Oystershell scale - Reproduction

Oystershell scale has both male and female adults and relies on sexual reproduction. The sex ratio is about 1:1, although a high ratio of females to males has been reported in spring in Europe. Despite this, several males have been observed in New Zealand attempting to mate with the same female. The female produces a sex pheromone which attracts the winged males for mating. Pheromone release occurs during the day when males are active. Males fly for up to a few days and may locate females after flight or by walking over the bark of the host tree. The chemical components of the pheromone of oystershell scale have not been identified.

Male (and female) oystershell scale insects are able to mate almost immediately after emergence and multiple mating may occur. In mating, the female scale insect lifts her scale covering slightly to exsert a structure called the pygidium which contains the pheromone glands. There have been attempts to improve control of San Jose scale by timing sprays for this event and the presence of the males, whose mobility increases the chances of contact with insecticides. However, the timing relies on the use of pheromone traps and/or day degree summations and this will not be possible for oystershell scale until the pheromone is identified and temperatures for development are known. Timing of sprays for oystershell scale must rely on other methods in the meantime.

Female oystershell scale insects produce eggs (oviparous), unlike San Jose scale which produces live crawlers. Females have a period of about 30-50 days after reaching adult before they begin to produce eggs, but this continues for 2-3 months and results in large numbers of offspring (see fecundity). Over this period, female scale insects may be found protecting from 1-15 eggs and/or from 1-19 crawlers under their scale covering. This high rate of reproduction, combined with limited parasitism in New Zealand, enables populations of oystershell scale to build rapidly and cause severe damage to apple trees.