Pesticides and Pollution. Insects as Disease Vectors

The enormous economic impact of insect pests has led to the creation of a massive pesticide industry. And this in turn has enabled the continued development of an increasingly intensive agriculture. During the twentieth century, in particular after World War II, the industrialization of farming had a major effect on what was previously very much the pastoral ecology of cultivated land, and it changed the visual appearance of the farmed landscape as arable land increased.

Modern farming is also driven by mechanization and the availability of chemical fertilizers, but the widespread use of chemical pesticides has played a major role in changing the very look of the land.

Although pesticides have increased yields and improved crop quality to feed a burgeoning population, they have given rise to their own problems—pollution of neighboring ecosystems and poisoning of nontarget organisms, including humans.

The best-documented case is the rise in the use of dichlorodiphenyltrichlorethane (DDT) and its subsequent withdrawal because of its toxicity to wild birds, other animals, and humans. New synthetic pesticides continue to be developed, but at the same time many that have been in long use are regularly withdrawn because of new or increasing toxicity fears.

Recent moves away from chemical crop protection by the genetic modification (GM) of plants to induce them to synthesize their own internal insecticides are surrounded by controversy and widespread public skepticism. Fears of the impact of insecticidal GM genes "escaping" into the wider environment are still great, primarily because the possible side-effects are presently unquantifiable.

A smaller number of insect pests attack stock animals, but they can nevertheless wreak major economic havoc. Blood-sucking lice, flies, and fleas spread disease and reduce stock viability. The veterinary pesticide industry is also a major market, and increasing use of chemicals on farm animals and household pets threatens a new ecological side-effect.

Systemic pesticides called "ivermectins," injected into the flesh of the animal or given orally against intestinal worms, create what is in effect insecticidal dung. The normal dungfeeding community of beetles and flies that harmlessly recycles this waste material cannot feed, and large quantities of animal excrement build up. Some experts worry that this situation echoes an ecological problem experienced in Australia in the middle of the twentieth century.

Native Australian dung beetles, accustomed to feeding in marsupial droppings, were not removing the cow and horse dung that threatened to smother the fertile meadowland. It was only the deliberate introduction of various African dung beetle species that reversed the build-up of ungulate (mammals with hooves) droppings.

Insects as Disease Vectors. Insects cause vast damage to property and plant and animal foods, but that damage pales beside the sickness and enormous loss of human life caused by insect vectors (transmitters) of disease. At the end of the twentieth century, the World Health Organization (WHO) estimated that malaria, transmitted by mosquito bites, affected 267 million people in 103 countries, killing 1-2 million each year.

WHO further estimated that the top five tropical diseases would kill 4 million people a year by 2010. Four of these five diseases— malaria, leishmaniasis, sleeping sickness, and lymphatic filariasis—are spread by insects. (The fifth, schistosomiasis, is carried by freshwater snails.)

Advances in medicine and hygiene have all but eradicated many insect-borne diseases, at least in the West. But historically, and today in some undeveloped countries, there are even bigger killers. Plague, also called the "black death," killed two-thirds of the population as it swept through Europe during the Middle Ages (eleventh to fourteenth centuries). It was transmitted by the tropical rat flea (Xenopsylla cheopsis) and remains a medical hazard in many tropical regions today.

The humble housefly (Musca domestica), breeding in filth and excrement and then visiting human food in dwelling places, is a prime candidate as the most dangerous animal on the planet. It has been recorded as carrying over one hundred pathogens and transmitting sixty-five of them, including bacteria, helminth worms, protozoa, and viruses. It is mainly implicated in the spread of diseases such as dysentery, polio, typhoid, cholera, diphtheria, and tuberculosis.

Widespread sanitary reform during the nineteenth century and the arrival of the refrigerator in the twentieth revolutionized domestic life in the developed world. They created a modern environment where the housefly is surprisingly scarce in the urban homes where it was once a scourge. However, the housefly and close relatives like the bush fly (Musca sorbens) remain major nuisances in Africa, Asia, and Australia.