PTB monitoring information

Monitoring Peach Twig Borer Development with Degree-Days
by Richard E. Rice, Frank G. Zalom, & Jay F. Brunner

The development of the peach twig borer Anarsia lineattela Zeller (PTB) can be monitored using pheromone traps and local temperature.  The relationship between climate and the insect's biology above a minimum developmental threshold and below a maximum threshold is measured in degree-days.  Degree-days (°D) are heat units that drive the insect from one point in its life cycle to another.

Biology

PTB is a major pest of almonds and stonefruits.  It also attacks other treefruits, but is has not been reported as a major pest on hosts other than Prunus species.

PTB overwinters as a first or second instar larva in cells (hibernaculae) under the thin bark in limb crotches or in bark cracks.  The presence of hibernaculae on trees can be determined by observing the small "chimneys" of frass and wood chips constructed on the back surfaces by feeding larvae.  During bloom and petal fall, overwintered twig borer larvae emerge from their hibernaculae and migrate up the small limgs and twigs to begin feeding on young leaves and buds.  A single larva may attack and damage several buds or small terminals.  As terminal growth develops, maturing larvae will establish themselves in a single shoot, causing a wilting or "Flagging" of several terminal leaves.  Upon maturity, these larvae will leave the mined shoot and seek a protected place, normally on the tree, for pupation.

Adults from the overwintered larval generation usually begin emerging in April.  Moths of this generation lay eggs primarily on terminals or young leaves, but in some instances they may also lay eggs on green fruit.  Twig borer larvae can develop equally well in green shoots or immature fruit.  The first generation larvae develop during May and June and give rise to the next moth flight in late June and early July.  During this flight and during the one following in August, many moths lay eggs on maturing fruit.

Some of the larvae that are produced from the moth flight in August an September go into hibernaculae for overwintering; others continue to develop on fruit or twigs and give rise to a generation of moths in October.  These moths lay eggs that produce larvae for the overwintering (fourth) generation that emerge as moths the following spring.

Monitoring

Adult twig borer populations can be monitored with pheromone traps.  The wing-style traps and pheromone dispensers are available commercially.  Instructions for their use are provided with the traps from the manufacturers.  Traps should be placed in orchards from about March 15 in the south to April 10 in the north to detect moth emergence, and should be positioned in the northern or eastern quadrant of the tree at a height of 6 to 7 feet.  Traps should be serviced and moths, counted at least once--or preferably twice--per week, particularly during peak flight periods.  Trap bottoms should be replaced wherever they become obviously dirty, or when about 250 moths have been collected and removed from a trap.  Pheromone dispensers should be replaced every 4 to 6 weeks.

Developmental thresholds are 50°F minimum and 88°F maximum.  Accurate use of degree-days to monitor PTB is initiated with the collection in a pheromone trap of the first male moths that emerge each year.  Approximately 220 °D accumulate from first moth catch to egg hatch in the next generation.  Larval development requires an additional 507 °D.  Approximately 1060 °D are required for completion of a generation.  Preliminary studies suggest that optimum timing for first generation larval treatments ("May" sprays) should be between 400 ° and 500 °D after first moth collections in spring.