Oystershell scale - Life cycle

The oystershell scale has one generation per year. It occurs only in the southern regions of the country and its life cycle has been studied in both Canterbury and Otago. The timing of events in the life cycle are similar in these two regions but may be a little more advanced in Canterbury. Temperatures in spring and summer also influence the timing of the different stages which may be advanced or delayed by 1-2 weeks in some seasons.

Oystershell scale spends the winter primarily as second instar (=stage) scales, with a few settled first instars. Production of adults begins in late September and a large part of the population becomes adult through October and November. The sex ratio is about 1:1. The winged males live up to a few days and occur only during late September, October and early November, with a peak of well-synchronised activity in October. The long-lived immobile females persist right through the summer and gradually die off through February to May/June. The time of first egg production by the females is quite variable depending on location and host plant. It has been seen as early as late November on apple or as late as the first week of January on plum in Otago. Peak egg production occurs in late December or January and continues through to March. The egg stage is brief (15 min to 5 hours), so that the timing of crawler production follows closely behind egg laying, and the crawlers (first instars) settle from December or early January through to the start of winter. Almost all of the settled first instar scales have moulted to the second instar by that time.

The rate of development of oystershell scale depends on temperature, and is very slow in winter. Winter dormancy (diapause) is obligatory. In summer, the first instar is completed in 45-80 days. Females contain young eggs within 5 days of becoming adult but egg laying does not begin for 4-6 weeks. Adult females then produce eggs over several months until late autumn. Further information is available on the life history of oystershell scale.