Competition for food and allied phenomena in sheep-blowfly populations

Abstract

Formerly in charge of Parasite Laboratory, Division of Entomology, Department of Agriculture, Pretoria, S. Africa, where this investigation was carried out. Competition for food among larval populations of sheep blowflies constitutes an important factor limiting the general fly population in nature. Both intra- and interspecific competition take place on carrion. In South Africa the main species of flies concerned are Lucilia sericata Mg., L. cuprina Wied., Chrysomyia chloropyga Wied., C. albiceps Wied., and C. marginalis Wied. The degree to which these species suffer loss of population from the effects of intra- and inter-specific competition on a carcass is determined mainly by their inherent growth characteristics and by the degree to which they are adapted to withstand the adverse conditions engendered by over-crowding of the larval populations on the available food supply. In this paper the species listed above have been examined on the basis of rigid experimental tests to ascertain their respective responses to competition of varying degrees of intensity. It has become evident that the species of Lucilia, which are the most important in relation to strike in sheep, are better adapted than are their competitors to withstand the effects of straightforward competition for food. Adaptations to meet adverse conditions consist of the rapidity of larval growth and the time at which the most rapid growth period occurs, the ability to form viable pupae at a comparatively low final growth weight and a general reduction in the size of the individuals in a population in favour of an increased total number of survivors. In these respects Lucilia spp. are able to achieve greater success than the Chrysomyia species. They are therefore always more successful under adverse conditions. An exception is found in the case of C. albiceps in which the larvae have predatory habits and are therefore able to destroy competitors on the carcass. C. albiceps is always characterized by a lower field population than occurs with the other species and is therefore limited in its usefulness as a controlling agent. It is more exposed to the attack of parasites and predators during the vulnerable pupal stage and the general fly population is thereby very considerably reduced during the summer months. In South Africa, blowflies are seasonal in their appearance on carrion and can be divided into two well-defined groups of summer feeders and winter feeders. These two groups correspond with the classification into secondary and primary blowflies respectively. Overlapping between the two groups occurs at the times when the populations of both are at their lowest ebb. Competition for food is therefore less intense between the primary and secondary groups than it is between species occurring within the same group. Secondary fly populations cannot therefore materially affect those of the primary group. For this reason and for others which concern its growth characteristics, Chrysomyia marginalis cannot appreciably influence populations of Lucilia and is therefore not a controlling factor for the latter as has been claimed. When Lucilia and C. chloropyga occur together on a carcass, the former is always able to survive and to reduce the population of the latter species. Competition in field populations of blowfly larvae is regulated by a number of factors, some of which have no direct connexion with the fly populations themselves. The quantity of carrion available, for example, is governed by the population dynamics of the animals from which it is derived and by those of the predatory and scavenging groups which prey upon them. During the 20 or 30 years over which the blowfly problem has become more acute in South Africa, it is noteworthy that a considerable change has occurred in the balance between predator and prey populations due to the activity of man. The latter have increased as the former have gradually been destroyed because of their depredations on domestic stock. The amount of carrion, derived from small animals, has therefore increased, providing additional food for blowfly larvae. From experiments and from field observations it is clear that competition for food can never serve as an efficient controlling factor for blowfly populations. This is especially true for populations of the important primary species (Lucilia spp.). Competition acts to produce fluctuations in the populations of flies subjected to it, so that certain years are marked by abnormally large populations of adult flies which produce an increased incidence of strike in sheep. Such years are well known to farmers as 'bad fly seasons'. Reduction of blowfly breeding grounds by means of the destruction of carcasses in the field is not likely to prove efficacious. It is clear that we require to know a great deal more about the relationships between the populations of flies, their environment, their natural controlling factors and their fluctuations as related to the factors which influence competition in the field before we can hope to evolve any satisfactory measures which are designed as a frontal attack on the blowflies themselves.