Q. It’s the time of year for peach twig borers. What do I look for?
A. Peach twig borers (PTB) overwinter as first or second instar larvae in chambers beneath the bark. The chambers are called hibernacula. To locate them, look for tiny “chimneys” of sawdust-like material (frass). Generally they are found in the crotches of 1- to 4-year old limbs. In older trees, it is easier to look at the base of large suckers. Hibernacula are most visible in the spring when the larvae are rapidly feeding.
Q. How do I determine when PTB have emerged?
A. Search for a hibernaculum formed with fresh frass. Use a sharp knife and remove a small v-shaped wedge of bark that contains the hibernaculum. Carefully cut it open and look for an internal tunnel that the larva has created. If the larva is present, it can easily be seen within the tunnel. Overwintering larvae have a dark head and chocolate brown body segments. They are about ¼ to ½ inch in length. If the larvae can’t be found, it is considered to have emerged.
Q. When do I start making my Bt sprays?
A. For best results, we recommend that Bts be applied when scouting indicates that 20% to 40% of the PTB larvae have emerged from their overwintering hibernacula. Bt sprays should be repeated at 80% to 100% emergence.
Bt products are larvicides. They are only effective against the larval stages of PTB. Bt spray deposits must be present on the plant surface when larvae are actively feeding and will ingest the Bt. If applied too early before larval emergence, the Bt deposits will degrade before feeding begins. Too late and the sprays will be wasted against non-susceptible pupal and adult stages of PTB.