Management of Pest Populations by Manipulating Densities of Both Hosts and Parasites through Periodic Releases

Abstract

Before a biological control program utilizing the concepts of population manipulation can be undertaken, a great deal of ecological information about the pest species must be known. Of primary concern are such basic, but often unknown, factors such as pest abundance and seasonal distribution and the role natural enemies play in the population regulation of the pest. In-depth studies were undertaken at the USDA Biological Control of Insects Research Laboratory at Columbia, Mo., on the population dynamics of the imported cabbageworm, Pieris rapae (L.) (Parker, 1970). Results of these studies indicated that the indigenous parasites were ineffective in reducing or holding the host population below the economic injury level. Only one parasite, Apanteles glomeratus (L.), was found numerous enough to contribute even partial control of larval stages of this pest, and this species was introduced into the United States from Europe around 1890. Parker (op. cit.) attributed the cause of parasite ineffectiveness to several facts: (1) Populations of A. glomeratus were not synchronized with early spring populations of P. rapae, as parasites emerged approximately two weeks before the host. Apparently, this braconid merely hibernates, with portions of the overwintering population emerging during warm periods in the winter; cocoon masses brought into the laboratory during the winter yielded adults within a few days. The host, on the other hand, enters diapause in late Sept.-Oct., and does not emerge until the following spring. (2) Host density in the first two generations was too low for parasites to increase soon enough and at a rate which could suppress subsequent host populations. In 1967 first generation egg densities of P. rapae were only 1 per 27 plants at its peak and only 1 per 4 plants during the peak of the 2nd egg generation. These peaks were separated by periods that extended to 25 days, with an even lower egg density. Host egg densities in the spring of 1969 were similar (Fig. 1).