insecticide containing codling moth granulosis virus with 5 gallons of water. This virus kills the larval stage of codling moths. Spray the infested tree with the insecticide until the foliage and fruit is wet in late spring or early summer at three- to 10-day intervals.
Dilute 1 1/2 to 3 3/4 fluid ounces of carbaryl insecticide concentrate in 1 gallon of water and spray to ensure thorough coverage of the leaves. This amount of insecticide treats a 1,000 square foot area.
Spray the walnut tree with the diluted carbaryl insecticide on a calm day when temperatures are below 90 degrees but above 40 degrees Fahrenheit.
Reapply at 14-day intervals if needed. Do not apply more than four applications a year.