Biological Control

Lloyd A. Andres, research entomologist, Plant Protection Research Unit, Western Regional Research Center, Agricultural Research Service, Albany, CA.

Looking over the large grassy meadow in northern California's Humboldt County, it is hard to imagine that in 1945, the colorful but poisonous weed St. Johnswort literally held this prime grazing land in its grasp. Property values had dropped to the point that ranchers could not afford the recommended, albeit marginally useful, control treatments.

The delicate and beautiful green lacewing adult, along with its voracious larvae, feeds on a variety of soft-bodied insect pests, including the pink bollworm.

That year, in an effort to break the weed's hold on the land, University of California and U.S. Department of Agriculture entomologists introduced a shiny blue-green beetle, Chrysolina quadrigemina, into the area. The scientists knew from studies in Europe, the native home of St. Johnswort, and in Australia, where the weed also had become a problem, that the beetle fed exclusively on St. Johnswort, or closely related plants, and would not become a pest on agricultural crops. Once released, the beetle adapted quickly to California conditions and by 1950, St. Johnswort was well on the way to being controlled.

Tried and True Method

Deliberate introduction of natural enemies to control pests goes back to the first U.S. project in 1888. Since then, it has been proved time and time again that introduced parasites and predators can control their pest hosts and, in fact, have saved farmers and ranchers millions of dollars in control costs and losses.

Although U.S. researchers will soon celebrate the 100th anniversary of that initial biological control project, the method has not lost its relevancy, even in today's high-technology agriculture. In fact, a recently completed census of insect pest species introduced to North America notes that the reservoir of foreign pests in the United States is increasing. Each year, an estimated 11 new immigrant species take up residence in the United States, 7 of which will become pests of some importance. New weed arrivals further compound the pest problem. Biological control research will play a vital role in the future to meet the new threats to our agriculture and environment.

Biological Control Concept

The introduction of natural enemies into the United States from a pest's native area is a deceptively simple concept. It is based on the fact that each weed or insect pest has a complement of parasites, predators, and diseases that are part of that pest's coevolved natural checks and balances. When a plant or insect is freed from these checks by being moved to new areas without its natural enemies, the new arrival enjoys a competitive advantage over already existing plant and insect species. This advantage often allows the newcomer to increase to pest population levels. Biological control workers seek to reduce this advantage by traveling to the pest's place of origin, collecting its former parasites and predators, and then introducing them into the pest's new home.

Research Needs

The process of locating the origin of the pest and then finding and introducing new natural enemies, although simple in concept, poses logistical and ecological challenges. For example, to locate the origin of leafy spurge, a weed of importance in the northern Great Plains, early plant collection records were reviewed to determine when and where this spurge first appeared in the United States. The localities noted indicated that spurge could have arrived with early immigrants. A further check of immigration and church records revealed that a number of the settlers to these sites came from farms in Eastern Europe and the Ukraine.

Another scenario indicates that spurge may have arrived as a contaminant with shipments of new plant germplasm from Eastern Europe and Asia. About the time that spurge first appeared, two USDA plant explorers were collecting crop and ornamental plants, one working in China and one in Russia, that would be well adapted to our northern Great Plains. While on vacation, a USDA entomologist who was intrigued as to the origin of our weed checked botanical collections in Japan and Korea for records of leafy spurge in Asia. His search turned up plants resembling the North American pest species, substantiating the possibility that our weed could have originated in Asia or was a multiple introduction from both Asia and Europe. The process of obtaining and matching plants from Eastern Europe, Asia, and the United States to verify this hypothesis is still under way.

Traveling to the pest's point of origin to survey for parasites and predators is the next step. Success in searching out and obtaining natural enemies hinges on the support and close cooperation of like-minded scientists and foreign governments around the world.

Once the natural enemies have been found, studied, and cleared for release, the final challenge is to establish them in the highly automated and artificial setting of today's agricultural world. Pesticides, ongoing disturbances of the habitat by agricultural operations, and the removal of noncrop vegetation that might otherwise offer food and shelter to the natural enemies are some of the obstacles that have hindered establishment and control.

Matching Pests, Climates, and Natural Enemies

Projects often fail when the natural enemies are poorly adapted to the U.S. pest or climate. For example, early shipments of the mite Aceria chondrillae to the Western United States to control rush skeletonweed, Chondrilla juncea, failed to establish. Incompatibility between the mites, which originated in Greece, and the U.S. plants was suspected. A second mite shipment from Italy readily established on American plants.

In another case, a small parasite Trioxys pallidus introduced to California from France to control the walnut aphid Chromaphis juglandicola established well in coastal areas but failed in the warmer interior valleys. Introductions of the same parasite succeeded when it was obtained from an area of Iran where the climate more closely approximated the interior areas.

To improve the matchup between populations of weed-feeding insects and their weed hosts, chemists are "fingerprinting" the invading weeds by isolating and identifying key chemical components and comparing these with similar isolates from foreign populations. Entomologists on the other hand are identifying differences between insect populations by comparing enzyme systems. Some re searchers have suggested that a reevaluation of early biological control attempts with these new techniques would help to explain some of the early failures.

Increasing the Numbers of Natural Enemies

Natural enemies sometimes fail when their numbers remain at levels too low to properly control their host. Artificial diets are being formulated and methods developed to produce and disseminate parasites and predators to increase low field populations. For example, the indigenous fungal pathogen Colletotrichum gloeosporioides f. sp. aeschynomene is now being cultured inexpensively and sold to control northern jointvetch, a weed native to southern rice fields. The fungus can be applied to the weed with conventional pesticide application equipment.

Similarly, forms of Bacillus thuringiensis, a bacterium that has proved valuable in the control of important chewing insects and mosquitoes, are being produced and marketed to farmers. New microbes and improved implementation strategies are being sought.

Genetic Improvement

Although the idea to improve performance of natural enemies by introducing or developing new genetic Strains has been around for several Years it wasn't until the recent development and field colonization of a new strain of the predatory mite Metaseiulus occidentalis, which is resistant to several insecticides, that this has proved practical. The mite is used to control spider mites feeding on crops.

Genetic modification of beneficial micro-organisms could facilitate artificial production as well as increase virulence and environmental tolerances. Fortunately, many species of natural enemies already have a high degree of genetic variability. Successful control is often a matter of finding these variants and introducing them to the problem areas.

Habitat and Behavioral Modification

Planting of cover crops, providing nectariferous plants and sources of alternate hosts in and around fields, and interplanting different crops to provide habitat diversity are all ways of encouraging natural enemies into fields and enhancing biological control.

Much of an insect's behavior is mediated by chemicals emanating from the environment. Entomologists, now aware of this, apply extracts from tomato to corn plants to increase the parasitization of corn earworm eggs. Food supplements applied to alfalfa and cotton can increase the number of green lacewing predators in the crops. Other environmental modifications are being studied to improve biological control.

Although the views on biological control and the perception of its potential to regulate insect and weed pests have changed over the past 100 years, the idea of somehow bringing together a pest and its parasites remains. It is to every society's advantage to use the pest controls that nature has provided. Biological control, a method that derives its energy directly from the pests themselves, is the best.