Temperature-Dependent Size and Development of California Red Scale (Homoptera: Diaspididae) and Its Effect on Host Availability for the Ectoparasitoid, Aphytis melinus DeBach (Hymenoptera: Aphelinidae)

Abstract

Developmental rates of individual stages of California red scale, Aonidiella aurantii (Mask.) and Aphytis melinus DeBach were determined at 17, 20, 25, 26.7, 30, and 33°C. A linear degree-day model estimated the scale's developmental time in the field more closely (4–7% of observed) than did either the polynomial (16–19%) or thermodynamic models (17–21%). The model overestimated male duration by 20%. Although actual duration of California red scale stages that are susceptible to parasitism by A. melinus decreased with increasing temperature, their durations relative to other stages increased. The size of non-growing stages (second molt and prepupa) decreased with increasing temperature. High temperatures also reduced the size of growing stages (second and third instars) and therefore affected the period during which scales were vulnerable to A. melinus . High temperatures also reduced the proportion of the scale population available to A. melinus for female progeny production. When developing scales were exposed to variable temperatures, average temperature during the developmental period predicted the size of second molt and prepupal stages. Based on developmental time, A. melinus has several generations to everyone of its host, thus its effectiveness as a biocontrol agent depends on a continuous supply of suitable scale stages to serve as hosts, especially for daughters.