Growing Fruit Trees in Maine - Insect Pests
Insects can devastate a crop of fruit in an unsprayed orchard since there are no varieties with resistance to insects and cultural practices are mostly ineffective in preventing insect damage. However, most damage can be prevented with a combination of a few well-timed insecticide applications.
Plum curculio is a major insect pest of apple, plum, apricot and cherry, and a minor pest of pear and peach. Plum curculios overwinter under leaf litter at the edges of woodlots. They emerge in May and slowly move into orchards where they mate. After bloom, adult curculios begin to lay eggs in the flesh of the developing fruit causing a characteristic crescent-shaped scar. Egg-laying continues for six to eight weeks, until mid July. Larvae feed inside the fruit and tunnel toward the center, causing infested fruit to drop in early summer. Plum curculios will also feed on young fruit causing a tiny hollowed out cavity. Plum curculio larvae move from fallen apples into soil, where they pupate and emerge as next generation adults from late July into September. These next generation adult curculios feed on apples before finding overwintering sites. Late summer fruit feeding damage appears as irregularly shaped holes that do not have time to heal before harvest. Feeding holes are often on the blossom or calyx end of the apple.
To prevent damage by plum curculios, an insecticide spray can be made shortly after petals fall from the blossoms or at the first sign of damage. For a high degree of control, a second application can be made about ten days later. If cool weather occurs, a third application may be needed when a high level of control is desired. The plum curculio is not likely to cause damage after mid July.
To protect bees, always wait until bloom is completely over to apply any insecticide. Products containing carbaryl (Sevin™) or phosmet (Imidan) are effective against plum curculio. Products containing pyrethrin may be certified organic, but are much less effective and will require reapplication at three- to seven-day intervals for effective control. The efficacy of azadirachtin, another organic insecticide, has not been fully tested for plum curculio.
Surround™ is a clay-based repellent that is also certified for organic use. In order to be effective, Surround™ coverage on fruit should be thorough beginning shortly after petals fall and re-applied weekly to maintain a white coating on the fruit and foliage until early July. Surround™ protection is designed to repel plum curculio until the egg laying period is over.
European apple sawfly lays its eggs on the base of the apple flower during bloom. The eggs hatch into larvae that initially feed on the surface of the fruit. This initial feeding creates a winding scar on the surface of the apple. For a high degree of control, apply an insecticide as soon as the petals fall from the tree. Delay of two or three days provides much less control of damage to the first infested fruit, but later sprays will help prevent movement of larvae into a second or third fruit. Starting at about two weeks after petals fall, larvae that were not killed with a petal fall spray will begin moving from the first infested apple to a nearby apple. The sawfly larvae tunnels into these apples, leaving a hole in the side of the fruit. These fruit will soon drop off the tree. Sawfly can cause much of the fruit to be shed. Insecticides and repellents that are applied for plum curculio also prevent damage by sawfly.
Codling moth larvae tunnel into apples and pears, feeding on the fruit and seeds. An
entry hole with sawdust-like excrement (“frass”) may be visible at the blossom end or on the side of the fruit. The damage to the fruit resembles the damage caused by European apple sawfly, but occurs later in the season when fruit are larger than one inch in diameter.
Codling moths overwinter in the orchard as larvae. Following pupation, they emerge as adults in May and June. They mate and lay eggs in summer. Their offspring pupate in July and emerge as adult moths in August.
Where insecticide sprays with carbaryl or phosmet have been used to prevent plum curculio damage, there is much less chance of severe problems with codling moth. If pyrethrin or Surround™ were the only materials used against plum curculio, then additional treatments targeted against codling moth will provide additional control. Organic options include Surround™ coverage maintained through mid July; or two to three applications at seven- to ten-day intervals of Bt (Bacillus thuringiensis), azadirachtin, or Entrust™ from mid June into July.
The apple maggot overwinters as pupae in the soil and emerges in late June and July. Approximately ten days after emergence, the females begin to lay eggs in apples. Larvae feed on the flesh of the fruit leaving a brown trail inside the apple.
