MOVEMENTS OF SWARMS OF THE DESERT LOCUST, SCHISTOCERCA GREGARIA (FORSK.), ACROSS THE NORTHERN RED SEA
ALISON STEEDMAN
Centre for Overseas Pest Research, College House, Wrights Lane, London W8 5SJ
(Received 25 August 1976)
Abstract. An account is given of two westward swarm crossings of the northern Red Sea. North-westerly surface winds over the sea suggest that this displacement is crosswind, contrasting with downwind movement reported or inferred widely elsewhere. A mechanism is described linking movement with easterly winds 1-2 km above sea level, where the channelling effect on wind by the Red Sea rift is absent.
INTRODUCTION
In the spring and early summer, swarms of the Desert Locust, Schistocerca gregaria (Forsk.), can invade Egypt from three general directions: from Libya, in the west from the Middle East and Arabia in the north and east and from Sudan and Ethiopia in the south (Fig. 1). This paper is concerned with westward invasions from Arabia, invasions which have been described on several occasions (e.g. McKillop & Gough 1916; Ballard, Mistikawi & El Zoheiry 1932; Hussein 1941; Rainey 1963; Karrar 1972), and which have even been mentioned in the Bible: 'The Lord brought an east wind upon the land all that day and all the night, and when it was morning the east wind brought the locusts and the locusts went up over the land of Egypt...' (Exodus 10 v. 13-14). There is evidence that some invasions cross Sinai (Omar 1965); but others are mare directly across the Red Sea judging by frequent sightings from ships of locusts over and in the water (e.g. Waloff 1960).
FIG. 1. - Study area and places mentioned in the text.
It has been well established that swarm movements in general flake place downwind (Rainey 1963). However, winds reported by ships on the Red Sea almost always blow from either north-west or south-east - a channelling effect of the Red Sea rift - so that winds are parallel to the shores and in directions that do not allow downwind movement across the Sea (see Royal Netherlands Meteorological Institute 1949). In addition, ;winds over the coastal plains are affected by onshore sea breezes, which develop daily a. few hours after dawn and persist into the evening. Thus, it appears that if a swarm were to cross the Red Sea during the day it would first have to fly against the sea breeze and then across the wind over the open sea. Crossings therefore do not seem to be down the winds at sea level. However, winds above the rift, about 1 km over the northern Red Sea, can blow across the sea on days during the migration seasons. In this paper an attempt is, made to determine the possibility of downwind movement at these higher levels during two periods in 1955 and 1968, when there seems to be strong evidence from locust reports that swarms crossed the northern Red Sea.
Variability of wind direction with altitude is illustrated by Fig. 2, a vertical time section of winds measured daily, at 1200 GMT during. June 1968 by balloons released from Hurghada on the Red Sea coast of Egypt (Fig. 1). Below 1 km the winds were mostly north-westerly*, but the flow at higher levels was often weak or from a different direction. For example, on 17 June north-westerly winds were present up to at least 3 km, the flow below 1 km being stronger (7.5-15 m s-1) than above (2.5-5 m s-1), whereas on 1 June a weak surface north-westerly flow was surmounted by north-easterlies between 0.9 and 3 km.
* N.B. - The wind direction is that from which it was blowing.
FIG. 2. - Vertical time section of daily 1200 GMT winds observed at Hurghada during June 1968.
For crossings of the Red Sea by swarms, not only must the wind direction be favourable, but wind speed must be strong enough to allow a complete crossing before locusts fall into the sea. from fatigue. In addition, the air must be warm enough to allow flapping flight. Laboratory experiments by Weis-Fogh (1952) have shown that flapping flight can be maintained for 6-17 h (at a speed of 3 m s-1), and Waloff (1946) has deduced that, during an unusual invasion of Portugal in 1945, Desert Locusts from southern Morocco must have flown continuously for 24 h. These "flight durations are, comparable with the 15-25 h needed to cross the 160-270 km width of the Red Sea in still air by locusts orientated across the sea. Durations are reduced, and probably become realistic for a greater fraction of the population which sets out on a crossing, when there are also tail winds with a component blowing across the sea. Such winds are essential for crossings by swarms in which the orientation distributions of either individuals or groups of locusts are. effectively random (Rainey 1963) rather than in a preferred cross-sea direction, although Waloff (1972) has suggested that locusts in high-flying swarms. are more likely to be orientated downwind than those flying nearer the ground.
Weis-Fogh (1956) has also shown that normal flight takes place at air temperatures between 25 and 40 °C with the threshold value between 22 and 24 °C, but flight has been observed at considerably lower air temperatures in the field, e.g. 17 °C (Rainey & Waloff 1951) and 14 °C (Gunn, Perry et at. 1948) in East Africa, and 9 °C in Morocco (Rungs 1946). Flight can occur where locusts are warmer than the air because they are being heated by absorption of sunshine or by generation of metabolic heat (Weis-Fogh 1964). This wide range of air temperature does not give any well-defined lower threshold for flight, but 20 °C is often taken. Air temperatures over land on either side of the Red Sea usually decrease with height during the day up to at least 3 km in May and June, and in these months the 20 °C level, based on routine temperature soundings, on average slopes downwind to the north from about 2.0-2.5 km over Khartoum to 1.0-1.5 km at Cairo. Although there is some day-to-day variation, temperatures very rarely exceed 20°C at 3 km. Flight over land at altitudes much above 3 -km therefore seems unlikely.
Over the Red Sea the vertical distribution of temperature is likely to be more complex. Because the sea is much cooler than the land during the daytime; a layer of cool air lies in contact with the sea, and above it there is likely to be a temperature inversion (Fig. 3). This cool air is, effectively trapped laterally by the mountains bordering the Red Sea, and its maximum depth will be close to the height of these mountains. Fig. 3 shows temperatures greater than 20 °C below 1750 m in April and below 2250 m in July, with the exception of a shallow layer near 500 in April, but it should be realised that this vertical distribution will vary somewhat from day to day.
FIG. 3. - Schematic vertical profile of temperature over the Red Sea, based on the following:
From these considerations of flight duration and air temperature, it is clearly feasible for swarms to make downwind crossings of the Red Sea at altitudes up to perhaps 3 km above sea level. Two well-documented seasons (1968 and 1955) are discussed below and evidence is presented to support such crossings.