Apples Insects Pest Management

Preparing for San Jose Scale Crawler Emergence

by Erica Pate

San Jose scale is becoming an ever- increasing pest in Ontario apple and pear orchards, resulting in damage to fruit at harvest. Scale can be found on both limbs or fruit. A heavy infestation of scale insects can seriously reduce tree vigour, growth, and productivity- severe infestations can even kill whole limbs. Feeding on fruit is most abundant around the blossom and stem ends of the fruit, and appears as small gray patches on apples surrounded by a small red inflamed area (Figure 1).

Scale feeding on fruit

Figure 1. Scale feeding on fruit

San Jose scale overwinter on limbs as immature scales, remaining dormant until the tree sap begins to move in the the spring. These scales will be full grown by late May, at which time the active males emerge from their scales and seek females for mating. Females are immobile and remain under scales throughout their lives. After mating females begin to produce live young (crawlers) for six weeks, bearing up to 500 offspring in their lifetime. Crawlers are very small, bright yellow and are active for only a few hours (Figure 2). Crawlers will move to find a suitable place to develop on bark or fruit and will insert their mouthparts into the tissue to begin to feed. After three weeks the crawlers will molt and shed their skin. Scales are attached to bark by their sucking mouthparts. As the insect grows they build a waxy shell around them that protects from desication, predators, and pesticides. This makes the timing of insecticides tricky.

Bright yellow crawlers

Figure 2. Bright yellow crawlers

There are two full generations of scales each year. We are expecting the first generation of crawlers to begin to emerge soon, with the second generation emerging in August. Control of scale begins with prevention; orchards with a history of scale problems should use Superior oil before the tree breaks dormancy, when the scales have only a thin wax covering. A summer spray alternative is sometimes preferred, in which case determining when crawlers are active is critical.

We are currently following two degree day models to estimate the best timing for managing crawlers. The Cornell (NY) model uses a March 1st biofix, and the Washington State (WA) model uses the 1st adult male catch as the biofix.

Base T
 1st generation crawler emergence
 Cornell (NY)
March 1st
278 DD, or 29+/-12.5 days McIntosh PF
Washington State


 1st adult catch
222-249 DD, or 1st generation CM biofix

We are currently monitoring orchards in Norfolk and Middlesex county, where the first adult male catches on pheromone traps occurred May 21st and May 28th respectively.

Using the above biofix dates here are the predictions of crawler emergence as of June 18th:


In 2014 the Cornell model was approximately a week ahead of crawler emergence- the pheromone trap biofix (WA model) was more accurate in predicting emergence. We have not currently caught any crawlers, but are expecting to see activity soon. Summer insecticides Movento, Cloer, and TwinGuard should target these crawlers. Movento is a slower acting insecticide and should be applied 2-3 weeks before crawlers are active, with a second application 14 days later. Closer and TwinGuard should be applied when the crawlers are active. More information on insecticides can be found in Chapter 3 of Pub 360.

If scale infested fruit are observed after the first generation of crawlers, action against the second generation is recommended.

0 comments on “Preparing for San Jose Scale Crawler Emergence

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: