You've been waiting for months for that first perfectly ripe apple. You grab it off your tree and gaze at it in anticipation, only to discover that something else got there first. As anyone who grows fruit trees knows, you compete with several insects for each piece of fruit.
The key to winning the battle -- and to maximizing your fruit crop -- is to understand the pest's lifecycle and habits and to use a full arsenal of countermeasures to thwart their activity. Integrated pest management specialists have devised many ways to control orchard insects, including traps, lures, beneficial insect releases and low-impact sprays. With a little know-how, the home gardener can use any and all of these controls to grow an abundant fruit crop with minimal spraying. What follows is a sampling of the prominent pests of fruit trees and the strategies to control them.
No discussion of fruit-loving insects can avoid the codling moth (Cydia pomonella), the proverbial "worm" in the wormy apple or pear. Larvae tunnel into the fruit in search of its nutritious core, but in doing so, they destroy the flesh of the fruit that we cherish. Codling moths start the season within a silken cocoon, hidden in and around fruit trees.
The following spring, the first adult moths emerge, usually around the time of full bloom. Over the next few weeks, female moths lay eggs on the leaves and young fruit. Some of the newly-hatched caterpillars manage to make it into the developing fruit, although few are successful.
However, it's the second generation of the moth that worries apple and pear growers. This generation typically occurs in mid to late July. Eggs are laid directly on the fruit. One to two weeks later, eggs hatch and the young larvae tunnel into the fruit, usually at the calyx end or where two fruit touch--both sites offer these tiny insects a better grip as they try to cut through the fruit's skin.
Codling moth control usually involves repeated use of insecticides, so that plants are protected during the entire egg-laying period. If the activity of the moths is monitored--by the use of attractant pheromone traps, for example--then use of sprays can be limited to only those periods when the moths are most active. Otherwise, use a regular 10- to 14-day schedule of sprays, beginning as soon as flower petals fall, to ensure a near worm-free crop.
Sprays of the insecticide Bacillus thuringiensis (Bt) are moderately effective. Sprays of kaolin clay are also promising. At least three sprayings are needed, and timing is critical: Make the first spray 15 days after petal fall begins, and subsequently at 5-day intervals.
Many gardeners don't want to go through such a hassle and are willing to give up a few fruit to the caterpillars in return for cutting back on insecticide sprays. Several cultural controls can assist them in this regard.
Trap larvae. After larvae have finished feeding on the fruit, they wander to flaps of bark or other protected sites to pupate. Assist them in this by providing an "artificial bark flap", such as a band of corrugated cardboard or burlap, on the trunk. The caterpillars will use the band as a "resting" site to complete pupation. There they can be collected and destroyed.
Make sure that the bands are checked at least every two weeks, or the codling moth caterpillars may transform to the adult moths and escape before you get to them.
Trap the adults. Adults of both sexes can be attracted to fermenting mixtures and then trapped. Mix some molasses in water (about a 1 to 7 ratio is frequently suggested) and pour it into a cut plastic milk jug or other homemade trap and hang it on the tree.
(Note: Pheromone traps contain the sex attractant used by the female to attract male moths. They are highly effective in this regard but, unfortunately, trapping male insects doesn't do much in terms of population control. Instead the remaining males become more sexually active and females end up producing just as many little ones.)
Thin fruit. Thinning is often a desirable practice in promoting larger fruit and steadier production. It also controls codling moths. Young caterpillars have great difficulty holding on as they cut into the fruit. Areas where two fruit touch provide them leverage, but their success in tunneling is even more limited once these sites are removed.
Biological controls. Codling moths are attacked by several natural enemies, although these rarely are effective enough to prevent serious crop damage. Releasing additional natural enemies helps. Trichogramma wasps, tiny parasites of the moth's egg stage, are commonly reared by insectaries and sold for pest control. If you're trying out trichogramma, release them when there is evidence of codling moth eggs. Consider sequential releases at one to two week intervals throughout the egg-laying period. This can usually be easily arranged with your supplier.
This pest often rivals the codling moth as a fruit grower's headache, at least in the midwest and eastern states where this "snout beetle" is well established. Capable of damaging almost all tree fruits, plum curculio can cause several kinds of injuries throughout most of the growing season.
Early in the year, around the time when petals fall, the overwintered beetles cut semicircular scars in fruit as they feed. These puncture wounds scar and deform the fruit. The beetle then lays eggs in some of these wounds. The eggs hatch into the grublike immature forms that tunnel in the developing fruit, causing infested fruit to fall in late spring, well before it has ripened. For the rest of the season, the new crop of adult beetles is present, chewing on and pitting whatever fruit they choose to feed.
Again, a variety of precautionary measures is best. Pick up fallen fruit in late spring or early summer. This will clear up the main source of the new crop beetles, which are developing within the fruit. Prune and train the tree to open up and accept more light: plum curculio hates light and thrives where vegetation is most dense.
