Biological Methods of Weed Control. Rosenthal, S.S., D.M. Maddox and K. Brunetti. Biological control of weeds is the use of living organisms to lower plant pest populations to the point where the plants are no longer economic problems. It is an ecological approach to weed management. Plant and population dynamics and the herbivore-host plant relations involved are aspects of ecology partic­ cularly applicable to biological control. It differs from other meth­ ods of control in that it (1) does not necessarily kill the pest out­ right, but may only lower its competitive ability; (2) may be slow acting; (3) is relatively inexpensive; (4) as it must be employed it is very selective; (5) causes no harmful side effects; and (6) can be permanent. The use of biological control of weeds dates back to the 1860's, but it was little used until the 1920's. Some success with lantana and spectacul~r successes with the control of prickly pear and St. Johnswort, or Klamath weed, have more recently stimulated increased research efforts in biological weed control all over the world. Most of the biological cotrol agents used against weeds have been , but mites, snails, nematodes, fish, mammals, birds, plant pathogens, and even other plants can also be valuable. The procedure in developing a biological control project consists of (1) determining the suitability of the weed problem for biological control, (2) surveying the weed for natural enemies in both its naturalized and native homes, (3) studying the biologies and host relations of the natural enemies to determine if and how they may be used to solve the problem, (4) implementing their use in weed control, and (5) evaluating their effectiveness. The most common method of implementing biological control has been the classical one, i. e., through the introduction of new natural enemies. Other ways of implementation are by the manipulation or augmentation of already existing natural enemies and by the use of allelopathy. Biological control of alligatorweed and waterhyacinth demonstrate the successful use of the classical approach against aquatic weeds. Both of these pests are natives of South America. They rapidly reproduce vegetatively making them difficult to control by conventional means. Alligatorweed is now controlled by the leaf-feeding flea , hygrophila, complemented by the stem-boring moth, Vogtia malloi, both introduced from South America. The most important biological control agents used against waterhyacinth are the weevils, Neochetina spp., that feed externally as adults and tunnel in the petioles as larvae; the moth, Samcodes albiguttalis, that burrows in the petioles as a ; and the mite, Orthogalumna terebrantis, that feeds within the leaves and petioles. All these organisms are from South America. Two terrestrial weeds effectively controlled biologically are puncturevine and Klamath weed. These plants originated in Europe and the Mediterranean Area, as did many of our agricultural weeds. Puncturevine is not only difficult to control by usual procedures, but occurs extensively on low value land, where it is not feasible to use other control methods, as well as on crop land. The natural enemies used against it are the seed weevil, Microlarinus lareynii, and the stem weevil, M. lypriformis imported from Italy. They have given

78 good control in warmer parts of the United States and the cold-hardy strains that have developed in north Texas and the Great Plains since their introduction in 1961 should prove somewhat successful in other cool winter areas. Klamath weed is a toxic range weed that rapidly increased once it was introduced into the western United States. Before the attempt at its biological control it was predicted to become one of the worst range weeds in that area. The control of this weed with (particularly) the leaf feeding beetle, Chrysolina quadrigemina, the root boring beetle, hyperici, and the gall midge, Zeuxidiplosis giardi, has been an amazing success. Classical biological control is most applicable to dominant, perennial, introduced weeds in undisturbed environments. It is especially suitable on low-value, inaccessible rangeland or in aquatic situations where other control methods may be too expensive or unfeasible. Augmenting the action of existing natural enemies may be utilized for the management of weed species that are natives, annuals, or are pests of cultivated crops, i.e., situations where classical biological contol is limited. Northern jointvetch is just such a weed that may be controlled using an anthracnose caused by the fungus, Colletotrichum gloeosporioides f.sp. aeschynomene. This is a native fungus that does not normally control the weed because it exists at too low an inoculum level until late in the season and has poor dispersal power. These constraints are overcome when the fungus is mass reared on artificial medium and sprayed, as a mycoherbicide, on weed-infested fields. This fungus gives excellent control of northern jointvetch infesting rice and soybean fields in Arkansas when it is applied early in the season. The possibility of using allelopathy in weed control is a tempting area of research, yet few practical applications of it are available right now. Allelopathy is the reason "smother" crops, such as barley, are able to suppress weed growth and germination. By screening the known germplasm of crop species allelopathic individuals may be recognized. Thus far the world collections of cucumbers and oats have been screened in this way and accessions with greater ablity to release allelopathic chemicals have been found. Their weed resistance may eventually be incorporated into crop varieties by plant breeders. When allelopathic chemicals are found there is also the possibility of using these potent phytotoxins as herbicides. Biological weed control is particularly valuable because it is ecologically sound. Even though it can be very effective and have a high cost:benefit ratio biological control should not be thought of as a panacea. For many weed species and situations it can only alleviate weed problems caused by overgrazing or other poor land management practices, it cannot solve them. Biological control is not any more appropriate than is any other method to suppress all weeds in all situations, but should be considered as an option wherever possible. Biological control of weeds should be as much a part of integrated pest management systems as is the biological control of . S.S. Rosenthal, D.M. Maddox: USDA Biological Control of Weeds Lab., 1050 San Pablo Avenue, Albany, CA 94706; K. Brunetti, Cal. Dept. of Food and Agric., 1220 N Street, Sacramento, CA 95814.

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