Peach Twig Borer

The peach twig borer is widely distributed throughout California and is found on several hosts. The adult moth is about 0.3 to 0.5 inch long, with steel gray, mottled forewings. Young larvae are almost white with black heads. Mature larvae are about 0.5 inch long and have black heads and dark brown bodies with white portions between each body segment, giving the appearance of stripes. The peach twig borer overwinters as a larva in a tiny cell called a hibernaculum, located in limb crotches of 1- to 4-year-old wood or in roughened areas of the trunk. There may be three to four generations each year, but the later generations occur after apricot harvest.

This pest damages in two ways. Larvae burrow down into tender shoots and kill the tip, which may cause problems in training young trees. They also feed on fruit, primarily at the stem end (early-harvested varieties are less susceptible than later-harvested ones). Either feeding damage or the presence of larvae will cause a fruit to be offgrade.

Within an IPM program, the preferred management strategy for peach twig borer is well-timed treatments of environmentally sound insecticides around bloom. These include Bacillus thuringiensis, spinosad (Entrust, Success), and diflubenzuron (Dimilin). Bloom 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 harm beneficials and nontarget organisms less and do not leave residue on fruit.

Alternatively, peach twig borer can be controlled with a dormant insecticide spray of oil plus spinosad (Entrust, Success), Intreprid, or diflubenzuron (Dimilin) to kill overwintering larvae in the hibernacula. Oil plus an organophosphate or pyrethroid insecticide is the most environmentally disruptive dormant spray option, as it raises water quality concerns and may pose some risks to raptors, aquatic invertebrates, beneficials, and other nontarget organisms. Dormant sprays of oil alone or oil combined with an effective insecticide have the advantage of controlling some other stone fruit pests, especially mites and San Jose scale. Oil alone does not control peach twig borer, and some apricot cultivars are sensitive to dormant oil sprays. Mating disruption can also be used effectively in early harvested orchards to supplement dormant sprays.

Mating disruption with sex pheromones can be used to supplement dormant or bloom sprays. The main practical use for mating disruption is where the crop is harvested before July and in organic systems. For later-harvested fruit, mating disruption has not been reliable against peach twig borer when used alone and should be supplemented with a bloom treatment of Bacillus thuringiensis or spinosad.

Mating disruption is most effective in orchards with low moth numbers that are not close (a mile) to other untreated peach twig borer hosts or almond orchards. Efficacy is reduced by small orchard size (especially if located near outside sources of moths; if a small orchard is isolated, then size is not a major factor), 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 numbers below economically damaging levels. Other commonly found natural enemies in California are the chalcid wasps, Paralitomastix varicornis and Hyperteles lividus, the braconid wasp Macrocentrus ancylivorus, and the grain or itch mite, Pyemotes ventricosus.

Use sprays of Bacillus thuringiensis, sprays of the Entrust formulation of spinosad, and hand-applied mating disruption during bloom for peach twig borer management on organically grown apricots.

Before bloom

Delayed dormant applications target the overwintering larvae in hibernacula and are best applied immediately before bloom. Spinosad (Entrust, Success) or diflubenzuron (Dimilin) pose less risk to water quality and provide the same level of control as organophosphate or pyrethroid insecticides. These can be combined with red bud fungicide sprays, but the preferred management strategy is well-timed applications of environmentally sound insecticides at bloom.

Bloom

Monitor peach twig larvae during bloom and when shoots are emerging. Look for feeding at the base of flowers. Damaged shoots do not wilt; therefore damage may not be obvious.

If larvae or their damage are observed at this time, two sprays of Bacillus thuringiensis (Bt) or a single treatment of spinosad (Entrust, Success) 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.

Sprays for peach twig borer are often combined with sprays for powdery mildew and brown rot.

In-season

If delayed dormant or bloom sprays were not applied or if numbers are high, an in-season spray may also be needed. Install pheromone traps in orchards by March 15 in the San Joaquin Valley and Central Coast and April 1 in the Sacramento Valley. Results from trap catches and degree-day accumulations are used to determine the timing. 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.

Calculate degree-days for peach twig borer in apricots 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.

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, treat at 300 DD. If Bacillus thuringiensis is used, however, two sprays should be applied: one at 300 to 350 DD and the other at 450 to 500 DD.

Take weekly fruit samples after color break to detect any developing problems in the orchard and 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 SAMPLING AT HARVEST). Record results (example form—PDF).

• Degree-day calculator
• Degree-day table
• Harvest fruit damage evaluation form (PDF)
• Using degree-days to time insecticide applications