Thorarinsson, K. 1988 . Interactions Between the Cottony-cushion Scale, Icerya purchasi Maskell (Homoptera: Margarodidae), and its Parasite ... University of California, Davis Davis, California 78 pp.
Notes: [Title continues: Cryptochaetum iceryae (Williston) (Diptera: Cryptochaetidae). Ph.D. Thesis; Lester E. Ehler, Advisor.] Field experiments were conducted to test the 'spatial-density-dependence hypothesis', according to which successful biological control is stabilized by parasitoid foraging behavior, such that increasing rates of parasitism accompany increasing local host (pest) density. Experimental colonies of varying densities of the cottony-cushion scale, Icerya purchasi Maskell, were established on the terminal shoots of mock orange, Pittosporum tobira, and then exposed to the natural population of parasitic flies, Cryptochaetum iceryae (Williston). After 11-15 days of exposure (depending on the experiment), the colonies were harvested and brought to the laboratory, where the scales were dissected to determine whether or not they were parasitized. In the first such experiment, the developmental stage of each scale was recorded; in the remaining two experiments, the length of each individual scale was also recorded. All data were recorded on a leaf-by-leaf basis. No relationship was detected between parasitism rate and scale population density on either spatial scale. This result supports the view that successful biological control is not necessarily characterized by spatial density dependence. Parasitism rate was found to be dependent on host size: Larger scales suffered higher rates of parasitism. Also, parasite loads, the number of parasite individuals per parasitized host, were found to increase with the size of the host. This result is perhaps not surprising, since gregarious development of C. iceryae can be successful in adult hosts and the larger nymphal hosts. It would, however, be premature to conclude from this result that parasites adjust the number of eggs to the size of the host. Instead, this result may simply reflect parasite preference leading to repeat parasite attacks on the larger hosts. The size attained by C. iceryae pupae was found to depend on the developmental stage of the host in which pupation took place: The further developed the host, the larger the contained pupa. This result suggests that the endoparasitic form of C. iceryae is food-limited in the smaller hosts, and that the apparent preference of C. iceryae for oviposition in larger hosts is to the parasite's advantage.