Numerical changes in a population of the desert locust, with special reference to factors responsible for mortality.
Abstract
The following is virtually the authors' summary. A small static population of Schistocerca gregaria (Forsk.) confined to an area of Heliotropium aegyptiacum on the Red Sea coast of Ethiopia was subjected to a detailed study of population changes and their relation to rates of predation and parasitism. Examination of the colour patterns of the nymphs and the morphometrics of the adults established that the population was the result of breeding by a small swarm and that during the period of study it was tending towards phase solitaria. Samples were taken every two days from a 1 m2 quadrat in 12 of the 24 squares, 50 × 50 m in area, into which the bulk of the habitat was divided. From the locusts counted during sampling, 36 adults or nymphs in each instar were dissected for parasites. Bird predation was observed from masked platforms placed 6 ft above ground in three of the remaining 12 squares, and other predators were counted by examining two series of quadrats in these squares, one by day and one by night. As hatching and fledging occurred throughout the period, analysis of the population and estimates of mortality rates could not be made by an established method. Independent data on the duration of each instar were obtained from a caged population of the locust, from which the development of each of the sub-populations (those hatching between sampling days) was worked out, so as to predict the number of each instar expected in the absence of mortality. A comparison with the observed numbers in each instar in the samples gave a measure of mortality. The mortality for each instar was different and also the mortality for the whole population varied with time.
The parasite and predator studies showed that there was a single parasite, the Calliphorid Blaesoxipha filipjevi Rod., that affected all instars except the first. Kestrels (Fdco spp.) and the isabelline wheatear (Oenanthe isabellina isabellina) were the chief bird predators, and, except for spiders preying on the early instars, other predators were not sufficiently numerous to have much effect. From data on the last appearance of parasite larvae in cages containing parasitized locusts and the peaks of parasite instars in the dissection data, the larval stage of B. filipjevi was estimated to last seven days [cf. RAE A 55 323]. An estimate of the duration of the third instar required for estimating mortality was also obtained from the peaks in the dissection data. The pupal stage lasted 12-14 days. Although up to six larvae were found in an adult locust, the maximum in young nymphs was less. Most parasitized locusts contained only one larva. The random distribution of larvae of different ages in those hosts with larvae of more than one instar indicated that B. filipjevi oviposits at random. In attempting to account for the observed mortality, it was necessary to treat nymphs in the third and later instars and adults separately from nymphs in the first and second instars and also, owing to the small size of the population, to consider separately events towards the end of the study. Estimated deaths from B. filipjevi, predation and failure to moult accounted for 40, 8 and 4% of the mortality during the earlier period. Recoverable dead for which the cause of death was unknown made up a further 6%, leaving 42% of the mortality unaccounted for. It is suggested that much of this is due to death during the first 48 hr. after hatching. This mortality also occurred in caged samples and so was not due to parasitism or predation. Towards the end of the study, parasitism was declining, but the importance of predation increased. The role of the different mortality factors is discussed, and it is concluded that Blaesoxipha is able to contribute a high level of mortality only under the special circumstances of more or less prolonged breeding by locusts and other Acridids. Predators, although not of great significance as a mortality factor, affect the outcome of breeding. In the absence of parasites and predators, the multiplication rate would have been of the order of ten times rather than the 0.03 times observed.