Control of adult beetles, as they awaken and move to trees in spring, is also recommended. Usually they begin to show up after the first warm spell in spring. (Three days with an average temperature of 60° F and a bit of rain is a perfect scenario for plum curculio kick-off.) Shake the branches to dislodge the beetles, which tend to drop when disturbed. They can be collected on sheets placed under the tree. Insecticides are frequently used, although plum curculio is difficult to control with many of the more commonly available products. Phosmet (Imidan) is one of the few insecticides that work well against this pest.
Fruit flies (Rhagoletis), such as the apple maggot and the cherry fruit flies, are also common orchard pests. Apple maggot, also called the "railroad worm," tunnels inside apples (less commonly hawthorn, cherries, pears and most other fruit) and creates meandering brown streaks that often cause decay. Similarly, cherry fruit flies tunnel and destroy cherry fruit. Damage by fruit flies is also caused when the adult insect lays eggs in the fruit, causing puckered wounds on the fruit surface.
Yellow panels are very attractive to the flies during the first couple of weeks after they become active. Use yellow sticky cards to detect when the insects are active, and when sprays are most appropriate. These traps also can be used to control these insects. Later, other traps can be used, notably the "Super Apple" a red sticky sphere resembling a somewhat oversized apple. Hung on the tree, this serves as a super attractant to the female flies as they seek out places to lay eggs.
Cleaning out infested fruit is also important when managing fruit flies. Pick over cherries as completely as possible and destroy apples showing the "stings" of apple maggot so that developing insects in fruit are destroyed.
Fruit flies also breed on the fruits of wild plants. Apple maggot prefers hawthorn, and cherry fruit fly prefers wild black cherry. If these pests are problems, consider destroying any of these plants that a nearby.
Pears not only get hit by the codling moth, but can suffer a special plague: pear psylla (Psylla pyri). This small insect, a distant cousin of the aphids, sucks sap from pear leaves. It damages the tree in several ways. Most notably, the insect excretes a sticky honeydew that covers the leaves and fruit and promotes the growth of black sooty molds. Pear psylla infestation stunts the growth of leaves. It can even cause all the leaves to fall, a condition known as psylla shock. Finally, pear psylla can transmit phytoplasma, a type of bacteria that causes pear decline, a potentially lethal disease. This typically occurs on trees grafted to Pyrus pyrifolia or P. ussuriensis root stock.
Pear psylla spends the winter off the tree, but adults return in late winter or early spring as pear buds begin to swell. At first, the insect lays eggs on the bark, near buds; later in the season, eggs are laid directly on leaves. Nymphs, or immature psylla, spend much of their time immersed in a droplet of sticky honeydew and only move out of it to prepare for the adult stage. Typically, three generations are produced during the season.
Controlling pear psylla is a real challenge to commercial orchardists, mostly because the insect is quickly resistant to new insecticides. However, in backyard plantings (or in commercial orchards where spraying is minimal), a host of biological controls is available and can provide a high level of natural control.
Several cultural practices will help prevent pear psylla problems. Perhaps most important is limiting the amount of succulent new growth produced during the year. This denies pear psylla the tender shoots that allow them to increase their numbers. Limit new growth by using only modest amounts of water and fertilizer. Also, prune to avoid flushes of regrowth. Pull off -- don't cut -- water sprouts that grow from the base of the trunk in late spring. Pulling them off prevents regrowth. Finally, using two applications of horticultural oil prior to bloom will delay the pest's egg-laying cycle.
Aphids are abundant on all types of fruit trees. Usually they infest the shoots and the young leaves in spring. One species, the woolly apple aphid, is found on the bark of trunks and branches or on the roots, where it produces knotty growths. Another problematical aphid is the rosy apple aphid, which injects a toxic saliva into the plant, causing fruit damage.
Aphids found on fruit trees spend winter on the plant as eggs, usually laid around dormant buds. They hatch just as the buds break. After a spring feeding, the winged aphids move to a second host plant. Alternation of hosts is common among aphids. For example, the rosy apple aphid alternates between apple and narrow-leafed plantain; green peach aphid alternates between stone fruits such as peach, apricot or plum and summer hosts of various garden vegetables and weeds.
Late in the season, however, offspring return to the winter host plant--your fruit tree.
Horticultural oils should do the trick. One or two applications prior to bud break will cover and smother the overwintering eggs and prevent spring attacks. These treatments also help to control other pests that overwinter on the tree, such as leafrollers and mites.
A host of other insects visit backyard orchards -- various caterpillars, pear slugs, and leafhoppers to name a few. However, their needs are usually modest and don't seriously threaten your harvest. Even if a few insects do get into the fruit, the transformation of your fruit to jam or juice does wonders toward eliminating this worry.
Whitney Cranshaw is professor and extension entomologist at Colorado State University at Fort Collins.
Photography by National Gardening Association
Article published on June 23, 2008.