Intraspecific Competition in the Population Dynamics of Bracon hebetor Say (Hymenoptera: Braconidae)

Abstract

Studies of the dynamics of insect populations are often concerned with the interaction between a host and its parasite or predator, potential agents of biological control that may contribute significantly to host mortality in a natural population. Early attempts at modelling the host-parasite inteiaction were confined to simplified situations where other host mortality was neglected; Nicholson(1 933) and Nicholson & Bailey (1935) emphasized the importance of parasite searching efficiency, and Thompson (1922, 1939) considered the fecundity of the parasite to be most important; models have recently been reviewed by Royama (1971). Life tables are often prepared for the host population (Harcourt 1969) but only rarely for the parasite; Varley (1970) stressed the need for these. While the ability of the parasite to find the host may be important for the host population, so is the effect of the host on the dynamics of the parasite population. Three aspects of these relationships have been extensively studied: (1) the parasite's ability to detect and accept a suitable host; (2) the degree of synchronization between host and parasite life cycles (Fisher 1971); and (3) the ability of the host to resist attack by the parasite and the ability of the parasite to escape the defence reactions of the host (Salt 1970). The majority of studies, however, have been confined to endoparasites where often only one parasite can develop to maturity in a single host. In gregarious parasites, varying numbers of progeny are able to survive in a single host and parasitism may be internal (e.g. Apanteles glomeratus (L.)) or external (e.g. Bracon hebetor Say). More parasite progeny per host can lead to their increased mortality, reduced size, and changes in their sex ratio. The effects of superparasitism on mortality, size and sex ratio (Narayanan & Subba Rao 1955; Chacko 1964; Wylie 1966a, b; Shiga & Nakanishi 1968a, b) have been studied but not from the point of view of population dynamics. This paper quantifies certain aspects of the interaction between B. hebetor and Cadra cautella (Walker) and examines the importance of these factors in the population dynamics of the parasite.