VARIATION IN THE NUMBER OF EGGS PER POD
There is considerable variation in the number of eggs laid per pod by the Desert Locust. Data from a number of egg fields examined during the past ten years in Eastern Africa and Arabia were considered in relation to the history of the population, environmental conditions, and the number of times the females had already laid.
Oviposition in low-density populations (solitaries) or by swarms that had a solitary ancestry produced the highest numbers of eggs per pod; averages were 95 to 128. Pods laid by females from continuous swarming populations had averages of less than 81 eggs per pod. Females from swarms laid significantly fewer eggs at the second oviposition than the first. If a third laying took place, then there was a further reduction in the number of eggs per pod. There was evidence that swarms originating from different areas laid significantly different numbers of eggs per pod. These differences may have been related to the times taken to reach maturity, rapidly maturing locusts laying more eggs per pod.
In a season of low rainfall and poor growth of vegetation the number of eggs per pod was significantly reduced.
THE NUMBERS OF HOPPERS IN BANDS
Many bands of Desert Locust hoppers were deliberately and completely killed, and all the hoppers were collected and counted, or the number of hoppers was estimated by volume. The area covered by the hoppers at different times of day was calculated from maps of the bands drawn to scale. From these data the average area densities of the hoppers in the bands were calculated.
Although hopper density within bands varied, it was found that the average area density tended to be similar for hoppers of the same instar, engaged in the same activity. This applied over a range of vegetation types.
Various methods of estimating the numbers of hoppers in bands without killing them were tried and the results were compared with those obtained by killing and counting. The best estimates of living hoppers obtained were calculated from the area covered by the band and the average area density appropriate to instar and activity. This method generally gave results with errors of less than 35%. Experienced observers were found to be able to make rapid visual estimates of the numbers of hoppers within bands, the errors being generally less than 40%.
The numbers of dead hoppers in bands completely killed were estimated by sampling and the estimates were compared with complete counts. The position of the first sample was chosen at random and the others taken at regular intervals from this. On the whole, sample areas of one square yard were found to be the most convenient. Using the arithmetic mean of the samples and the total area covered by the hoppers, estimates of the total number present were often very good, the errors being less than 5%.
Methods of making rapid estimations of the total area of a hopper band were investigated. The width of the band across the front and its length through the centre were the most useful measurements.
NATURAL MORTALITY IN GREGARIOUS POPULATIONS
Detailed studies were made on nine egg fields laid by swarms of the Desert Locust in various parts of eastern Africa.
Examination of the egg pods at various stages during incubation provided data on mortality factors. The percentage of eggs surviving to produce hatchlings varied from egg field to egg field; the extremes were 94.7% and 31.7% and the mean was 72.3%. The number of eggs per pod surviving incubation varied from 72.5 to 18.6, the mean being 46.5. The 5.3% to 68.3% loss of locust eggs during incubation could be attributed to several factors.
Just over 3% of apparently healthy eggs containing embryos failed to hatch successfully. This accounted for 1-3.2% of the eggs laid. The cause of this failure to hatch is unknown.
Nearly all pods contained a few shrivelled eggs. The number of these rose during incubation and there were variations between egg fields. Under conditions such that soil moisture was adequate, the percentage of shrivelled eggs per pod was generally below 10. The cause is unknown. Overall mortality due to it was estimated to vary between 0.9% and 18.7%.
The egg predator Stomorhina lunata (Fab.) was found in six of the nine egg fields. The percentage of pods infested by it varied from 5 to 46 in the different egg fields. Overall mortality due to Stomorhina varied from 7.5% to 46.2%. The number of eggs attacked per larva varied in an irregular way, but was reduced as the number of larvae per pod increased. Stomorhina larvae damage far more eggs than they eat; the data suggested that only 10-20 locust eggs were necessary for one larva to complete its development.
Beetle larvae, including Trox procerus (Har.), were found in seven of the egg fields, but the degree of infestation was low. The estimated loss of eggs due to beetle larvae varied from 0.4% to 3.3%.
The predator Systoechus somali (Oldroyd) was present in six of the nine egg fields. The proportion of pods infested by it varied from 5% to 62%, but the destruction of eggs by it was always small and varied from 0.6% to 10.3%.
There was some evidence that Stomorhina and Systoechus were relatively more abundant in egg fields laid by large swarms than in those laid by small and scattered swarms, and in the case of Stomorhina, it appeared that the first egg fields to be laid in any one season were particularly prone to heavy attack.
There were often heavy losses amongst hatchlings, due to cannibalism. Hatching hoppers were eaten by those that hatched on the previous day or earlier. Even when there was plenty of tender annual vegetation and humidities were high, some 30% of hatchlings were probably lost in this way. A shortage of annual vegetation, due to inadequate rain, led to a greater degree of cannibalism. Cannibalism occurred, but less frequently, amongst older hoppers.
Of many different predatory animals that devoured locust hoppers, birds were probably the most important. In one instance, birds were mainly responsible for the destruction of 8 million young hoppers in 14 days. Mammals, reptiles and a variety of arthropods took a steady toll.
When locust breeding had been heavy over a large area, the predators were not sufficiently numerous to control the infestation, even though they destroyed many millions of locusts. On the other hand, small hopper bands that resulted from scattered layings were completely eliminated by predators, as were fairly large bands when limitation and localisation of rainfall caused the predators and the locusts to be concentrated into the same restricted area.
go to previous section of this publication