Mortality of San José scale (Diaspidiotus perniciosus Hemiptera: Diaspididae) on an apple tree

A population of San José scale (Diaspidiotus perniciosus) on an unsprayed ‘Red Delicious’ apple tree (Malus×domestica) was sampled fortnightly from September 1969 to September 1970 at Nelson, New Zealand. With the exception of crawlers and live scale insects containing young endoparasitoids, all live, dead and parasitised stages of San José scale were counted on samples of bark of known surface area from the trunk, leaders, main branches, twigs, fruit spurs and new shoots. Samples were randomly taken from strata below, within and above five identified scale insect infestations. The scale insect stages on the leaves and fruit of the sampled twigs, fruit spurs, and shoots were also counted. At harvest, the total number of scale insects on each fruit in each stratum was recorded. Annual and generational mortalities were calculated for each stage from the percentage of dead scale insects, and compared with the net losses between stages. Annual stage mortalities increased with each instar (first – 28%, second – 48%, adult female – 59%), except for males (34%), and were less than calculated net annual losses (first – 30%, second – 57%). Population density increased with each generation, resulting in a 3-fold and 8-fold annual growth of female and male population densities respectively. Parasitism primarily affected second instar and adult female stages and was mainly inverse density dependent. It varied greatly between samples (0–100%) and was only a small component of the overall mortality in each generation (mean parasitism range, 5% to 14%) despite the known presence of Encarsia perniciosi, E. citrina, Aphytis mytilaspidis and A. diaspidis. San José scale infested 76% of the harvested fruit, demonstrating the current need for chemical intervention to assist biological control and the need to find new ways to improve its role.