How to Manage Pests
Pests in Gardens and Landscapes
Figure 6. Frass, a mixture of feces and food fragments, fills tunnels that codling moth larvae have bored into this apple.
Codling moth, Cydia (Laspeyresia) pomonella, is a serious insect pest of apples, pears, and English walnuts.
Codling moth adults are about 1/2 to 3/4 inch long with mottled gray wings that they hold tentlike over their bodies (Figure 1). Their appearance blends well with most tree bark, making them difficult to detect. If you are trapping the adults, you can distinguish codling moth from other moths by the dark, coppery brown band at the tip of their wings.
The larvae are white to light pink “worms” with a dark brown head (Figure 2). They are one of the few caterpillars likely to be found inside pear or apple fruit. Navel orangeworms also might be found in walnuts, but these can be distinguished from codling moth larvae by the crescent-shaped markings on the second segment behind the orangeworm head and by the excess webbing they leave in the nut.
Codling moth overwinters as full-grown larvae within thick, silken cocoons under loose scales of bark and in soil or debris around the base of the tree. The larvae pupate inside their cocoons in early spring and emerge as adult moths mid-March to early April. The moths are active only a few hours before and after sunset, and they mate when sunset temperatures exceed 62°F.
After mating each female deposits 30 to 70 tiny, disc-shaped eggs singly on fruit, nuts, leaves, or spurs. After the eggs hatch, young larvae seek out and bore into fruit or developing nuts. After completing development they leave the fruit and drop from the trees to search out pupation sites and continue the life cycle in the soil or on debris under the tree; some crawl back up the tree to pupate in bark crevices (Figure 5).
The rate of development will vary with temperature, proceeding more rapidly in warmer weather and climates. Depending on the climate, codling moth can have two, three, and sometimes four generations per year.
On apples and pears, larvae penetrate into the fruit and tunnel to the core, leaving holes in the fruit that are filled with reddish-brown, crumbly droppings called frass (Figure 6). If left uncontrolled, larvae can cause substantial damage, often infesting 20 to 90% of the fruit, depending on the variety and location. Late maturing varieties are more likely to suffer severe damage than early varieties.
In walnuts, larvae feed on the kernels (Figure 7). Nuts damaged early in the season when the nuts are quite small will drop off trees soon after damage occurs. Nuts damaged later in the season will remain on trees, but their kernels are inedible. Walnuts aren’t as favored a host as apples and pears, and untreated trees might incur very little to modest damage (10 to 15% of the nuts), depending on the variety and location.
Codling moth can be very difficult to manage, especially if the population has been allowed to build up over a season or two. It is much easier to keep moth numbers low from the start than to suppress a well-established population. In trees with low levels, codling moth often can be kept to tolerable levels by using a combination of nonchemical management methods; however, it is important to begin implementing these measures early in the season.
Where populations are moderate to high and many infested trees are nearby, insecticide applications might be necessary to bring populations down to low levels. To be effective, the timing of insecticide spray applications is critical, and several applications are necessary, especially with newer, less toxic pesticides. In most backyard situations, the best course of action might be to combine a variety of the nonchemical and/or low toxicity chemical methods discussed below and accept the presence of some wormy fruit. If eating wormy fruit, be sure to cut out damaged portions, because they might contain toxins (aflatoxin) generated by mold. It is ideal to make codling moth management a neighborhood project, because your trees can be infested by moths from your neighbor’s trees, despite your own best efforts at keeping populations of this pest down.
Several methods are available for reducing codling moth that don’t require using insecticides. Selecting varieties that are less susceptible to damage, such as early-maturing apples and pears and late-leafing walnuts, can greatly reduce the potential for damage. This can be especially important in the hot Central Valley climates that have additional generations and result in higher population pressure.
Once trees are planted, nonchemical control methods include sanitation and fruit bagging. These methods are described below. Thinning out and removing infested fruit on the tree is an especially important part of an IPM program for codling moth. Pruning trees to a height where the canopy is easy to reach also will facilitate management of this pest.
If a backyard tree or orchard has a very high moth population, it might be impossible to satisfactorily reduce codling moth without using pesticides. Also, nearby orchards or backyard trees in which no control program is in place can serve as a continual source of codling moth and can make it even more difficult to limit damage through nonchemical means alone.
