Description of the Pest

Small larvae of peach twig borer 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, bands contrasted with the brown body distinguishes peach twig borer from other larvae found in stone fruits. Mature larvae are about 0.5 inch (13 mm) long. There are 3 to 4 generations per year.

Pupae are 0.25 to 0.40 inch (6–10 mm) long, brown, 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 (8–10 mm) long with steel-gray, mottled forewings. The long, narrow forewings are lightly fringed; the hindwings are lighter gray and more heavily fringed. Prominent palpi on the head give the appearance of a snout. The bluntly oval eggs are whitish to orange and are laid on twigs, leaves, or on the fruit surface.

Peach twig borer overwinters on the tree as an early-stage larva within a tiny cell, called a hibernaculum, that is usually located in the 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 feed on newly emerged leaves, shoots, and blooms. 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, later than the same pest in almond orchards. 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.

Damage

Peach twig borer damages prune by feeding on 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 feed just under the skin. Brown rot often invades this entry site. Peach twig borer damaged fruit usually drops before harvest.

Management

The preferred management strategy for peach twig borer is well-timed treatments Bacillus thuringiensis ssp. kurstaki (Bt; DiPel). Bloom-time applications integrate well with brown rot treatment, reducing application costs. Bloom sprays are preferred over in-season sprays because they are less harmful to natural enemies and nontarget organisms.

Peach twig borer can be controlled with a dormant spray of an organophosphate or pyrethroid insecticide plus mineral oil to kill overwintering larvae in the hibernacula. These sprays also control (using organophosphate plus mineral oil) or suppress (using pyrethroid plus oil) San Jose scale. However, organophosphate and pyrethroid insecticides pose surface water quality concerns and may pose some risks to birds of prey, aquatic invertebrates, natural enemies, and other nontarget organisms.

Dormant or delayed dormant sprays of mineral oil plus spinosad, methoxyfenozide, or diflubenzuron can effectively control peach twig borer and do not present the same level of environmental problems as treatment with organophosphates or pyrethroids. The oil in these mixtures applied in dormant or delayed controls European red and brown mite eggs and low to moderate San Jose scale populations. (Note: Oil alone does not control peach twig borer.)

In-season monitoring should be done regardless of pre- and postbloom treatments to avoid surprise infestation. Postbloom treatment may be necessary if 400 degree-days postbiofix monitoring (see details below) shows high peach twig borer numbers. Sprays of an organophosphate (diazinon) or a pyrethroid (esfenvalerate) are not recommended, as these pesticides can disrupt biological control of scales and spider mites. Appropriate pesticides for this timing include Bacillus thuringiensis ssp. kurstaki, chlorantraniliprole, methoxyfenozide, or spinosyn.

Mating Disruption

Mating disruption with sex pheromones can be used to supplement dormant, delayed dormant, or bloom-time sprays. The main practical use for mating disruption is postbloom treatment in organic orchards where other pesticides 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.

Monitor the orchard regularly for shoot strikes at the end of each generation to verify effectiveness of the mating disruption program. Also monitor fruit from the tops of trees regularly for signs of larvae or damage; monitor more frequently during the final 4 weeks before harvest. Treat with insecticide if there are more than an average of 3 to 5 shoot strikes per tree after the first moth flight or if larvae are found in green fruit.

If you are catching more than 5 moths per pheromone trap per week within one generation of harvest, treat with an insecticide rather than replacing pheromone dispensers.

Biological Control

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 kill a significant portion of larvae, but by themselves generally do not reduce twig borer numbers 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).

Organically Acceptable Methods

Use bloom-time sprays of Bacillus thuringiensis ssp. kurstaki (Bt) and mating disruption in a certified organic crop. Mating disruption has not proven to provide reliable control on its own but may be used to supplement bloom sprays.

Monitoring and Treatment Decisions

Bloom Treatment Timing

• Monitor peach twig borer 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 can be applied.

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 to the fresh fruit market, 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 (see Pheromone Traps), and record results on a monitoring form (PDF). Once the first moths are captured on two consecutive nights (biofix), begin accumulating degree-days (DD) using a lower threshold of 50°F and an upper threshold of 88°F. (For assistance in calculating degree-days, see Degree-days.) Do not use trap numbers and timing from traps placed in almond orchards. Pest emergence in each crop is timed to match with bud break of that crop.
• When 400 degree-days 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 that are damaged or have larvae present 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.

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.

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.

Dormant and Delayed-Dormant Sprays

Dormant or delayed dormant applications of many pesticides applied within 48 hours prior to rain events carry increased risks of surface water contamination resulting from orchard runoff, especially when the ground is saturated. Follow the Dormant Insecticide Contamination Prevention regulations when applying during this time of the year. Be especially careful when applying pyrethroid (MOA 3A) or organophosphate (OP; MOA 1B) insecticides, because they are highly toxic to aquatic life.

Pesticide spray drift risk is highest during dormant or delayed dormant spray applications due to lack of leaves to slow the progress of spray through the canopy. Growers should consider using Gear Up, Throttle Down or other spray practice to reduce spray drift including low drift nozzles, reduced sprayer fan speeds and use of synchronized sprayers to restrict spray movement. Currently, spray drift from pyrethroid and, to a lesser degree, organophosphate pesticide applications are of major concern regarding spray drift impacting surface water quality.

Pesticides and Natural Enemy Releases

Important Links

• 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