Eggs and larvae are protected from insecticides and repellents since they are inside the apple. This means that control methods should be in place just before egg laying occurs. Insecticide applications, starting in mid-July, with a renewal application in late July or early August, and a final application in mid August, are usually adequate to prevent apple maggot infestation. The final application should be omitted for the earliest ripening varieties, and the first application maybe omitted for the latest ripening varieties. Insecticides should not be applied right before harvest. Esfenvalerate cannot be used within 21 days of harvest. Carbaryl and phosmet cannot be applied within a week of harvest.
Red spheres the size of a large apple and covered with the sticky substance Tangletrap™ can be used to trap adult flies. At least one trap for every 100 apples is needed to adequately prevent infestation. Use one trap on a dwarf tree or four traps on a semidwarf tree. Place traps in the orchard the first week of July. Traps should be completely covered with Tangletrap™ and hung from a branch in the outer part of the tree, surrounded by fruit, but not hidden by leaves.
The repellent Surround™ can be used in conjunction with traps or on its own. However, Surround™ applied to fruit in August may still be on the fruit at harvest and should be washed off before consuming the apple.
Different aphid species commonly occur on fruit trees, but are usually an insignificant pest. Aphid populations are usually reduced by their natural enemies later in the growing season, so insecticide sprays are not warranted. One exception to this is for very young trees where a heavy aphid population can stunt growth. There are many fruit tree spray products to control aphids if needed. However, on established trees, spray application is typically not needed.
Pear psylla feed on pear leaves and secrete honeydew which accumulates on foliage and branches. Black sooty mold grows on the honeydew. When they feed on leaves, pear psylla inject a toxin with saliva, and this toxin can cause defoliation.
Adult pear psylla overwinter in cracks on the tree and in ground litter. Early in the spring before buds open, psylla adults become active and begin to mate on days with temperatures approaching 50°F. The earliest eggs hatch as the first foliage appears. The first generation egg hatch is largely complete by the end of bloom.
The primary defense against pear psylla is dormant oil application before or just as the buds swell and temperatures reach 50°F in early spring. A second dormant oil application seven to ten days later is also recommended. Apply oil at a rate of 2%, or three fluid ounces of oil per gallon of water. For best control, completely cover the tree with the dormant oil spray. After foliage appears, when dormant oil can damage foliage, insecticides labeled for pear psylla can be used, but may not be effective because of resistance. A horticultural grade oil may be used instead, but can cause leaf damage when applied with Captan or sulfur fungicides. Mature pear psylla are more difficult to control than eggs and young nymphs.
Earwigs feed on peach, nectarine and apricot fruit as they ripen in summer. Earwigs also feed on nectarines soon after bloom causing a large scar on the surface of the fruit at harvest. They have an elongated, dark brown or reddish brown body with pincers at the end.
As fruit ripen, they move from the ground cover into trees. Removing shoots that are in contact withweeds and tall grass will inhibit their movement into trees.
Monitor their presence in the orchard by placing a rolled up newspaper in the orchard several weeks before fruit begin to ripen. This should be examined for earwigs once a week. As they begin to move into the tree, carbaryl insecticide can be applied to the trunk. The efficacy of organic insecticides for control of earwigs has not been tested.
Trunk borers are insect larvae that feed on bark and wood inside the trunk. Trees less than ten-years-old, and trees with a trunk diameter less than four inches are particularly vulnerable. Many young and dwarf fruit trees in home plantings are killed by borers. Once borer larvae are feeding inside the trunk, they are protected from natural predators and insecticide sprays. Preventing borer infestation and killing borer larvae before they tunnel deep into the trunk are the keys to protection.
The roundheaded apple tree borers lay their eggs on the lower trunk usually near the ground from late June into August. Upon hatching, the larvae burrow into the lower trunk. The tunneling can kill the tree by structural damage to water conducting tissues and in severe cases will cause the trunk to break off completely. Small pinholes with reddish frass indicate the presence of boring larvae. Pencil-sized holes in the trunk indicate that the borer has done extensive damage and has already left the trunk. Flatheaded apple tree borers feed primarily on the sapwood of trunks and larger branches on trees with poor growth, and trees with damaged bark. The dogwood borer, which feeds near the surface of the trunk, is less devastating.