Sanitation. Sanitation should be the first step in any codling moth control program, and it is even more important for those wishing to use primarily nonchemical management approaches. Every week or two, beginning about six to eight weeks after bloom, check fruit on trees for signs of damage. Remove and destroy any infested fruit showing the frass-filled holes. Removing infested fruit before the larvae are old enough to crawl out and begin the next generation can be a very effective method for reducing the population. Thinning out the infested fruit has the added benefit of encouraging the remaining fruit on the tree to grow larger. It also might improve spray coverage, if sprays are used.
It also is important to clean up dropped fruit as soon as possible after they fall, because dropped fruit can have larvae in them. Removing infested fruit from the tree and promptly picking up dropped fruit from the ground is most critical in May and June but should continue throughout the season.
Bagging fruit. Excellent control can be achieved by enclosing young fruit in bags right on the tree to protect them from the codling moth. This is the only nonchemical control method that is effective enough to be used alone and in higher population situations. However, it is quite time consuming to apply the bags, so this method is most manageable on smaller trees with fewer fruit. You can bag all the fruit on the tree or just as many fruit as you think you will need. Keep in mind that unbagged fruit are likely to serve as a host and increase the pest population, so it would be prudent to employ sanitation to keep the population in check.
Bagging should be done about four to six weeks after bloom when the fruit is from 1/2 to 1 inch in diameter. Prepare No. 2 paper bags (the standard lunch bag size that measures 7 1/4 inches by 4 inches) by cutting a 2-inch slit in the bottom fold of each bag. Thin the fruit to one per cluster. Slip the thinned fruit through the 2-inch slit so that it forms a seal around the stem and staple the open end shut.
It is difficult or impossible to bag certain varieties with very short stems such as Gravenstein. Also late-developing varieties might be attacked by codling moth even before they are 1/2 inch in diameter, so they might not be protected. Some gardeners have found success with cotton tie string bags; nylon bags, however, aren’t effective.
This technique won’t affect the maturity or quality of the fruit, but it will prevent full color development on red varieties. You’ll need to open some bags to check for ripeness as harvest time approaches. Some people open the bags up a week or two before harvest to allow color development, but the fruit still might be attacked if codling moth eggs are being laid. Other benefits to bagging include protection from sunburn and larger fruit as a result of diligent thinning.
Trapping. Hanging traps in each susceptible fruit or nut tree might help to reduce codling moth populations on isolated trees but isn’t a reliable way to reduce damage. If it works at all, this method likely will have the most effect where trees are isolated from other trees harboring codling moth (e.g., apple, pear, or English walnut) and when several traps are placed in a tree. Use in combination with sanitation and other control methods for the best effect and expect damaged fruit.
Codling moth pheromone traps are important for monitoring flight activity of moths to help time insecticide treatments. Traps are available from many commercial sources, such as hardware stores, garden centers, or online. These traps usually have a sticky cardboard bottom and are baited with a pheromone (sex attractant) lure. The lure mimics the scent of a female moth, attracting males to the trap. Traps should be put up in mid-March in the Central Valley and by the end of March in coastal areas. They should be hung as high as possible in the tree canopy. Check them every few days for moths. Only one trap is required if you are using them to monitor moth flights to time insecticide treatments. See the Insecticides section for more information.
Trunk banding. A traditional, nonchemical method for controlling codling moth is to trap mature larvae in a cardboard band as they climb the trunk seeking a place to pupate. Banding works best on smooth-barked varieties such as Red Delicious apple, which don’t provide good alternative pupation sites. Scaly-barked varieties such as Newtown Pippin and most types of pears have so many crevices on the trunk that many larva will pupate before they get to the banded area. However, even in the best situations, banding will control only a very small percentage of the codling moth, because many pupate elsewhere on the tree or in the ground. Additionally, if bands aren’t removed and destroyed in a timely fashion, they could increase the population, so banding no longer is recommended for control in home gardens.
Biological control. Although a few predators such as spiders or carabid beetles might feed on codling moth larvae or pupae, naturally occurring biological control isn’t effective. In commercial walnut and pear orchards, releases of the tiny wasp Trichogramma platneri have been used successfully to manage codling moth in combination with mating disruption or soft pesticides. This method hasn’t been successful in commercial apples and hasn’t been tested in backyards.