Peach Twig Borer

Small peach twig borer larvae are almost white with a distinct black head. As larvae mature they become chocolate brown with alternating dark and light bands around the abdomen. The light, intersegmental membranes contrasted with the brown body distinguishes peach twig borer from other larvae found in stone fruits. Mature larvae are about 0.5 inch long.

Pupae are 0.25 to 0.4 inch long, brown in color and lack a cocoon. Pupation takes place in protected places on the tree and occasionally in the stem cavity of infested fruit.

Adult peach twig borer moths are 0.3 to 0.4 inch long with steel gray, mottled forewings. The long, narrow forewings are lightly fringed; the lighter gray hindwings are more heavily fringed. Prominent palpi on the head give the appearance of a snout. The bluntly oval eggs are yellowish to orange and are laid on twigs, leaves, or on the fruit surface.

Peach twig borer overwinters on the tree as a first- or second-instar larva within a tiny cell, called a hibernaculum, that is located in crotches of 1- to 3-year-old wood, in pruning wounds, or in deep cracks in bark. The overwintering site is marked by a chimney of frass and is especially noticeable when first constructed or before winter rains set in. Larvae emerge in early spring, usually just before and during bloom, and migrate up twigs and branches where they attack newly emerged leaves, blossoms, and shoots. As shoots elongate, larvae mine the inside, causing the terminals to die back. Dead shoots are known as shoot strikes or flags.

Adults from the overwintered generation usually begin emerging in April or early May. 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 twigs or fruit depending on fruit maturity and population density.

Peach twig borer can damage stone fruits by feeding in shoots and causing shoot strikes, or by feeding directly on the fruit. Shoot damage is most severe on the vigorous growth of young, developing trees because feeding kills the terminal growth and can result in undesirable lateral branching. As fruit matures, it becomes highly susceptible to attack; damage is most likely to occur from color break to harvest. Twig borer larvae generally enter fruit at the stem end or along the suture and usually feed just under the skin.

Within an IPM program, the preferred management strategy for peach twig borer is well-timed treatments of environmentally sound insecticides around bloom time. These include Bacillus thuringiensis, spinosad (Entrust, Success), methoxyfenozide (Intrepid), and diflubenzuron (Dimilin). Bloom time applications integrate well with brown rot treatment, thus helping to cut application costs. Bloom sprays are preferred over in-season sprays in an IPM program because they have less adverse impact on beneficials and nontarget organisms.

Alternatively, peach twig borer can be controlled with a spray in the delayed dormant season to kill overwintering larvae in the hibernacula. Organophosphates and pyrethroid insecticides have traditionally been used but these should be avoided because they pose surface water quality concerns and may pose some risks to raptors, aquatic invertebrates, beneficials, and other nontarget organisms. Dormant sprays of oil plus spinetoram (Delegate) spinosad (Entrust, Success) or diflubenzuron (Dimilin) do not present these environmental problems. Dormant sprays of oil alone or oil combined with an insecticide, however, have the advantage of controlling some other stone fruit pests, especially mites and San Jose scale. (Oil alone does not control peach twig borer.) Mating disruption during the growing season can also be used to supplement dormant sprays.

Mating disruption with sex pheromones can be used to supplement dormant or bloom time sprays. The main practical use for mating disruption is postbloom treatment in organic systems where other materials are not available. Mating disruption has not been reliable against peach twig borer when used alone. It is most effective in orchards with low moth populations that are not close to other untreated peach twig borer hosts or almond orchards. Efficacy is reduced by small orchard size, uneven terrain, reduced pheromone application rates, applying too low in the tree, improper timing, and high insect pressure. Follow timing guidelines given in the treatment table below.

Peach twig borer has about 30 species of natural enemies. The gray field ant, Formica aerata, preys on peach twig borer during spring and summer. In some years these natural enemies destroy a significant portion of larvae, but by themselves they generally do not reduce twig borer populations below economically damaging levels. Other commonly found natural enemies in California are the chalcid wasps, Copidosoma (=Paralitomastix) varicornis and Hyperteles lividus, the braconid wasp Macrocentrus ancylivorus, and the grain or itch mite, Pyemotes ventricosus.

Bloom time Bacillus thuringiensis sprays, sprays of the Entrust formulation of spinosad, and mating disruption are organically acceptable methods for peach twig borer management.

Bloom

Monitor for peach twig borer larvae and its damage during bloom (see EARLY SEASON MONITORING) when shoots are emerging, to determine if the pest is active. When emerging shoots are about 1 inch long, look for wilted leaf shoots and feeding at the base of flowers.

If larvae or their damage are observed at this time, a single treatment of diflubenzuron (Dimilin), spinetoram (Delegate), spinosad (Entrust, Success), methoxyfenozide (Intrepid), or chorantraniliprole (Altacor) can be applied or two sprays of Bt.

Shoot strike monitoring

Monitor all orchards from bloom onward for shoot strikes at the end of each generation. Shoot strikes first appear when the degree-day accumulation from moths in traps approaches 400 DD but more will be evident around 700-800 DD. Treatment threshold is three strikes per tree. See SHOOT STRIKE MONITORING for additional information.

Pheromone traps and degree-day accumulation

Install pheromone traps in orchards by March 20 (see PHEROMONE TRAPS). If in-season sprays are necessary (as determined by the shoot strike sample in the previous generation), you will need results from trap catches and degree-day accumulations to time them. Once the first moth has been trapped, begin accumulating degree-days (DD) using a lower threshold of 50°F and an upper threshold of 88°F.

Research has shown that best control can be achieved when treatments are applied about 400 DD from the beginning of the flight if the fruit is still green; if fruit has begun to color, however, treat at 300 DD. If Bacillus thuringiensis is used, two sprays should be applied: one at 300-350 DD and the other at 450-500 DD. If Intrepid or Altacor is used, a single application is effective and should be applied at 300 DD.

Calculate degree-days for oriental fruit moth in nectarine for your location using the peach twig borer model or degree-day table. To learn more about using degree-days to time insecticide applications, watch the degree-days video.

Fruit samples

Examine fruit on trees every other week after color break (see PREHARVEST FRUIT SAMPLES) to detect any developing problems in the orchard and take a fruit damage sample at harvest to assess the effectiveness of the current year's IPM program and to determine the needs of next year's program, see FRUIT EVALUATION AT HARVEST. Record results (PDF) for harvest sample.

• Degree-day calculator
• Degree-day table
• Fruit sampling for pest damage in harvest (PDF)
• Using degree-days to time insecticide applications