The science of chemistry has developed to the point where very small quantities of material -- even the few micrograms that might be isolated from insects -- can be isolated and identified. Many scientist now are using this capability to investigate the chemical basis of biological phenomena.
An intriguing problem being studied at the University of Iowa (by Dave Wiemer and Steven Hubbell), is the question of food selection by a group of herbivorous insects, the leafcutter ants. This quest has uncovered natural chemical defenses by which some plants repel the ants. And this, in turn, suggests a possible way of controlling these agricultural pests without using dangerous pesticides.
Leafcutter ants, knows scientifically as attine ants, may be the most important plant-eating insects in the New World. They are very common throughout Central America and northern South America and are found from Texas and Louisiana to northern Argentina. Leafcutters harvest leaves from many different types of plants but show a special preference for many agriculturally important trees.
The ants appear to eat very little of the leaves directly. Instead, they carry collected leaf fragments back to a central colony where the leaves are pulped and used as a culture medium for a specific species of fungus.
This fungus appears to be the major food of the ant larvae and an important food for the adult ants. The symbolic relationship of the ants and their fungus makes a variety of plants which might otherwise be indigestible useful to the ants.
Individual ant colonies can grow in a few years to immense size, numbering 3 million to 5 million ants and measuring 10 feet across. As many as 20 such colonies can exist in a single acre, each one harvesting an enourmous amount of plant material -- 20 to 50 kilograms of fresh leaves per day.
Plants found in the natural tropical forest seem to have developed ways of protecting themselves from these ants. Nearly two-thirds of the native plants are not attacked by them, presumably because of some defense mechanism evolved over long exposure to the ants' predation.
Commercial plantations are another story. Among the commercial plants of the tropics -- citrus fruits, banana, papaya, oil palm, mango, manihot, cacao, coffee, etc. -- nearly all are heavily attached by the ants, again presumably because they have had no opportunity to develop natural defenses to leafcutter ant attack.
When we first began to look at the native plants to see how they managed to fend away the ants, we considered mechanical, physical, biological, and chemical defenses. Probably the easiest defense to imagine and the easiest for plants to develop would be a chemical defense -- a substance produced by the plants which would make their leaf material indigestible, unpalatable, noxious, or toxic to the ants or, possibly, toxic to the fungus grown in the ants' gardens.
In theory, if such defensive chemicals do exist within a plant, it should be possible to isolate and identify them. These chemicals might then be synthesized and applied in some way to commercial crops to protect them. Their use might save millions of tons of food for human use and save millions of dollars in loss to farmers. The use of more dangerous chemicals in tropical lands might also be curtailed or slowed down. Perhaps most important, the development of such chemicals might serve as a model for the discovery and synthesis of other agents for safely controlling other pests.
For the past several years, supported primarily by the US Department of Agriculture, we have been searching for defensive chemicals in some of the plants of Central America. We have found, through elaborate procedures, at least half a dozen ant-repellent compounds.
Finding one specific unknown compound among the thousands contained within the average plant is a difficult task. One approach uses solvent to break the plant material down into large sets of compounds, testing each set for the properties we are seeking, then breaking the large set into smaller and smaller sets until only the effective compound is left. We keep a captive colony of leafcutter ants on our campus specifically for this testing procedure.
Once a pure compound effective in repelling ants is isolated, our task is to identify the chemical structure using various spectroscopic techniques.
The final step is synthesis of the chemical. If we can manufacture the compound in the lab and it repels ants, then we know we have successfully isolated and identified an effective compound. Field tests in Central America are then conducted both to gauge the practicality of the compound -- whether or not it will actually repel ants in the natural environment -- and to attempt to determine exactly how the compound works.
One compound isolated in our lab last year is especially interesting. This compound is strongly antifungal -- it will kill fungi similar to the one that the ants grow in their gardens. As yet we do not know whether this is a general phenomenon of all plants' defensive chemicals. It is possible that some repellents are antifungal agents and the ants have learned to avoid them, while other compounds are irritating or toxic to the ants themselves. In either case, the plants is protected.
It remains to be seen whether studies such as ours will result in safe, practical insect repellents. However, by exploring the chemical ecology of this and other insect communities, it is quite possible that we will discover some better methods of controlling insect pests.
For the past 60 to 70 years, control of insects in the field has been attempted solely through the use of chemicals. Among the first widely used pest-control chemicals were naturally occuring poisons such as lead and arsenates and, more recently, synthetic insecticides such as DDT, aldrin, and dieldrin. All the chemicals are broad-spectrum, persistent poisons, toxic to a wide range of insects and animals, and dangerous even to humans. Indiscriminate use of these chemicals has not only resulted in damage in the environment, but the insects have in some casese actually developed a resistance to the poisons.
As we learn more not only about the dangers of some of these chemicals but about the insects themselves, the pressing need for new, safer, and more effective means of insect control becomes more and more apparent.
Today, broad-range toxic chemicals, where they are deemed necessary, can be used in carefully time applications for maximum effectiveness against specific pests with minimum impact on the environment. More significant, some new methods of controlling pests either do not use chemicals at all, or use chemicals so specific in their effect that they have little or no side-effect on the environment.
To achieve such specific control, however, requires a good deal more research and careful planning than the comparatively simple task of spraying broad-spectrum poisons. As the work with leafcutter ants illustrates, effective control of a particular pest, with minimum damage to the environment, requires detailed knowledge of the pest and the ecosystem in which it occurs, including the basic chemistry underlying various biological phenomena.