How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

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.

DAMAGE

This pest damages in two ways. Larvae burrow down 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.

MANAGEMENT

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), 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 and do not leave residue on fruit.

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 effectively in early harvested orchards to supplement dormant sprays.

Mating disruption. Mating disruption with sex pheromones can be used to supplement dormant or bloom time 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 populations 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.

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 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, the braconid wasp Macrocentrus ancylivorus, and the grain or itch mite, Pyemotes ventricosus.

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

Monitoring and Treatment Decisions
Monitor peach twig larvae during bloom and when shoots are emerging to determine that it is active. When emerging shoots are about 1 inch long, look for feeding at the base of flowers. Because damaged shoots do not wilt as they do in other stone fruit crops, detection can require a bit more care.

If larvae or their damage are observed at this time, two sprays of 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.

Install pheromone traps in orchards by March 20 in the San Joaquin Valley and Central Coast and April 1 in the Sacramento Valley. If in-season sprays are necessary (as determined by damage 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. For assistance in calculating degree-days, see "Degree-days ".

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 300-350 DD and the other at 450-500 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 (sample form—115 KB, PDF).

• Summary of Important Links

IMPORTANT LINKS

• Degree-day calculator
• Degree-day table
• Harvest fruit damage evaluation form (115 KB, PDF)

Back to management

PUBLICATION

UC IPM Pest Management Guidelines: Apricot
UC ANR Publication 3433
Insects and Mites
W. W. Coates, UC Cooperative Extension, San Benito County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
K. R. Day, UC Cooperative Extension, Tulare County
K. A. Kelley, UC Cooperative Extension, Stanislaus County
J. L. Caprile, UC Cooperative Extension, Contra Costa County
Acknowledgment for contributions to Insects and Mites:
L. C. Hendricks, UC Cooperative Extension, Merced County

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