The intensity of parasite attack in relation to the abundance of Cardiaspina albitextura (Psyllidae)

Abstract

In areas where outbreaks of the psyllid, Cardiaspina albitextura, were taking place the percentage destruction caused by the three primary parasites (Psyllaephagus gemitus, P. xenus, and P. discretus) was usually low, and varied inversely with psyllid abundance. The numbers of primary parasites were limited greatly by hyperparasitism and sometimes also by breakdowns in host-parasite synchronism. In areas in which outbreaks did not occur and in areas where psyllid numbers remained low after outbreaks, effective parasitism varied independently of psyllid abundance according to a uniform seasonal pattern. The mean mortality in nymphs that escaped destruction by other agencies was 30-40% in the spring and summer and about 75% in the autumn. It appears that there was a limit (75-85%) to the fraction of psyllid nymphs that could be parasitized and that this limit was approached frequently in the autumn. Although the parasites did not operate as "density-governing agents" it is probable that in some areas they contributed materially to the prevention of psyllid outbreaks.