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The overall pattern of total cultivated area and annual frequency distribution of Desert Locust swarms is shown in Map 3, and the comparative areas infested by swarms and hopper bands over the 25-year period 1939-63 according to annual frequency groups are given in Table 1.

TABLE 1
Comparison of cultivation densities with annual frequencies of Desert Locust occurrence

Average frequency per degree square 5.9 years out of 25.
*Area of an average 'degree-square' in the Desert Locust invasion area was taken to be that occurring at lat. 23°N, i.e. 2.816,000 acres.

Nearly 400 million acres were cultivated annually during the period 1955-60 in areas where they could be expected to have been attacked by swarms and hopper bands on the evidence of the annual frequency maps. Over the 25-year period 1939-63 swarms are known to have infested 2,100 degree-squares of which 40% were devoid of cultivation.* The vulnerable acreage of cultivated land therefore represents 6.6 % of this area of 924 million sq. miles. This area is almost certainly an underestimate since no records of swarms have been received from 350 degree-squares in desert areas of the Sahara and Arabia which have almost certainly experienced migratory swarms at some time during the 25-year period; the probable gross area liable to infestation by the Desert Locust is therefore about 10.25 million sq. miles, of which 6.1 % is cultivated land and 53 % is uncultivated desert.
*Since the smallest practical plotting unit of cultivated area was 10,000 acres on the basic plotting maps and areas of 5,000-10,000 acres were plotted as one unit, the term "devoid of cultivation" implies less than 5,000 acres of cultivation in a degree-square.

The areas of high annual frequency of occurrence of swarms and hopper bands, are, in both cases, areas with a low density of cultivation; where the swarm frequency was greater than 15 years out of 25, the mean cultivation density was 2-4%. and where the hopper frequency was greater than 10 years out of 25, the mean cultivation density was 1.3 %. The proportion of the gross area infested that was devoid of cultivation was 52 % for hoppers and 40 % for swarms (or 53 % for swarms if the inferred gross area is accepted), while the proportion of these areas in which the cultivation density was greater than 20 % was 13 % for swarms and only 4 % for hoppers. Thus the area of cultivation liable to attack by hoppers is very much smaller than that liable to swarm attack: 159.3 million acres against 391.3 million acres, the former being only 41 % of the latter; and the hopper area was also less densely cultivated, 4.2 % compared with 6.5 %.

Even a cursory examination of frequency distribution maps (e.g.-Maps 1, 2 and 3) shows that the frequency values tend to be highest on desert margins and to diminish towards the wetter and therefore usually more densely cultivated areas. While the damage potential of the Desert Locust is large, as is indicated by the 400 million vulnerable cultivated acres representing about 11 % of the total world cultivated acreage, it is becoming even larger as cultivated areas increase as a result of increasing population pressures and new or expanded irrigation schemes. Cultivation on the desert margins is especially risky.

The data used in this section refer to the total acreage of crops grown in the course of one year and the annual frequency of Desert Locust occurrence. The acreage in any one constituent area represents the total acreage of a complex of different crops. There is no evidence that the Desert Locust is either attracted to or deterred from settling and feeding upon cultivated plants in general. It is a highly polyphagous species; nevertheless it does sometimes show preferences between plants at the specific and the varietal levels, so that, for example, some varieties of sorghum and sugar-cane are not seriously attacked and the leaves of tangerine and coffee are usually undamaged. Feeding preference is obviously dependent upon the availability of a number of different plant species and varieties within the immediate food-searching range of the locust, so that no rule concerning the invulnerability of plant species and varieties could be expected to be universally applicable. On average, plants cultivated by man probably constitute only about 6-7 % of the total food intake of swarming Desert Locust populations, but its proportion may rise as cultivated areas are increased.

These data could form the basis for the calculation of an index of vulnerability of crop areas to Desert Locust attack were there not a marked seasonal variation in the distribution of both crop areas and Desert Locust infestations (e.g. see Maps 1 and 2). As both are, to a large extent, governed by the seasonal availability of rainfall, their distributions show similar broad patterns in time and space. But as irrigation is carried out in many areas; making cultivation practical during the dry seasons, crops grown at such times are, in general, unlikely to experience a Desert Locust attack.

Therefore, in order to allow for the seasonal variation in the distributions of crop areas and Desert Locust infestations in the calculation of an index of crop vulnerability, it is necessary to use data referring to time units smaller than the year.

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