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Helping maintain nature’s balance

July 1, 1993

When man encourages one organism to the detriment of another, the practice is called biological control. This concept was first used successfully in this country in 1889 when the cottony cushion scale insect was devastating the citrus groves of southern California. A species of scale-eating ladybird beetle was imported from Australia, reared in large numbers, and then released among the orange, lime, and lemon trees. In a relatively short period of time, the destructive scales were all but exterminated by the ladybirds, whose appetite for them was insatiable.

Have you ever wondered what becomes of the myriads of plant lice that infest your rose bushes or the host of pudgy caterpillars that destroys your tomato plants in the garden? Well, parasitic wasps destroy a significant number of these economically important pests and many others. The tiny wasp that destroys the pestiferous plant louse is only about 1/12th of an inch in length. The female wasp selects a plant louse as her prey and then jabs her needle-like ovipositor into her victim and deposits an egg. The egg hatches into a larva that feeds on the body fluids of the louse. When it is ready to emerge as an adult, it cuts a circular hole in the louse’s body and escapes to the outside world. The actions of the parasitic wasp and its offspring fatally damage the plant louse.

The tomato hornworm is a robust caterpillar, with which all of us who raise tomatoes are familiar. A single hornworm can completely devour the stems and leaves of a tomato plant in 24 hours. Frequently, we are happy to see 50 to 100 whitish bodies on the surface of a hornworm and know another biological control agent is working for us. These whitish bodies are the cocoons of another parasitic wasp that has evolved an affinity for the hornworm. Eggs are injected into the body of the worm and hatch into larvae. The larvae feed on the internal tissues and then burrow to the outside to spin their silken cocoons. The hornworm is so injured by this activity that it eventually dies.

The science of biological control has made tremendous strides in the past 75 years, and especially since the danger of the widespread use of potent insecticides has been recognized. The use of microorganisms (bacteria, viruses, fungi) as biological control agents has developed tremendously in the last few years.

For a number of years a bacterium (Bacillus thuringiensis) has been commercially produced and used as a means of controlling a number of economically important species by disrupting their physiology. The gypsy moth and the cabbage and alfalfa caterpillars are especially susceptible to this microorganism.

Another related bacterium, discovered in New Jersey in 1933, attacks the grubs of Japanese beetles. Because the infected grubs turn white, the infection is called the milky disease. Between 1939 and 1955 the USDA, in cooperation with state agencies, distributed 230,000 pounds of dust containing spores of this organism in sites in 14 states east of the Mississippi. Much of the reduction in Japanese beetle populations can be attributed to the slow but effective spread of this control agent, but insecticides are still needed to keep this serious pest under control.

Myxomatosis is a viral disease that is spread by mosquitoes and fatal to rabbits. It is normally found only in Brazil where rabbits have developed a partial immunity to it. When rabbits were first introduced into Australia about 100 years ago, they found no natural enemies and multiplied like wildfire. Within a few years, the sheep industry was threatened as the rabbits were destroying the grazing grounds. After many frustrating attempts to control the runaway rabbit population, the Australian government decided to introduce the myxomatosis virus. The Australian rabbit problem was all but eliminated in a few years. However, the story does not end here: in the early 1950s a physician near the city of Chartres, France became very upset when rabbits repeatedly raised havoc with his garden. He somehow obtained a supply of myxomatosis virus and injected it into a few trapped rabbits and released them in the countryside. In a matter of five years the rabbit population in Western Europe was all but exterminated. The French love “lapin” for the dining table, and the doctor’s life was threatened on several occasions. When I lived in France during that period, I frequently hunted Hungarian partridge and pheasant with French friends who repeatedly apologized for the absence of rabbits because of that (expletive deleted) crazy doctor living 60 miles west of Paris.

Biological control is but one weapon in our arsenal for the control of economically and medically important organisms. But, before employing a biological agent to damage another organism, we must know all the possible ramifications of such an action. To do otherwise is to court disaster.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

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