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San Jose scale - Life history |
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San Jose scale populations live primarily on the bark of their host trees. The adult female scale insects produce live young called crawlers, which spend a short time under the scale of their mother, and then disperse and settle on the bark, leaves and fruits. Male crawlers settle in greater numbers on leaves than the female crawlers. Crawlers are also carried by the wind and settle on other host plants. After settling and inserting their long feeding stylets into the plant, they secrete a wax cap over their bodies which is initially white and then turns black. The cap hardens into a scale during this process and the scale insects become immobile. They then moult to the second instar (stage) with a grey scale and, while females undergo only one further moult before becoming adult, the males pass through prepupal and pupal stages before emerging as winged adults. The males back out of their scale cover during emergence and then live for 1-3 days. The longer-lived females continue to grow larger as adults. The life cycle in New Zealand varies with latitude but the winter is spent mainly as immature second instars. There are 2-3 generations in the south and 3-4 generations in central and northern New Zealand. Overseas research suggests that there is high mortality of overwintering scale insects in the south if they have developed beyond the first instar black cap stage at the onset of winter.
Research in the U.S.A. on stonefruits has shown that the threshold minimum and the maximum temperatures for San Jose scale development are 10.6oC and 32.2oC respectively; after the first spring male is caught in pheromone traps, about 225 day degrees (DD) are required for the start of crawler production and 301 DD for half the females to reach this stage, 128 DD to complete the first instar, and 122 DD for the second instar. A generation is completed in 583 DD. Studies in Australia on apple indicated that a generation is completed in 741 DD above a threshold of 10.5oC. The release of first generation crawlers on apples commenced 261 DD after capture of the first male in spring. However, a similar study on apple in Greece using the same threshold indicated only 200 DD were required for this. Such variations may indicate strain differences between scale insect populations or host plant and variety effects on their development.