During the winter months there is an accumulation of egg pods in the soil. As there is sufficient water, the embryos in the eggs laid by females at the beginning of winter have developed and are ready for hatching. At the onset of summer, when temperatures increase, there is synchronised hatching of eggs.
The emerging hoppers come into contact with each other in sufficient numbers to cause gregarisation. These hoppers then form bands which start marching in search of green food. The gregarious hoppers have a distinct brown and black colour pattern. They will develop into gregarious adults which are yellow-orange in colour with distinct black markings. These develop into flying swarms which are swept along by the prevailing winds into agricultural areas. These locusts have the potential to cause damage to cereal crops.
How can farmers protect their crops?
The best way to cope with locust swarms is to try to prevent them from increasing in size. Surveillance is important. This is where people are employed to monitor known outbreak areas so that early control measures such as targeted chemical control can be employed to keep the population below the threshold to trigger gregarisation. The threshold has previously been estimated to be about 2,000 adults per hectare. Unfortunately many of the areas where the locusts lay eggs are remote and inaccessible.
Though these locusts are consumed by many predators such as birds, reptiles and mammals, they don’t make an impact on the swarms.
The most effective means of controlling locusts is to prevent the swarms from forming. The only successful control measure has been chemical control, which is expensive and environmentally unfriendly. There’s research into using the hormones that cause the locusts to aggregate as a chemical attractant to get the locusts to congregate in a place where they can be killed. This research is in the early stages.
Frances Duncan is a professor from the school of animal, plant and environmental sciences at the University of the Witwatersrand.