By John Wood, Certis USA Regional Manager
The time for peach twig borer (PTB) is almost here and scouting is in order. PTB overwinter as first or second instar larvae in chambers beneath the bark, called hibernacula. You can locate them by looking for small chimneys of sawdust-like material commonly referred to as frass. They are most often found in the crotches of 1- to 4-year-old limbs. In older trees, look for hibernacula at the base of large suckers. They will be the most visible when the larvae start to feed rapidly.
To best determine if PTB have emerged, look for a hibernaculum that is formed with fresh frass. Take a sharp knife and remove a small v-shaped wedge of bark that holds the hibernaculum. Cut it open and look for a tunnel inside. If the larva is present, you’ll see it in the tunnel. Overwintering larvae have dark heads and chocolate brown body segments and are generally ¼ to ½ inch long. If you don’t find larvae in any tunnel, you can consider them to have already emerged.
When scouting indicates that 20 percent to 40 percent of the PTB have emerged, it is time to apply Bt sprays. These sprays should be repeated at 80 percent to 100 percent emergent.
Deliver® biological insecticide contains a strain of the Bt subspecies kurstaki as its active ingredient and is effective against newly hatched larvae. It produces a higher concentration of the highly PTB-active Cry1Ac proteins than other Bts.
Keep in mind that Bt products are only effective against the larval stages of PTB and must be ingested by the larvae to be effective. That means Bt spray deposits must be present on plant surfaces when the larvae are actively feeding. If you apply it too early – before the larvae emerge – the Bt deposits may have degraded before the larvae start to feed. If you wait too late, the sprays will be wasted as the pupal and adult stages of PTB are not susceptible to Bt.
Best wishes for a successful scouting program. Until next time.