Apple trees sprayed with foliar insecticides during June, July and August are much less likely to be attacked by trunk borers. For trees not receiving foliar insecticide sprays, insecticide application to the trunk at two-week intervals from late June to mid August will reduce the likelihood of borer attack.
To prevent borers from laying eggs, a loose-fitting barrier such as mosquito netting or window screen can be wrapped around the lower trunk and closed at the top by tying it with a cord and closed at the bottom by mounding soil over it. Barriers should be in place by the end of June, but loosened or removed after harvest to prevent girdling. When a cord or trunk guard girdles a tree, it kills the tree and defeats the purpose of the trunk guard. Paper trunk wraps and white spiral plastic wraps encourage trunk borers and should not be used on fruit trees grown in Maine.
White latex paint, diluted with water at a ratio of one part paint to one part water, can be painted on the lower two feet of the trunk to repel borers. Other types of paint may harm trees.
Careful inspection of the lower trunks in September and May can reveal sunken wood and loose bark where borers have invaded. Borer larvae feeding near the surface can be removed with a sharp cutting tool. A stiff wire is useful to kill borers that have tunneled further into the trunk.
Read and follow all safety instructions before using any insect control product. Use only products that specify the type of fruit you wish to spray, or “tree fruit” on the label. “Organic” in this context refers to products allowed by organic certification programs.
Azadirachtin, an extract of neem oil, is a repellent with mild insecticidal activity. Azadirachtin has limited effectiveness for control of apple pests. Product names include Bioneem™, Neemix™ and Aza-Direct™.
Bt is produced by the bacteria Bacillus thuringiensis and is effective for control of moth and butterfly caterpillars, such as codling moth and leafroller. Bt has relatively short effectiveness because it is quickly degraded by sunlight. Protection lasts no longer than about one week. Thus multiple applications are needed for adequate control when relying solely on Bt. It is sold as Dipel™ and many other product names.
Dormant oil can effectively control spider mites, pear psylla and scale insects, but not other insect problems. Commercial formulations contain an emulsifier that enables oil to be mixed with water. While spraying dormant oil, agitate the oil-water mixture to keep them from separating. Do not mix oil with captan or sulfur fungicides, as these combinations burn leaf and fruit tissue. Do not apply dormant oil after foliage emerges in spring.
Entrust™ is an insecticide that controls codling moth, leafrollers and apple maggot. It is a naturally occurring compound with insecticidal activity, but a low degree of toxicity to mammals and birds. It is a broad spectrum insecticide meaning it works against many types of insects including bees, so it should not be used when trees are in bloom.
Pyrethrin is a botanical insecticide effective against many apple pests, but degrades quickly. Frequent applications are needed for reasonable effectiveness. It is a component of some multipurpose sprays for fruit trees.
Surround™, a clay product, acts as a repellent thereby preventing insects from damaging fruit. For maximum effectiveness, Surround should be applied weekly to achieve full coverage of the fruit.
Permethrin, resmethrin and esfenvalerate are synthetic pyrethroid insecticides effective against a broad range of tree fruit pests. They are sold under many product names.
Carbaryl (Sevin™) controls most pests and can be used as a fruit thinner of apple. Apply it one to three weeks after bloom for fruit thinning. This will reduce the number of fruit on the tree and will increase flowering the following season. When applied just before hot weather (>85°F), it can over thin. Carbaryl causes pears to become deformed when applied in the month after bloom.
Fruit tree spray mixes typically contain a mixture of carbaryl or pyrethrum insecticide and the fungicide captan. It is meant to be used for both insect and disease control.
Phosmet (Imidan) is very effective for control of most pests of apple. It is useful when a high degree of insect control is desired.
Rotenone is a botanical insecticide which provides short term control for some apple pests. It is more acutely toxic than other botanical insecticides such as pyrethrin and azadirachtin.
Where company or brand names are used, it is for the reader’s information. No endorsement is implied nor is any discrimination intended. Always consult product labels for rates, application instructions and safety precautions. Users of these products assume all associated risks.