Peach twig borer—Anarsia lineatella
The larva of peach twig borer (family Gelechiidae) is a pest of almond and stone fruits.
Young larvae of peach twig borer are whitish with thin brown bands and may appear pale brown or whitish overall. They have a distinct black head and grow up to 1/2 inch long. As they mature, they become chocolate brown with alternating dark and light bands around the abdomen. The pale membranes between each segment contrasted with the brown body distinguish peach twig borer from other larvae found in almond and stone fruits, which include codling moth and Oriental fruit moth.
Pupae are oblong, brown to orangish, and 1/4 to 2/5 inch long. They occur without any cocoon in organic litter beneath the tree and in protected places on the tree, such as bark crevices and occasionally in the stem cavity of fruit where they fed as larvae.
Adult peach twig borers (moths) have forewings that are dark gray mottled with whitish. The hind wings are paler gray and fringed with numerous hairlike scales. The moths are 1/3 to 2/5 inch long. Prominent elongated appendages (palpi) project forward from the head, giving adults the appearance of having a snout.
Eggs are oval and orange, yellowish, or whitish and about 1/30 inch (0.8 mm) long. They are laid on the surface of fruit, leaves, and twigs.
Peach twig borer develops through 4 life stages: egg, larva, pupa, and adult. It overwinters on the tree as a first- or second-instar larva within a tiny cell (hibernaculum) it excavates beneath thin bark in the crotches of 1- to 3-year-old wood, in pruning wounds, or in cracks of bark. At the opening to its hibernaculum, a larva forms a chimneylike pile of reddish brown frass (excrement). Hibernacula are especially noticeable when first constructed in the fall and before winter rains wash away the frass. If frass is washed away larvae rebuild their frass pile on warm winter days.
Larvae emerge from hibernacula in early spring, usually just before and during bloom. They move up branches and twigs where they chew on new blossoms and succulent leaves and shoots. As shoots elongate, larvae chew and tunnel inside, causing the terminals to die back. Dead shoots are known as flagging or shoot strikes. Note that larvae of Oriental fruit moth also cause shoot strikes on the same hosts and both insects can occur on the same tree.
Adults from the overwintered generation of larvae usually begin emerging in late March through May. Adult females lay tiny eggs individually on fruit, leaves, and twigs during spring and summer. In the fall eggs are laid on small branches.
First generation larvae usually develop in twigs during May and June and give rise to the next flight of moths in late June or early July. Larvae from this and subsequent generations may attack either fruit or twigs. Once fruit begin to develop color the preferred feeding site of larvae is just under the fruit skin.
Larvae bore into the growing shoots of twigs and ripening fruit or nuts. Shoots and leaves wilt and die back one to several inches from the growing tips of twigs. Scattered dead shoots are generally not of significance to tree health unless the tree is very young. Shoot regrowth on heavily infested young trees can cause trees to develop poor structure.
The significant damage to almond and stone fruits is caused by larvae feeding under the fruit or nut skin. This can allow entry of decay pathogens and makes fruit and nuts unappetizing to consume. Note that codling moth and Oriental fruit moth also feed in fruit of stone fruits, but these pests bore deeper inside to feed around the pit.
About 30 species of parasitoid (parasitic) wasps and predators feed on and kill peach twig borers during spring and summer. The most common natural enemies are the Chalcidoidea wasps Copidosoma (=Paralitomastix) varicornis and Hyperteles lividus, the Braconidae wasp Macrocentrus ancylivorus, and the grain or itch mite, Pyemotes ventricosus. Other wasp parasitoids include Bracon gelechiae, Toxophoroides albomarginatus, and Spilochalis species. Trichogramma species egg parasitoids commonly kill the eggs.
Insect predators include of adults and nymphs of assassin bugs and minute pirate bugs, larvae of green lacewings, and spiders. In some years these natural enemies destroy a significant portion of larvae, but by themselves do not completely prevent damage by peach twig borer.
Control ants, reduce dust (e.g., periodically hose off small plants), and avoid the application of broad-spectrum, persistent insecticides for all pests on the trees to increase the effectiveness of natural enemies. See Protecting Natural Enemies and Pollinators for more tips on conserving parasitoids and predators.
If peach twig borer damage has been intolerable during previous growing seasons, there are two timings to insecticide sprays that can be effective for residential fruit and nut tree growers. The most effective is Bacillus thuringiensis (Bt) or spinosad applied during bloom to thoroughly cover petals and shoot tips. If applying spinosad, make the application at petal fall and in the late afternoon when honey bees are no longer active because spinosad is toxic to bees for several hours after its application. If applying Bt, make the first application just after the popcorn stage, when blooms first open. Make a second application 7 to 10 days later before petals fall. If there is prolonged cool weather a third application 7 to 10 days later, but before petal drop, may be warranted.
Alternatively horticultural oil and spinosad combined can be applied during the delayed-dormant season (once buds have begun to swell but before they open) to completely cover shoot terminals. The combination spray of oil and spinosad has the advantage of also killing other pests such as overwintering mites and scale insects.
Note that sprays of oil alone do not control peach twig borer. Any applications of products available to home gardeners after petal fall are not likely to be very effective. If peach twig borer is causing damage, wait until the next delayed-dormant season or bloom and then make the applications as described above.
Adapted from Integrated Pest Management for Stone Fruits, Pest Management Guidelines: Peach, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).