By Mark M. Green
Robert L. Metcalf (1916-1998), one of the most influential entomologists of the 20th century, was one of the first to recognize the danger of synthetic pesticides and the growing potential for insect resistance against these unnatural methods of insect control. And this was at a time of enormous enthusiasm for the new chemical based methods of insect control.
In a tribute to Professor Metcalf by the University of Illinois, following his death (copied below) one finds an interesting insight into his prescience and courage to stand against widely held beliefs.
“Reader’s Digest stories trumpeted the ability of “entire towns” to “abolish flies;” Time Magazine proclaimed in 1947 that “the flies in Iowa can now be counted on the fingers of one hand” as a result of using the new pesticides. Robert Metcalf was among the first few entomologists prescient enough to recognize the dangers of excessive enthusiasm. Most importantly, he was among the first to document meticulously and incontrovertibly the phenomenon of insecticide resistance. In 1949, he documented DDT and lindane resistance in house flies in southern California.”
Rachel Carson’s bombshell, “Silent Spring,” which detailed how chemical pesticides damage the environment, was not published until 1962, long after Professor Metcalf’s warnings.
But Professor Metcalf was not one to criticize without offering new approaches as one can see in the title of his Founders Memorial Lecture presented at the 1978 Meeting of the Entomological Society in Houston, Texas: “Plants, Chemicals, and Insects: Some Aspects of Coevolution.” In this distinguished lecture he pointed out the many hundreds of millions of years in which plants developed what are called secondary plant chemicals whose purpose was, and is, to counter phytophagous arthropods, insects and other arthropods that fed on the plants. These insects and ticks among other small members of the animal kingdom also feed on us leading to the idea that study of the methods used by plants to protect themselves could be of value to us. Considering the arthropod vectors carrying the bacterium causing Lyme disease, which is a tick, and that carrying the Zika virus (so much in the news now) the Aedes aegypti mosquito, it seems that Professor Metcalf’s point-of-view should be listened to as we run out of pesticide solutions.
Wikipedia has the following definition: “A birdbrain refers to a stupid person. Often it refers to a person with a short attention span.” Perhaps we need a new definition. In 1984, a young biologist, Peter Wimberger, published a paper, which is widely cited by ornithologists (biologists interested in birds) with the title “The Use of Green Plant Material in Bird Nests to Avoid Ectoparasites.” An ectoparasite is a life form attacking the outer part of its host’s body. In Wimberger’s paper, which appears to have initiated a field under the heading “nest protection hypothesis,” he introduced the idea, supported by his investigations, and now the scientific work of numerous other biologists, that many bird species use odor rich green plants to repel arthropods, which feed on the birds. Peter Wimberger obtained a Ph.D. degree from Cornell University where the highly regarded entomologist, Thomas Eisner (1929-2011) was a professor who wrote an article “Catnip: its Raison d’Être” published in 1964 in “Science,” which nicely demonstrates the plant-insect connection. In the beginning of the article Professor Eisner wrote: “Surely a mint plant derives no benefit from an ability to stimulate cats!” The paper goes on to report testing the essential oil of the catnip plant (Nepeta cateria) against a wide range of insects therefore discovering its powerful repellent properties. Clearly a catnip plant is to cats as the Opium Poppy or the marijuana plant (cannabis) is to us: the effect on us is not the intention of the plant.
Much is known and made use of following on the insect plant connection. Consider one example, chrysanthemum which yields the pesticide pyrethrum.
Working together with Donald Sutherland (1930-2009), former president of the American Mosquito Control Association and Craig Hibben (1930-2014), chief botanist at the Brooklyn Botanic Garden we added to the plant-insect connection in discovering that tiny amounts of the essential oil of a species of marigold in water kills the larvae of Aedes aegypti, the mosquito transmitting Dengue, Yellow fever and the Zika virus (Science, 277, 623 (1997). This mosquito is a local vector, meaning it is born in a small accumulations of water (containers, leaf axils, gutters, creases in tarpaulins, treeholes and the like) near to where it bites you. The controlling plant, the Mexican Marigold (Tagetes minuta), is an easily and widely grown weed from which the essential oil can be obtained in one’s kitchen by simply boiling the cut up plant in water, as one would a vegetable. Pouring a small amount of that water in the water near where you live in which the larvae might be developing (as outlined above) will yield a method of blocking the Zika virus. Professor Metcalf would have likely looked on happily.