The best known plague in human history is probably the Black Death which swept through Europe in the 14th century. This infectious disease was spread to humans by fleas carried on the black (or ship’s) rat.
Insects are not the only things that harm people or do great damage to the environment. Other pests include micro-organisms such as fungi, as well as worms, birds and mammals such as deer and rabbits.
PESTS AND DISEASE
Insects that spread disease include the female Anopheles mosquito, whose bite causes malaria, a disease that affects more than 260 million people worldwide.
Mosquito control was given a major priority in afflicted areas in the 1950s and 1960s. The main weapon was the insecticide DDT. At first, the number of cases was greatly reduced, but it rose again when the campaign to eliminate the mosquito slackened and when the Anopheles mosquito became resistant to DDT.
Other programmes have been undertaken to reduce sleeping sickness, which is spread by the tsetse fly in Africa. But there have been some undesirable consequences. Alter areas of land had been sprayed and the tsetse flies eliminated, farmers moved in. Their animals overgrazed the land, causing soil erosion. The wildlife, which is immune to the tsetse fly, has been forced into ever smaller areas.
Worms and fungi also cause human disease. For example, parasitic worms cause river blindness, which affects around 17 million people, mostly in Africa, while fungi cause ringworm and athlete’s foot.
COMPETING FOR RESOURCES
As the world’s population has grown, so the number of species classed as pests has increased. Many of these pests threaten human interests by competing for such resources as pasture or crops.
Some pests, such as locusts have been recognized throughout history. According to the Old Testament, Moses brought a plague of locusts to Egypt in an attempt to persuade the pharaoh to release the Israelites from slavery. Locusts are short horned grasshoppers which undergo periodic population explosions and migrations during which they devour every plant in sight. The conditions that cause these swarms in the African and Arabian deserts are not fully understood, but they can cause great devastation to crops.
Several methods are used to combat locusts. In the late 1980s, for example when locusts threatened the Sahel (the dry grasslands south of the Sahara which were already hit by drought), aircraft and helicopters sprayed large areas with insecticides. This successfully stemmed the advance of the locusts.
Another pest, sometimes called the locust bird, is the red-billed quelea, a small brownish bird which lives in thorn scrub country in Africa. The quelea feeds on grass seeds, but with the development of agriculture, it has, like various species of blackbird in other continents, found new food supplies, notably large fields of grain. It now occurs in millions which destroy grain crops. Its numbers are often so great that when the birds roost on trees, they can bring down branches. Many methods, including poisons and napalm, have been used to control them. Millions are killed every year, but crop destruction continues.
The human introduction of species into new environments can cause havoc. When goats were taken to St Helena after the discovery of this island in the 16th century, the animals stripped away the plant cover which protected the island’s soils. Within 200 years, most of the island’s fertile soil had been washed into the sea.
The rabbit became a pest on the main- land of Australia after 24 were released near Geelong, Victoria in 1859. They multiplied and spread rapidly, damaging crops and pasture and reaching southern Queensland by 1886 and Western Australia by 1895. Attempts to control them failed until the virus disease myxomatosis was introduced in the 1950s. This disease, which is fatal only to rabbits, wiped out huge numbers. But some rabbits developed resistance to the original strain of the virus and new, stronger viruses have since been developed.
Another Australian animal, the dingo or wild dog, was introduced about 7000 years ago by the Aboriginal people. Sheep farmers regard dingos as pests, because they kill lambs and sometimes attack sheep.
Pest control techniques fall into several groups. Some involve the burning of rubbish dumps or the destruction of hztbituts, such as swamps, where insects breed.
Many countries have quarantine systems to prevent the accidental import of pests from one area to another. In parts of Africa, for example, the insides of vehicles leaving a tsetse fly-infected area are sprayed at road- side control points. Other techniques involve improved farming methods, such as crop rotation, because different pests are associated with different crops. Another method involves the planting of crops which are not harvested at times when particular pests are at their most numerous.
The most familiar methods include the use of poisons, including insecticides. However, their use can have serious effects on the environment. In southeast Asia, for example, the heavy use of insecticides necessary to protect new and highly productive strains of rice has poisoned the ponds in the paddy fields where fish were reared. Pesticides also damage the soil by killing micro-organisms, and they harm humans when they get into the food chain.
Biological control involves the use of predators, parasites and diseases to control pests. Mosquito-eating fish, for example, are introduced into ponds where insects breed in parts of lndia, while ladybirds were imported into California to control the cot- tony cushion scale, a pest which damages lemon and orange groves.
But biological control is a hit-and-miss affair. Scientists considering the introduction of a new species must be sure that it is not likely to become a pest itself. The cane toad, for example, was taken to Australia in the 1950s to control beetles in the sugar-cane fields of Queensland. The toads have since become pests in their own right throughout much of Queensland and north- eastern New South Wales.