The peach twig borer has three complete generations in Washington in most years. It overwinters as first or second instar larvae in cells, known as hibernacula, under the thin bark in limb crotches or in bark cracks. To detect the hibernacula, look for small chimneys of frass and wood chips built up by larvae feeding under the bark.
During bloom and petal fall, overwintered larvae emerge from their cells, migrate up the small limbs and twigs and begin to feed on buds and young leaves. As terminal growth develops, a larva will enter a single shoot, boring down the center, causing the terminal to wilt or flag. When mature, the larva leaves the mined shoot in search of a protected place to pupate. Adults from overwintering larvae usually begin to emerge in mid- to late May. Females each lay between 80 and 90 eggs on fruit, shoots or the undersides of leaves next to veins. The eggs, which are laid singly, hatch in 5 to 18 days, depending on temperature.
The larvae can develop equally well in shoots or immature fruit. The first summer generation larvae develop during late May and June. The next adult flight is in early July. During this flight and the following one in late August, moths prefer to lay eggs on maturing fruit. Some larvae that develop from the eggs laid in August go into cells to overwinter. Others continue to develop on fruit and shoots and produce a partial third summer flight of moths in October. These moths lay eggs that produce larvae that overwinter and emerge as moths the following spring.
The first sign that the peach twig borer is in the orchard may be wilting, or flagging, of new growth in the spring. As buds open and new leaves begin to grow, the overwintering larvae burrow down the tender shoots, which then wilt and die. Twig or shoot damage may be more severe on young trees. One overwintered larva may attack more than one shoot. In high numbers they can cause extensive damage to young trees or nursery stock.
Larvae of the succeeding generations feed on shoots or fruit. They attack fruit at the stem end, where two fruit touch or where leaves touch the fruit. They also may feed along the sides of the fruit, disfiguring it.
Shoots should be examined in late April or early May to determine if the pest is in the orchard. Wilted shoots are easy to spot, and several trees throughout the orchard should be examined. Damage to shoots by oriental fruit moth larvae looks the same as that caused by the peach twig borer. Wilted shoots should be opened to determine if peach twig borer larvae are inside. Examine shoots again in mid-June to look for larvae of the first summer generation.
Adult twig borers can be monitored with pheromone traps, which should be placed in orchards by early May to detect emerging moths. Moths in the traps should be counted and removed once a week. Trap bottoms should be replaced after 50 moths have been captured.
A tiny wasp, Pentalitomastix pyralis, is a parasitoid of the peach twig borer egg. After hatching, the twig borer larva develops normally until maturity. Then, the parasitoid egg in the body of the twig borer is activated and divides several times, producing 25 to 50 larvae. Adult wasps are active in the orchards at shuck fall.
Early sprays aimed at young larvae give the best control of twig borer. The best times to spray are the pre-bloom and petal fall stages. However, these sprays may disrupt natural enemies of the green peach aphid, which are active at this time.
Delaying sprays until first summer generation larvae are present can help conserve natural enemies of green peach aphid and improve control of that pest. Larvae of the overwintering generation of peach twig borer do not feed on fruit and, unless numbers are extremely high, cause little economic injury to trees.
A degree-day model can be helpful in timing insecticide treatments against the summer generation of peach twig borer. The developmental thresholds of the peach twig borer are 50ºF and 88ºF, the same as the codling moth. The same method of calculating degree-days is used so that the degree-day look-up table for codling moth can be used for both species.
Start accumulating degree-days at first moth capture in a pheromone trap. The first moth is captured at 400 to 410 degree-days after March 1. The best timing of insecticide treatments is between 400 and 500 degree-days after first moth capture. The duration of a generation is about 1060 degree-days. If additional sprays are required, they should be applied at 1400 to 1500 degree-days after capture of the first moth of the overwintering generation. A table is available showing the relationship between degree-days after first moth capture and moth flight and egg hatch.