How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Peach twig borer is a major pest in several tree crops. Adult moths have steel gray mottled forewings. Females lay eggs on twigs, fruit, and leaves. Eggs hatch in 4 to 18 days. Larvae are small, brown caterpillars with white intersegmental bands and a black head capsule. They go through four to five instars. Overwintering larvae are sheltered in tiny cells (hibernacula) that they bore under the bark of limb crotches on 1- to 4-year-old wood or in bark cracks on larger limbs and the trunk. Pupae are dark brown, without a cocoon and are found in tree crevices, between hull and shell, curled leaves, or in debris on the ground. There are four generations per year.

DAMAGE

Larvae damage both growing shoots and nuts, causing shallow channels and surface grooves on the nutmeat. Peach twig borer damage can be masked by navel orangeworm feeding, which often occurs on nuts previously damaged by peach twig borer.

MANAGEMENT    Degree-day calculator    Degree-day table
Some orchards will require a treatment for peach twig borer. Use past history or harvest samples to determine if your orchard will require treatment. Preferred treatment timing is at full bloom and petal fall. Treatments during the dormant season with environmentally sound insecticides such as spinosad (Entrust, Success) and diflubenzuron (Dimilin) are also acceptable. Avoid applications of organophosphates during the dormant season as these applications threaten water quality when they run off during winter rainfall. At the beginning of bloom, monitor hibernacula to determine when larvae are emerging. Place pheromone traps out around April 1, and monitor for shoot strikes to catch any inseason problems.

Biological Control
Peach twig borer has about 30 species of natural enemies. Among those commonly found in California are the chalcid wasps, Paralitomastix varicornis and Hyperteles lividus. Another commonly found parasite is Macrocentrus ancylivorus, which attacks both peach twig borer and Oriental fruit moth. In some years and orchards, these natural enemies destroy a significant portion of larvae, but they may not reduce twig borer populations below economically damaging levels. Ants, Formica spp., also can be found preying on peach twig borer larvae.

Organically Acceptable Methods
The use of Bacillus thuringiensis or spinosad (Entrust) sprays at bloom, preceded by a delayed dormant oil treatment for the control of scale and mite eggs are organically acceptable methods of managing this pest.

Monitoring and Treatment Decisions
Peach twig borer can be successfully managed with environmentally sound insecticides such as spinosad (Entrust, Success), diflubenzuron (Dimilin), Bacillus thuringiensis, methoxyfenozide (Intrepid), and spinetoram (Delegate). Although dormant treatments with organophosphate insecticides are effective and easy to time, they should be avoided because of their potential negative impacts on water quality.

Bloom sprays. To effectively time bloom sprays, monitor to determine when larvae are emerging from overwintering hibernacula on limb crotches or on tree trunks. The first application must be made when 20 to 40% of larvae have emerged and the second application 7 to 10 days later or when 80 to 100% of larvae have emerged from overwintering hibernacula. If emergence is spread out, a third spray may be needed when emergence finally reaches 80 to 100%. A less precise method is to apply the first spray between popcorn and full bloom on Nonpareil and a second spray at Nonpareil petal fall, unless there is an extended bloom period in which case a third application may be needed.

Peach twig borer can be very damaging to developing scaffolds in the second growing season so a dormant spray is recommended in the first dormant season when monitoring indicates peach twig borer hibernacula.

Spring sprays. Spring sprays are usually not needed if the dormant spray or bloom spray has been successful or there is no history of peach twig borer problems. If broad-spectrum insecticides are applied in May, they can cause outbreaks of mites and other secondary pests. Softer pesticides (spinosad-Entrust, Success; methoxyfenozide-Intrepid; and diflubenzuron-Dimilin) have recently become available that have less impact on natural enemies.

Place peach twig borer pheromone traps in orchards, one per 20 acres (but never less than two traps in smaller orchards) by March 20 in the southern Central Valley and April 1 in northern areas. Hang traps 6 to 7 feet high in the northern quadrant of the tree, 1 to 3 feet from the outer canopy. Monitor twice a week; replace pheromone caps at the interval recommended by the manufacturer and trap bottoms after 100 moths have been counted and removed.

If shoot strike monitoring (below) indicates treatment is necessary, use trap catches and degree days to time a May spray. Optimum timing for first generation larvae (the May spray) is between 400 and 500 degree-days after the first male is trapped in April when using spinosad (Entrust, Success), spinetoram (Delegate), or the organophosphates; when using an IGR (Intrepid), make the application at 300-400 degree-days. Accumulate degree-days for peach twig borer using a lower threshold of 50°F and an upper threshold of 88°F. (For assistance in calculating degree-days, see "Degree-days".)

Monitor trees of any age for shoot strikes in mid-April. Shoot strikes are easiest to see on young trees and on water sprouts. If several strikes are seen in each tree by late April, a spring spray timed with pheromone traps and degree-days may be required. When examining shoot strikes, determine if they are caused by peach twig borer or Oriental fruit moth by looking for the larvae. Although Oriental fruit moth larvae mine deeper into a shoot than peach twig borer larvae, distinguishing the damage can be difficult. Cut the shoot lengthwise to find the larva and identify it: Oriental fruit moth larvae are white or pink with a brown head and peach twig borer larvae are dark brown with white portions between each body segment and a black head. Also, peach twig borer shoot strikes occur earlier in the season than do Oriental fruit moth strikes.

If navel orangeworm is also a problem, it may be possible to time the May spray to control both pests if navel orangeworm egg hatch occurs at about the same time as optimum time for the peach twig borer spray. If not, wait and spray the navel orangeworm at hull split and time the May spray for peach twig borer. (Control of peach twig borer during hull split is difficult.)

• Summary of Important Links

IMPORTANT LINKS

• Degree-day calculator
• Degree-day table
• Harvest sample
• Monitor hibernacula
• Monitor shoot strikes in spring
• Pheromone Traps

PUBLICATION

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431
Insects and Mites
F. G. Zalom, Entomology, UC Davis
C. Pickel, UC IPM Program, UC Cooperative Extension, Sutter/Yuba counties
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
D. R. Haviland, UC IPM Program, UC Cooperative Extension, Kern County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
R. E. Rice, Kearney Agricultural Center, Parlier
L. C. Hendricks, UC Cooperative Extension, Merced County
R. L. Coviello, UC Cooperative Extension, Fresno County
M. W. Freeman, UC Cooperative Extension, Fresno County

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