San Jose scale - Life cycle

The life cycle of San Jose scale in New Zealand varies with latitude. In the North Island and central New Zealand, there are three to three-and-a-half generations each year depending on temperatures. Further south in Central Otago, there are two generations, and sometimes a partial third generation, per year. In Central and northern New Zealand, overwintering is accomplished mainly by second instar (= stage) scales, accompanied by some settled first instars. A few adult females may also survive through the winter in the north, and occasionally release crawlers. Adult males are very rare in winter. In the south, stages older than first instar (with black scale caps) are unlikely to survive the winter.

In central and northern New Zealand, almost all scales are second instars by August, resulting in a well synchronised first generation of adults in September-November. Crawler release reaches a maximum in November. In February the crawlers of the second generation are released, and during late summer there is considerable overlap between stages during the third generation. The life cycle has been less studied in the south but information to date indicates that the overwintering scales produce a highly synchronised emergence of males in late October/November; the longer-lived females are also produced at this time and they survive, producing crawlers, through to the end of January. Second generation adults emerge from February, with male flight continuing into early April while the females survive and produce crawlers through to early winter. In warm summers, the second generation flight of males occurs in January/February and this is distinct from a partial third generation flight in March/April.

The rate of development of San Jose scale depends on temperature, and is very slow in winter. There is an interval of about 70 days between the peaks of the first two summer generations in central and northern regions. In summer, the first and second instars are completed in 40-45 days. Females begin producing crawlers about 30 days after becoming adult, and then live for a further 40-60 days. Further information is available on the life history.