Locusts

Pests of Field Crops in Southern Africa

LOCUSTS

(Orthoptera: Acrididae)

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Text Box: Four locust species occur in Africa, these being the desert locust, Schistocerca gregaria, the brown locust, Locustana pardalina, the red locust, Nomadacris septemfasciata, and the migratory locust, Locusta migratoria migratorioides. Of these, only the latter two species really occur in Zimbabwe. The desert locust is mainly found in the northern half of Africa, and the brown locust tends to occur in the semi-desert, karoo-type areas of South Africa. Exact identification of the locust species is not always a simple matter, as they all exist in more than one colour form. In general, they have a solitary, "non-outbreak" form, in which they behave as ordinary grasshoppers, and a gregarious form in which they behave as locusts. Unfortunately, there are several transitional forms between these two phases. The adult locust, or gregarious, phase is described here. The African migratory locust is 40 - 50 mm long and tends to be pale yellow in colour with fine, dark, lateral markings on the abdomen. The forewings are almost translucent but have small brown spots. There is often a covering of fine hairs on the undersurface of the thorax, particularly in transitional phases. The red locust is a larger locust, measuring up to 70 mm in length, and when swarming, it has a distinct reddish tinge, particularly on the body and the base of the hindwings. The forewings are characterized by bearing seven distinct brown bars, hence the specific name, "septemfasciata". Again, even within the gregarious phase, there can be considerable colour differences. The brown locust is about the same size as the migratory locust, yellowish-grey in colour and has very long wings, that are covered in blackish spots from about halfway along their length, and pale blue hindwings. Hoppers, as the juvenile stages are known, are obvious if only because of their existence in extreme numbers, and tend to be quite darkly coloured, in shades of black with red and yellow markings. TOP

Migratory and red locusts in outbreak swarms, especially after long flights, can and will consume almost any green plant material, but they show some preference for grasses, and hence their pest status on maize, and most other cereals.

Text Box: In the solitary phase, the grasshoppers feed on grass leaves, but damage is negligible. When in outbreak numbers, however, feeding can result in complete local defoliation. Even the hoppers, which can only move by crawling and hopping, will climb all over and destroy young crops. Leaves tend to be eaten from the margins inwards. TOP

Text Box: Migratory locusts: Like most grasshoppers, the eggs of the migratory locust are laid in the soil in batches called "pods". The female prefers sandy soils for egg-laying but diverse soil types can be used. She forces the tip of her abdomen into the soil forming a curved hollow about 6 - 7.5 cm long (the abdomen extending way beyond its normal length). The eggs are laid and the hole is filled with a frothy substance that dries into a porous pod. Sand is then kicked over the indentation to hide it. Each female can lay between 50 and 200 eggs, in four to five batches. Under very dry conditions, the eggs can lie dormant for well over a year. If conditions are warm and moist, hatching takes place in 10 to 20 days. The migratory locust hoppers are active from 24 hours of age and go through five moults before attaining adulthood. At each stage the hoppers are larger and the wings develop gradually. Under warm conditions with plentiful food, the hopper stage may last little more than five weeks, and each hopper will eat its own weight in food every day. The adults are soon able to fly short distances, and if their numbers are large, they may swarm to new feeding and breeding grounds. Under favourable conditions there may be two or three generations per year. Red locusts: The red locust life cycle is similar to that of the migratory locust, but longer, and the eggs are laid in sandy soils in flood plains, with up to 100 per batch, and in three or four batches. The traditional breeding sites of this locust are in the Rukwa Valley of Tanzania, and various marshy plains of Zambia and Malawi. Egg-laying takes place in the wet season (November to April) and hatching takes place in about 30 days. The development period is two to three months, and while there are seven (sometimes more) instars in the solitary phase, this is reduced to six in the gregarious phase. Adults probably live for about nine months, and there is one generation per year. The life cycle of the brown locust is the same as described above, but the eggs rest in diapause through the cold and dry Karoo winter until summer rains arrive. There are 2-3 generations per year, and the gradual build-up of the population forces the insect into gregarious phase every 7 to 11 years. Swarming in the migratory locust, unlike that in the red locust, may arise from local individuals. Solitary phase migratory locusts are widespread, and certain conditions may trigger gregarization and swarming from these indigenous populations. Red locusts, on the other hand, swarm into Zimbabwe from their breeding grounds in countries to the north and east. Hence the formation of a regional locust control organisation whose purpose is to deal with red locusts before they reach the adult swarming stage, which is much harder to control than the hopper stage. TOP

Text Box: As mentioned above, locusts are much easier to control while still in the hopper stage, than if they have already acquired the ability to fly. Any unusually high density of hoppers should immediately be suspect (anything more than about five per square metre), and identification should be sought from experts. A certain amount of government assistance may be available for locust control in the form of chemicals and equipment, but hoppers can be sprayed by ULV, knapsack or tractor and boom, applying the chemical in bands ahead of the direction of their progress. Hoppers should be driven towards the treated areas. This is possible with outbreaks of migratory and brown locust, which breed within the country. It may be necessary to control even fairly small outbreaks of migratory locusts while they are in the hopper stage, since, if left uncontrolled, widespread small bands can swarm together when they reach adulthood. Red locusts, however, only appear in Zimbabwe in flying swarms arriving from neighbouring countries and control is extremely difficult, especially from a logistics point of view. Should large swarms enter the country, aerial spraying is the only viable, although expensive, option. The locusts do not fly at night, and may be sprayed where they "roost" in vegetation, but a large swarm may cover an enormous area. This obviously requires a great deal of organisation on the part of authorities and farming groups. The flying swarms need to be tracked by day, and their roosting place noted (it may be far from road access) so that control measures can be taken before they begin to swarm the next day. For red locusts, which may invade inland countries like Zimbabwe in the cool season (June or July), the temperature often only warms up sufficiently for swarming flights to begin at about 10 am. The warmer the temperature, the earlier they fly, so that the daylight hours available for control are limited. Fenitrothion has been the official insecticide for locust control in Zimbabwe for many years, and due to the sporadic nature of the pest, no research has been possible to replace this. It is concentrated and highly toxic. Less dangerous insecticides such as carbaryl and malathion will give reasonable control. In other African countries, notably South Africa, synthetic pyrethroids have given good control and showed the same efficacy in one test that was carried out in the 1996 outbreak of red locusts in Zimbabwe. In South Africa, fipronil is also registered for brown locust control as an aerial or ground spray. It should be remembered that a good deal of natural control of locusts occurs, since they form a food source for a great variety of creatures. Birds can exert a substantial controlling influence on hopper numbers, especially when they are not extreme. This should, of course, be encouraged, and with this in mind, chemicals should be chosen and used with care. Another problem with locust control in African countries is that the locusts may be prized as a food source. Some chemicals with a rapid knockdown ability may serve merely to make the locusts easier to collect, a situation that is extremely undesirable. Current research in locust control includes much more environment-friendly methods, such as the manipulation and use of fungal diseases to which they succumb, and in future these may offer good alternative control methods. Indeed, in South Africa, the fungus Metarrhizium anisopliae var. acrinum is already registered for control of brown locusts. Among the many interesting predators of locusts is the larval stage of blister beetles (Family Meloidae). In the first instar, these small creatures are predacious on subterranean locust egg pods, and the beetles may be found in considerable numbers in locust breeding grounds, hunting out the oviposition sites and laying their own eggs in their proximity. TOP

Red locusts in flight.

Red locusts after aerial spray.

Red Locust. Note the distinct barring on the wings.

Blister beetle at locust oviposition site.

Migratory locust (top)

Red locust (middle)

Brown locust (bottom)