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 distinguish 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, especially in nectarines and peaches.

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 yellow white 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, usually 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 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 begin emerging in April or early May. First generation larvae 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 (first to third leaves), 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 feed just under the skin; the brown rot pathogen often invades this entry site. Peach twig borer damaged fruit usually drops before harvest and, therefore, may not be present at harvest.

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 dormant spray of an organophosphate or pyrethroid insecticide plus oil to kill overwintering larvae in the hibernacula; however, these sprays pose water quality concerns and may pose some risks to raptors, aquatic invertebrates, beneficials, and other nontarget organisms. Dormant sprays of oil plus 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 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, Paralitomastix varicornis and Hyperteles lividus, 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 treatment timing

Monitor peach twig larvae during bloom and when shoots are emerging to determine that it is active. Look for the chewing damage they leave on buds. (To identify caterpillars present at bloom, view photos.) If larvae or their damage are observed at this time, two sprays of Bt or a single treatment of spinosad (Entrust, Success), methoxyfenozide (Intrepid), or diflubenzuron (Dimilin) can be applied.

Bt sprays at bloom can also be timed by dissecting hibernacula regularly from late February through bloom. Look at young trees or 1- to 4-year-old wood near branch crotches to detect the tiny hiberncula. The increase in the number of empty hibernacula reflects the number of larvae that have emerged and can be controlled by Bt once foliage is present.

Fruit sampling to determine need to treat postbloom

In orchards where peach twig borer is not damaging every year and dormant or bloom treatments are not routinely scheduled, or if the crop will be marketed as fresh prunes, be sure to monitor for this pest. Place pheromone traps in the orchard March 20 in the San Joaquin Valley and April 1 in the Sacramento Valley. For more information, see PHEROMONE TRAPS and record results on a monitoring form (PDF). Once the first moths are captured on two consecutive nights (the biofix), begin accumulating degree-days (DD) using a lower threshold of 50°F and an upper threshold of 88°F.

When 400 DD have accumulated from the biofix, begin monitoring fruit by walking around trees looking for the presence of peach twig borer larvae and damage (i.e., entries into the fruit). Be sure to look where fruit contact one another and where leaves touch fruit. Examine 15 fruit from each of 80 trees for a total of 1200 fruit. For each tree, record the number of fruit with larvae present and/or the number of damaged fruit on a monitoring form (PDF). For dried plums, treat if more than 2% (or 24) of the fruit are found with peach twig borer larvae or damage. For fresh market prunes, treat if any larvae or damage is evident.

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

Late-season fruit damage sample

In mid-July, take a fruit damage sample to assess the overall effectiveness of the current year's IPM program and to determine next year's needs. For more information, see FRUIT EVALUATION AT HARVEST. Record on a monitoring form (PDF) the number of fruit infested by larvae, type of larvae present, whether the damage is surface feeding only or if the larvae penetrated the fruit.

• Degree-day calculator
• Degree-day table
• Final fruit damage evaluation form for caterpillars, scale, and aphids (PDF)
• Photos of leafrollers and other caterpillars
• Postbloom fruit sampling form for peach twig borer (PDF)
• Using degree-days to time insecticide applications