Numerical changes in a population of the desert locust, with special reference to factors responsible for mortality

A small static population of the desert locust 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 colour patterns of nymphs and morphometrics of adults established that the population was the result of breeding by a small swarm and that during the period of the study it was tending towards phase solitaria. Samples were taken every 2 days using a 1 m2quadrat in twelve of the twenty-four squares of 50 x 50 m into which the bulk of the habitat had been divided. Thirty-six individuals of each instar from those locusts counted during sampling were dissected for parasites. Bird predation was observed from hides placed in three of the remaining twelve squares, and other predators were counted by examining two series of quadrats in these squares, one taken by day and one by night. As hatching and fledging occurred throughout the study period the 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 was obtained from a caged population of the locust. From this the development of each of the sub-populations, i.e. those hatching between sampling days, was worked out, so as to predict the number of each stage expected in the absence of mortality. A comparison with the observed numbers of each stage 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 Blaesoxipha filipjevi (Rohd.) (Calliphoridae) affecting all instars except the first. Kestrels and the isabelline wheatear were the chief bird predators, and with the exception of spiders preying on the early instars, other predators were not sufficiently numerous to have much effect on the population. The feeding of kestrels could be observed directly and estimates of the numbers of locusts eaten by them were made from the number of successful stoops made by the birds in a given time. The rate of feeding of the wheatear had to be inferred from the amount of time spent feeding as its prey could often not be observed. Predation rates for spiders were estimated from the numbers of locusts caught by spiders observed in the predator sampling. From data on the last appearance of parasite larvae in cages