Excerpt from the Popular Garden Series magabook Orcharding with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Orcharding here.
Key pests of fruit trees include a number of the butterflies and moths, such as the Oriental fruit moth, peach twig borer, leafrollers, green fruit worms and codling moth. These orchard pest insects have a complete metamorphosis, which means they go through four distinct stages of development: egg, larva (caterpillar), pupa (cocoon or chrysalis) and adult. As larva, butterflies and moths are chewers and, as adults, they are nectar feeders. It is the larval stage that does the damage in this group of orchard pest insects.
The codling moth is “the worm apple.” Codling moth is widespread and very damaging, and besides apples also attacks pears and English walnuts. The adult moth is about 1/2-inch long, mottled gray with distinguishing copper-colored bands along the bottom edges of the wings. The larva is white to light pink with a dark brown head and reaches about 1/2-inch long by end of larval stage. A Codling moth overwinters in a cocoon under debris or bark scales, or in the soil. There can be up to four generations of codling moth per year depending on temperatures. As with all insects, the pace of codling-moth development is extremely temperature sensitive.
Disc-shaped codling moth eggs are laid singly on fruit, nuts, leaves and twigs in the spring. The larva hatch and burrow into the fruit to feed, tunneling around and eventually dropping out to pupate. The holes left by the codling moth are covered with brown frass (droppings) and are sometimes hidden on the blossom end of the fruit. On walnuts, codling moth feed on kernels, causing premature drop or inedible nuts. Damage is usually more severe on late varieties, so consider growing early maturing apples and pears.
Codling moths are monitored with pheromone-baited traps. The developmental cycle of this moth can be predicted fairly accurately by calculating the accumulation of degree-days from the time that moths are trapped and the time that the temperature at sunset is more than 62 degrees Fahrenheit (this is called the biofix). A degree-day is a unit that is used to measure pest development; an easy-to-use degree-day calculator is available at the USPest.org website.
Good orchard sanitation can help disrupt the reproductive cycle of the codling moth. Starting about six to eight weeks after bloom, check fruit for damage. Remove and destroy any with frass-filled holes (be sure to check blossom end). Always clean up fallen fruit. Keep orchards free from junk and debris to reduce the sites for pupation. Another tactic aimed at the pupa is trunk banding. Large-sized corrugated cardboard is wrapped around the trunk to lure the larva from the trunk into the cardboard to pupate. By most counts, this method is not highly effective, but the emotional satisfaction of starting a burnpile with the codling moth-filled cardboard might be worth the effort (just be sure to chase out your friendly spiders first).
Another approach to control is mass trapping. You can use commercial pheromone traps at a high rate, but these traps attract only the males (and it’s the female codling moths that do the damage). A homemade codling moth trap can be made from a 1-gallon milk jug containing 1 cup of cider vinegar, 1/3 cup dark molasses, 1/8 teaspoon ammonia and enough water to make 1 1/2 quarts. Cap the bottle and cut a 2-inch diameter hole. Hang one to three traps per tree.
labor-intensive, but extremely effective, barrier method of control is fruit bagging. Four to six weeks after bloom, when the fruit reaches 1/2- to 1-inch diameter, put bags over the fruit. Use standard-size lunch bags with a 2-inch slit in the bottom. Thin the fruit, then slip one bag over each fruit through the slit and staple the open end shut. Remove bags shortly before harvest to allow color to develop in red varieties.
A tiny parasitic wasp, Trichogramma platneri, is commercially available to help with pest control. Starting when egg-laying begins, you can release Trichogramma wasps in each tree, repeating every week or two. This method is often combined with mating disruption. In a mating disruption program, pheromone dispensers are placed all over the orchard and exude the alluring scent of female Codling moths. The idea is that the males will be so overwhelmed that they won’t be able to find the real females. Mating disruption is usually effective only in large, fairly uniform blocks of trees (more than 5 acres) some distance from other host trees.
Chemical control for Codling moths requires accurate timing. Sprays must be applied just before or as eggs hatch because the spray must reach the larvae before they burrow. Degree-days are used to time sprays; the first spray is applied around 250 to 300 degree-days. Spinosad, a bacterial insecticide, and summer oil, a horticultural mineral oil used as a fungicide and an insecticide, are both used for codling moths. Conventional pesticides are also used, but are more disruptive to natural enemies and bees.