Following last year’s harvest, a number of California almond growers suffered high crop reject levels due to shallow channels and surface grooves in almond kernels caused by peach twig borers.
That’s not the only threat these worms pose. Their feeding also kills newly-emerging leaf tissue and shoot tips after bloom. This pest can be very damaging to developing scaffolds in the second growing season.
This is where an effective program of monitoring and, when needed, treating orchards with an insecticide to control this almond pest can pay off.
If past history and samples from the previous harvest indicate treatment is required, the preferred time to spray is during the dormant season, report IPM specialists with the University of California Cooperative Extension (UCCE).
To protect water quality from run off of chemical residues during winter rainfall, they recommend not using organophosphates for dormant treatments. The specialists caution against using insecticides to treat for peach twig borer during bloom, even if labeled for bloom application, to prevent possible harm to bees.
However, they note, this pest can be treated safely with Bacillus thuringiensis (Bt) during bloom when pollinators are present, says John Edstrom, UCCE farm advisor Emeritus for Colusa County.
He explains more about this use of Bt in an article in The Almond Doctor blog dated Jan. 18, 2016. The Almond Board of California also recommends that no pesticides, except Bt, be applied at bloom.
In orchards that did not receive a dormant peach twig borer treatment, UC IPM guidelines base timing of Bt bloom sprays on monitoring the larvae emerging from over-wintering hibernacula – sawdust- or frass-covered cells bored under the thin bark of limb crotches or on tree trunks.
Most hibernacula are commonly found in the crotch of 2- to 3-year-old wood.
Usually, spring sprays are not needed for peach twig borer if a dormant spray or the series of Bt bloom sprays has been successful, or there is no history of peach twig borer problems. Otherwise, it may be necessary to using pheromone trap counts, shoot strike numbers, and degrees days to determine the best timing of a May spray.
“It all starts with a good program of monitoring your orchards and keeping accurate records to keep an eye on population levels and the amount of damage the pest causes,” says Kris Tollerup, UCCE IPM advisor, based at the Kearney Agricultural Research & Extension Center in Parlier.
Tollerup recommends placing pheromone traps in orchards at the rate of one per every 20 acres or at least two traps in smaller orchards by March 20 in the southern Central Valley, and April 1 in northern areas, and monitoring the counts twice a week.
Also, monitor trees for shoot strikes in mid-April. These are a part of a twig that wilted after being mined by peach twig borer or oriental fruit moth larvae. This can cause the affected portion of the twig to wilt. Shoot strikes are easiest to see on young trees and on water sprouts.
Look for larvae to distinguish between damage caused by peach twig borer from that caused by oriental fruit moth. To find the larvae, cut the shoot lengthwise. Oriental fruit moth larvae are white or pink with a brown head, while 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 the number of shoot strikes indicate treatment is necessary, use trap catches and degree days to determine when to make a May spray to target first-generation larvae. Optimum time is between 400 to 500 degree days after the first male is trapped in April.
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Accumulate degree days for peach twig borer using a lower threshold of 50 degrees Fahrenheit (F) and an upper threshold of 88 degrees F.
Keep in mind that applying broad-spectrum insecticides in May can cause outbreaks of secondary pests. However, softer pesticides including spinosad (Entrust, Success), spinetoram (Delegate), chlorantraniliprole (Altacor), and flubendiamide (Belt) can have a lesser impact on natural enemies, Tollerup notes.
If both peach twig borer and Navel orangeworm are present, a well-timed May spray can help to manage both pests, he adds.
More information on managing peach twig borer, plus tips for monitoring hibernacula and sheet strikes and using pheromone traps, is available online at www.ipm.ucdavis.edu/PMG/selectnewpest.almonds.html.