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How do I protect my peach trees from insects?

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If any of your peaches survived the spring freezes then you will definitely not want to share your precious harvest with the insects this year. There are two primary peach insect pests in Utah that attack the fruit, twigs and trunks. It is important for home gardeners and commercial orchardists to protect their crops as uncared for trees serve as a source of insects for neighboring peaches.

Peach twig borer is a small brown moth as an adult and a chocolate brown caterpillar in the young stage. This insect overwinters as a mature caterpillar on twigs and limbs in a small chimney-shaped cocoon. When new buds begin to swell with warming temperatures in the spring, caterpillars emerge and feed on buds and in elongating shoot tips. An excellent preventive control for peach twig borer is to spray peach trees with a dormant oil (highly refined petroleum oil) at first bud break and show of color in the spring. The oil will coat the limbs and suffocate the overwintering caterpillars before they become active. If the insect isn't controlled in the spring, and twig flagging (drooping terminal shoots) occurs, prune off injured shoot tips to remove insects before they complete development to an adult.

Subsequent generations of the peach twig borer caterpillars will bore into peach fruits in June through September, especially entering at the stem end where they are difficult to detect. The most effective way to protect fruit is to use a protective insecticide spray. Timing sprays is important to avoid more treatments than necessary and ensure effective control. The first spray is typically due in mid June in northern Utah. Check with your local USU Extension county office for the proper timing for your area. Two Utah State University web sites with information on home horticulture, timing of tree fruit pest controls and integrated pest management are http://www.slcoedcr.org/usu/html/horthome.html and http://extension.usu.edu/coop/ag/environ/ipm/index.htm. Effective insecticides include endosulfan (Thiodan), phosmet (Imidan), diazinon and methoxychlor for the home gardener and chlorpyrifos (Lorsban) for the commercial grower.

Another peach insect pest that can girdle and kill trees if left unchecked is the greater peachtree borer. This insect is a day-flying clearwinged moth with a steel blue body with yellow stripes. The adult moths lay eggs in cracks and crevices near the base of peach, apricot and nectarine trees beginning in early July in northern Utah. Cherry and plum are also hosts, but are not attacked as frequently in Utah. Upon hatching, the caterpillars bore under the bark and feed in the cambium layer. Repeated attack by borers can girdle and kill trees. Obvious signs of borers are sapping holes with a buildup of sawdust on or under the holes. The most effective control is to kill eggs and larvae with a protective insecticide treatment. Endosulfan (Thiodan) (Lorsban for commercial growers) should be applied only to the lower trunk (avoid limbs and fruit) the first week of July in northern Utah and repeated in the first week of August. A mechanical control for borers already present in trunks is to insert a wire and puncture the insect in the spring to early summer or fall. Use of pheromone mating disruption is very effective in orchards one acre or larger. Contact your local USU Extension office for more information.

Posted on 14 Sep 2006

Diane Alston
Hort-Entomologist Specialist

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