If caterpillars are eating your apples, they are almost certainly the larvae of the codling moth (Cydia pomonella). This is North America’s most important insect pest of apples, both in commercial orchards and home garden trees. My garden includes four dwarf apple trees, so I have codling moth concerns.
This pest can be difficult to eliminate completely in the home orchard, but it can be controlled to the point that the gardener will have plenty of fruit while sharing a small percentage with these vexatious invaders.
A recent recommendation in “Things to Do This Week” was to spray apple trees with carbaryl (sold as Sevin) a broad-spectrum insecticide. Correct use of this product requires careful timing, using a maximum-minimum thermometer and a degree-day chart, as noted by the University of California’s Integrated Pest Management Program (ipm.ucanr.edu).
This product is quite effective by over-stimulating the nervous systems of insects, resulting in the inability to contract breathing muscles and ultimately causing death.
Carbaryl is also effective in killing honeybees and other beneficial insects and quite toxic for people. The National Pesticide Information Center (npic.orst.edu) reports that brief exposure while spraying can cause weakness, dizziness, and sweating. Additional reports have included pinpoint pupils, lack of coordination, muscle twitching, and slurred speech. People could also experience headaches, nausea, vomiting, stomach cramps, diarrhea, or drooling.
Greater exposure can cause high blood pressure, decreased muscle tone, and seizures. Other serious signs include difficulty breathing, constriction of the airways, mucous production, fluid buildup in the lungs, and reduced heart and lung function.
Given these problems, I looked into non-chemical management of codling moths. As one might expect, this involves knowing the pest’s life cycle. The adult moth emerges right around now, mid-march to early April, is active for only a few hours before and after sunset, and mates when sunset temperatures exceed 62 degrees F.
The female deposits eggs on apple leaves or fruit. When the eggs hatch, the larvae bore into the fruit, causing the damage we don’t like to see.
After the larvae mature, they drop from the tree, sometimes still in apples. They continue their life cycle by pupating in the soil or debris under the tree, or in bark crevices until they emerge as adult moths.
The first step in non-chemical management of this pest is sanitation, which involves removing and destroying any fruit that the larvae have entered. Thinning the infested fruit in this way also helps the remaining fruit to develop.
Sanitation continues during May and June by removing any dropped fruit from the ground.
The next good management action is to bag the fruit when it is one-half to one-inch in diameter, using No. 2 lunch bags. The bagging method can be very effective, even when limited to the number of apples the gardener wishes to protect. The bags can be opened a week or two before harvest to allow color development, at some risk of late arriving larvae.
A relatively new insecticide called CYD-X has been found to be both effective and safe to use in the garden. This product is a naturally occurring granulosis virus that infects and kills the larvae of the codling moth. It is highly specific to the codling moth and is non-infectious toward beneficial insects, fish, wildlife, livestock or humans. The National Organic Program has listed CYD-X for use in organic orchards.
Spray application of this product must occur during the day or two after the codling moth larvae have hatched and before they penetrate fruit. This time period occurs from late May to mid-June. Precise timing requires the use of a degree-day model, which regretfully requires more explanation than this column can accommodate. For the home gardener, weekly applications during egg hatch throughout the season will be quite effective. Adding 1% horticultural oil to the application can improve effectiveness.
The larva must ingest the product to become infected with the virus. The product is extremely virulent, so it is effective at very low use rates.
Another safe and effective insecticide is Spinosad, a biological product made from a naturally occurring bacterium. It is a lower-toxicity material that is safe for most beneficial insects as well as for people, pets, and the environment although it is more toxic to beneficial insects than granulosis virus. Repeated applications during egg hatch for each generation are necessary for acceptable control.
The availability of non-chemical controls of codling moths enables gardeners to keep highly toxic chemicals out of their gardens and still enjoy pest-free apples.
Organic gardening is its own reward.