While relatively easy to manage, San Jose scale (Figure 1) is potentially the most destructive pest of peaches in Kentucky, and it is also a serious pest of apples. It is difficult to control the minute scale underneath its protective cap during much of the year. The key has been to time control efforts at the early-stage nymphs during the pink stage or newly-hatched crawlers near the end of May. The crawler is the only mobile immature stage and has no protective cap. This cap can protect the scale from some insecticides.
While feeding, San Jose scale injects enzymes that are toxic to the tree (Figure 2) and can cause limb dieback or even death of some peach and apple trees. Eggs of the first generation hatch in late May or early June underneath the adult female’s cap; hatching is temperature-dependent. There is a model on the UK Ag Weather Center Web site for estimating hatch timing for specific counties (http://wwwagwx.ca.uky.edu/ky/agmodels.php). Upon hatching, the crawler stage moves about the tree to find a suitable location to settle and begin feeding. Growers can use black tape applied with the sticky surface outward around infested scaffold limbs as a means of monitoring for the emergence of crawlers. Using a hand lens, check the tape twice a week for scale crawlers, which will appear as fine tan specks on the tape (Figure 3). Presence of crawlers necessitates the need to treat for San Jose scale.
San Jose scale nymphs and adults are protected from most insecticides underneath a waxy cap during most of the year. But while in the motile crawler stage, they are more vulnerable to some sprays. In terms of insecticides for scale control, Midwest Tree Fruit Spray Guide (ID-92), lists those chemicals that can be used on apple and peaches based on tree development stage. Sprays for San Jose scale crawlers will be listed on the cover spray section for the respective crops.
While there is an emergence of crawlers later in the summer, that generation is not synchronized and emerges over several weeks, making control difficult. Adequate control of the first generation eliminates the need to control this later generation.
By Ric Bessin, Extension Entomologist