When is Peach Twig Borer a Concern?

Peach twig borer (PTB) larvae damage new shoots and, if the timing of their generations is just right, can feed on almond kernels at hullsplit causing shallow surface groves on the kernel. Oriental fruit moth (OFM) damage looks similar and its best to monitor both pests with pheromone traps to determine if either of them have the potential for nut damage. Peach twig borer adult moths have gray mottled forewings. Females lay eggs on shoots, fruit, and leaves. Eggs hatch in 4 to 18 days. Larvae are small, brown caterpillars with white bands and a black head capsule. They go through four to five growth stages. Pupae are dark brown, without a cocoon and are found in tree bark crevices, between hull and shell, or in debris on the ground. They have four generations per year.

Non-bearing trees. Both PTB and OFM larvae prefer to feed in new shoots; young trees are particularly attractive to these pests. Damage to new shoots in early spring in first leaf orchards disrupts primary scaffold development, distorts growth, and makes scaffold selection in the first dormant season more difficult. The May spray timing for peach twig borer often adequately protects developing primary scaffolds on new trees from shoot strikes. Protecting new shoot growth on new trees in early spring is money well spent. Shoot strikes occurring at other times on vigorous non-bearing trees stimulates branching and doesn’t set trees back or warrant control.

May sprays are timed using pheromone traps and degree-days. Peach twig borer pheromone traps should be placed in orchards by March 15. Depending on the insecticide used, optimum timing for first generation larvae (the May spray) is between 300 and 400 degree-days after the first male is trapped in April. To calculate degree-days from your biofix, use the degree-day calculator on the UC IPM website (select PTB under Pest and plant models).

The second time PTB control is important is in the first dormant season. Primary scaffold branches are selected at the first dormant pruning. PTB can overwinter as larvae in the crotches of these selected primary scaffolds; overwintering larvae emerge during bloom and can graze on buds up and down along the primary scaffolds. When extensive, this feeding can disrupt the development of secondary branching along the primaries. If severe, the twig borer feeding can mimic the appearance of non-infectious bud failure because so many bud positions are eaten, thus failing to produce new shoots. A dormant insecticide spray following the first dormant pruning prevents this potential problem.

Bearing trees. In bearing orchards, PTB can be controlled with two well-timed treatments of Bacillus thuringiensis (Bt). For Nonpareil, we recommend the first Bt spray be applied between popcorn and full bloom, with the second spray applied at petal fall. If the bloom spray can’t be applied due to rain, a petal fall spray followed by a second application 7 to 10 days after petal fall is recommended. In cool years with an extended emergence period, or if there is a high population, the post-petal fall spray might also be considered. This timing has the least negative impact on the environment and natural enemies. Insecticides other than Bt should never be applied to almonds when bees are present in the orchard.

Understanding PTB behavior makes a difference in determining potential risk to the crop and the need for treatment later in the season. When pheromone trap monitoring indicates the bulk of the second generation will occur before hullsplit, hatching larvae will feed primarily on shoots rather than causing nut damage. If larvae hatch well after hullsplit and kernels have begun to dry, they are less attractive to PTB and damage may be avoided. Once nuts are drying, this pest is more likely to return to feeding on tender shoots.

If monitoring indicates hatching larvae of the second generation will coincide with early hullsplit, PTB may feed in the hull and move into the nut. When newly split hulls, shells, and kernels are moist, larvae are more likely to cause feeding damage on kernels. Timing is critical for good control and sprays should be applied at 1% hullsplit on sound nuts. This timing will also help reduce navel orangeworm damage and will pick up OFM if they happen to be present. PTB resistance to organophosphates and pyrethroids has been identified in the Sacramento Valley.

Nut damage from OFM is rare since they prefer moist tissue and the timing of a generation’s hatch must coincide precisely with early hullsplit. OFM larvae can often be found feeding in moist hulls after hullsplit, but rarely is the timing just right for kernel feeding to occur.

 

 

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