History of biological control

Biological control is the use of another organism to reduce or rid an environment of a microbe, plant or animal that is a disease, pest or weed. It is the alternative to a chemical pesticide, weed killer or even a medicine. However it is worth bearing in mind that some of the ‘chemicals’ used are refined forms of those that occur in microbes, plants and even animals, there is a very fuzzy line between natural and artificial.

Most histories of biological control exclude the development of medicines but many early forms of medicine are a type of biological control as well. Scientists worked out by observations that a disease occurred in one person but that another was immune, and from that further deduced that some factor was causing the immunity.

For example Edward Jenner was interested in the old wives’ tale that milkmaids could not get smallpox. He believed there was a connection between the fact that if milkmaids got smallpox at all it was only a weak version from which they quickly recovered. He made the connection that it was the pus in the blisters that protected the milkmaids. He tested his theory by injecting a boy who hadn’t had cowpox with the pus from the blisters. Then some time later he deliberately injected the boy with smallpox, the boy became ill but with a mild form of the disease and made a full recovery. This led to the production of a vaccine for smallpox and its eradication (we hope).

Observation is very important in finding a biological control or chemical and the study of the disease, pest or weed in detail is the first step. It is also important to study the control agent in detail particularly if it is not indigenous to the environment where you plan to release it. Many mistakes have been made. The most famous one was probably the introduction of the cane toad to Australia to control cane beetle in sugar cane (also an introduced species). Unfortunately it also started eating indigenous non-pest species and its population increased rapidly until it is now a pest itself.  It has taken over the habitat of native amphibians reducing the populations of native frogs and toads. The cane toad produces a toxin which it squirts from a gland if threatened, anything that eats it is killed and because of its large numbers it has seriously affected the biodiversity of native species in the ecosystem.

There are however many successes and biological control is certainly a viable option when it works without affecting the environment it is released into as well as successfully reducing or eliminating (though this level of control is unlikely) the target species.

The cottony cushion scale (Icerya purchasi) was introduced to North America accidentally in a shipment of Acacia plants from Australia . It wasn’t a problem pest in Australia but when it discovered the citrus orchards in California the population increased rapidly causing a lot of damage. The local predators were not interested in eating the cottony cushion scale but observations by a scientist in Australia led to the discovery of a biological control agent another beetle, Rodolia cardinalis. Later another control agent, a parasitiod fly Cryptochetum iceryae was also found to be effective and both control agents have proven to be highly effective control agents for cottony cushion scale.

Jude Ongeri