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The factors affecting locust populations in Cyprus have been discussed by Dempster (1957), who has stressed the importance of the duration of adult life in determining the number of eggs laid and hence the potential population in the following year. He pointed out that early hatching in 1955 resulted in the early appearance of adults, which, although fewer than in either of the two previous years, survived as late. More egg-pods were laid and the average number of eggs per pod was higher; this he attributed not only to the longer length of life of the adults but also to the availability of some green food on the site at the beginning of the period. If we consider the whole period 1951 to 1955 (that is to say, including the two years before Dempster began his detailed investigations) the importance of the supply of green food is confirmed. The relevant data are summarised in Table 39.

Table 39 The date of the first hatching and the first appearance of adult locusts and the total period over which they were observed at Mia Milea, upper site. (The data for the years 1953 to 1955 are from Dempster, 1957)

It will be seen that in 1951, hatching was early but adult life was short. This was presumably a reflection of the very dry winter and the consequent poor development of the annual vegetation, which died early. In 1952, after a very wet winter, the vegetation remained green late, adult life was long and very many pods were laid (Dempster 1957). Thus, while it appears true that a long adult life results in a large number of egg pods, yet early hatching does not necessarily mean that adults live long. A late supply of green food is essential. The conditions that would appear to be optimal are good winter rains followed by warm but not very dry weather in February. Dry weather at this time may lead to a heavy mortality among the eggs. If early March is cold and wet, hatching may be spread over a long period and there may be heavy mortality among the hatchlings as in 1952 and 1953. Nevertheless the effect of this early mortality among eggs or hoppers may be quite outweighed by an increased number of egg-pods laid per female, as in 1952 and 1955. Heavy winter rainfall, particularly if it begins early in the autumn, has the ancillary effect of increasing the abundance of the annual food-plants of the egg-predators and thus the predation rate. When green food is abundant, the numbers of eggs per pod tend to be higher. This effect on the potential numbers is of less importance than that of variation in the number of ovipositions, since variation in the mean number of eggs per pod is not large in the field, but the effect from one cause reinforces the effect from the other, since both increase with an increase in the available green food.

The presence of locust populations capable of rapid increase in numbers at Sotira, where the population of the surrounding area is generally more sparse and scattered, and at Larnaca may be connected with the unusual character of both sites. At Sotira the abandoned fallow fields probably follow a flush line and certainly have a vegetation which remains green long after the Stipa grassland of the open kafkalla. Similarly, although Larnaca airfield gives the impression of being an unsuitable habitat for large populations of locusts because of its restricted area of mosaic vegetation and the comparative unsuitability of its soils for oviposition, the grass Cynodon dactylon, which is a summer-green perennial, is common because of the high ground water table. Even in 1951, the total period over which adults were seen was 70 days, and until the recent alteration of the site, locust control was required there rather frequently.

At the site at mile 5 on the Nicosia-Famagusta road, although the mosaic pattern of the vegetation is developed, the soil is very thin over the kafkalla and the sparse vegetation has shown little change after three years' protection from grazing. Annual climatic fluctuations have much less effect on the vegetation here than at Mia Milea. Even in 1952, the total period over which adults were observed was only 56 days. Nevertheless, high-density populations can arise here and marching hopper bands were seen in 1947 and 1948.

Variation in the overall climate, by affecting the development of the annual vegetation, affects the distribution of locusts within a site. The proportion of short grass is increased in a dry year and that of the tall, dense vegetation decreased. Moreover, green food becomes scarce earlier in the year, so that there tends to be concentration of locusts during later instars. After a winter of high rainfall, the proportion of short grass is decreased and a succession of wet winters leads to a decrease in the amount of bare ground suitable for oviposition. The early instars then tend to be concentrated on these limited areas.

Numbers of locusts may increase during a succession of years with a high, well distributed rainfall, although oviposition will certainly occur in increasingly limited areas. If a year of low winter rainfall follows, the hoppers will not only be grouped on hatching because of the limitations on oviposition during the previous year, but will also tend to be crowded as they develop because of the scarcity of the taller vegetation. It is difficult to draw conclusions from Cyprus, where populations of rather low density are persistently baited, but it is worth noting that the occurrence of gregarious behaviour was far more widespread in 1951 than in any subsequent year. In 1950, the rainfall was unusually high (20.7 in. at Nicosia) and lasted well into the spring, while in 1951 it was unusually low (7.9 in. at Nicosia).

It will be seen that both mortality and fertility are profoundly affected by variations in climate, in particular the amount and incidence of the winter rainfall. Yet increase in numbers as a result of favourable weather conditions is limited by other factors and is considerable only where the vegetation has the mosaic structure discussed earlier in this paper. Dempster compared the locust populations in two neighbouring sites: Mia Milea, with a well developed vegetational mosaic and a rather large locust population, and Kaloyeros, without a mosaic structure and with a very small population. Unfortunately, numbers were so low at Kaloyeros that the sampling data allow few conclusions to be drawn concerning differences in mortality or fertility which might be related to the differences in vegetation. Very few locusts hatched at Kaloyeros in 1955, considering the number of adults on the site in 1954. No estimate of the numbers of egg-pods on the site in 1954-1955 could be made because of the difficulty of sampling such low densities. Dempster suggests that the low numbers hatching may have been due to high mortality among the eggs, possibly as a result of the dry weather in February, which resulted in the desiccation of many eggs at Mia Milea, where losses from this cause amounted to 30%. The possibly greater effect at Kaloyeros may be related to the very restricted area of bare ground suitable for oviposition.

We know, however, that locust numbers drop where protection from grazing results in an increase in the height and percentage cover of the vegetation and a reduction in the area of bare ground, as at Akhna. This must be the result of an increase in mortality or a decrease in reproductive capacity. The exact mechanism is uncertain, though several possibilities can be suggested. It may be that fertility is affected, or that the important pre-copulation parade of stridulating locusts, which takes place on open ground where visual attraction undoubtedly occurs, is stimulatory. A high concentration of pods resulting from restriction of oviposition sites might be expected to lead to increased egg-predation; there is little evidence of this, however, although the percentage predation rate at Akhna did rise steadily as locust numbers fell. Egg-pods laid in areas of marginal suitability may be more liable to damage by unfavourable weather either through desiccation or through being uncovered or buried by erosion. On a priori grounds it might be expected that the greatest effect of limitation of areas of bare ground or short grass would be to increase the mortality of the first- and second-instar hoppers, but there is no definite proof of this.

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