| Locust handbook |
|3. Other African locusts|
The Brown Locust is illustrated in Plate 4. Figure 112 shows the outbreak area and the invasion area for this locust. Between plagues solitary Brown Locusts can be found throughout the Karoo area of South Africa, some 250,000 km². This is a dry region where the rainfall is erratic and generally less than 300 mm annually. The vegetation is made up of dwarf grasses which are the main food of the locust, and many bare patches which become suitable basking and egg-laying sites.
As its name suggests, the immature adult is brown, but there are occasionally green forms. In seasons with good rainfall there can be three generations but in droughts the eggs can remain dormant for up to 15 months. Laying takes place in well-drained, loose soil and the eggs hatch after 10-20 days; sometimes there is a 1-3 month diapause. Both diapause and non-diapause eggs can be laid in the same pod but generally solitarious locusts are more likely to lay eggs which diapause than gregarious locusts. Females can lay up to five pods but the average is nearer two; each pod contains 10-82 eggs.
Solitary hoppers can be brown, green or grey; gregarious hoppers are black with orange markings in the later instars. There are 4-5 hopper instars for males and five for females. Development periods vary from 21-38 days for solitarious hoppers. Gregarious hoppers take longer, at least 42 days, because they are significantly larger than the other phase. The size difference between the two phases is greater than in any other species.
In the Karoo Brown Locusts overwinter in the egg stage (1-3 months). Hatching begins in September after the first rains and continues in October with fledging in December. In years of good rainfall further laying can occur in January to produce a second generation in March; this generation in turn can lay again, probably in April.
Solitary hoppers move away from other hoppers while gregarious hoppers group as with other locust species. Research has suggested, however, that within a single egg pod hoppers hatch with different behavioural characteristics. For example, reared under similar conditions, hoppers from the top of the egg pod are more likely to have gregarious characteristics than those at the bottom of the egg pod.
Bands of Brown Locust hoppers usually march in very long narrow columns and can cover up to 2 km during their life time, although up to 40 km has been recorded. Densities of hoppers in bands sometimes reach 3000-4000 hoppers/km².
Gregarious behaviour takes at least two generations to develop. Adults fledging from grouping hoppers fly in loose swarm formation and lay together some 15-50 km from the hatching site. The following generation produces more strongly gregarious characteristics and swarms (up to 43 km²) can fly hundreds of kilometres in a generally downwind direction.
Gregarisation occurs when the numbers of solitary locusts begin to rise over a wide area. As more contacts are made between insects phase transformation begins. This is further encouraged by good rainfall especially if it follows an early summer drought the previous season.
Plagues, upsurges and seasonal movements
Studies of the Brown Locust this century have suggested that periods of 7-11 years of great swarming activity are separated by recessions of similar length.
Neither chemical control nor exceptional periods of rainfall have effectively disrupted this periodicity of swarming and recession. This has led to the suggestion that there are other factors influencing changes in numbers and behaviour, such as an individual locust's response to density. Locusts not sensitive to density will not be active movers and if numbers increase locally these locusts will not move away but become part of a transient population. Such a population will provide a target for predators and will be reduced. Locusts sensitive to density will initially move away from contact with other insects. They will thus be difficult to find and will not be a prey to natural predators. Eventually the proportion of these adults in the total population will predominate. When overall numbers reach a critical level phase transformation takes place quickly and highly mobile swarms are formed. At the end of an active swarming phase it is thought that the density-sensitive adults are killed or die out leaving the remaining solitary population with a majority of locusts not sensitive to density. There must then be a period of build-up of locusts which are density sensitive before there will be another swarming period.
When swarms do form they can be displaced by the winds northward, northwestward and northeastward away from the recession area in the Karoo. This movement takes place in February (early summer) but later in the summer movements have an easterly trend both north and south. Control has been relatively effective so there have been few invasions outside South Africa in the last 50 years. Botswana and Namibia are the countries most likely to be invaded. Invasions of Mozambique, Angola and Zambia are not well documented. In 1924 swarms from northern Botswana invaded Zimbabwe. Breeding occurred in the extreme west in January and the resulting swarms moved eastward throughout the country from late April. By June maturation had occurred and laying took place. The eggs hatched in September when the rains came and the ensuing adults flew westward into Botswana in December.