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AQUATIC VEGETATION --Biological Control

(By Fish and Arthropods)

Fish--Exotic fish have been used for clearing aquatic vegetation from waterways, which has also resulted in excellent mosquito & chironomid midge control. In the irrigation systems of southeastern California, three species of subtropical cichlids, Tilapia zillii (Gervais), Oreochromis (Sarotherodon) mossambica (Peters), and Oreochromis (Sarotherodon) hornorum (Trewazas), were introduced and have become established over some 2,000 ha of Culex tarsalis breeding habitat (Legner & Sjogren 1984). These fish were imported to California for the biological control of emergent aquatic vegetation that provides a habitat for such encephalitis vectors as the mosquito Culex tarsalis Coquillet, and as predators of mosquitoes and chironomid midges.

Left Figure = Potamogeton weeds clogging an irrigation canal siphon at Blythe, California.; Middle Figure = Hydrilla weeds dredged from the All American Canal, Imperial Valley, California.; Right Figure = Potamogeton removed from the lower half of a canal in the Coachella Valley, California, causes a cascade of     water, 4-ft. high at the unremoved portion upstream.

       In this situation, mosquito populations are under control by a combination of direct predation and the consumption of aquatic plants by these omnivorous fishes (Legner & Medved 1973, Legner 1978a, 1983; Legner & Fisher 1980; Legner & Murray 1981, Legner & Pelsue 1983). As Legner & Sjogren (1984) indicate, this is a unique example of persistent biological control and probably only applicable for relatively sophisticated irrigation systems where a permanent water supply is assured, and water conditions are suitable to support the fish (Legner et al. 1980). There is a three-fold advantage in the use of these fish: (1) clearing of vegetation to keep waterways open, (2) mosquito control and (3) a fish large enough to be caught for human consumption. Some sophistication is necessary when stocking these cichlids for aquatic weed control, which is often not understood by irrigation districts personnel (Hauser et al. 1976, 1977; Legner 1978b). Otherwise competitive displacement may eliminate T. zillii, the most efficient weed eating species (Legner 1986).

       Although the degree of control achieved by the three fish species imported varied in different parts of the targeted area, circumstances beyond the control of researchers preempted a broader success. Careful studies under natural, but quarantine, areas in California showed that the different fish species each possessed certain attributes for combating the respective target pests (Legner & Medved 1973). Tilapia zillii was best able to perform both as a habitat reducer and an insect predator. It also had a slightly greater tolerance to low water temperatures, which guaranteed its survival through the winter months in southern California, while at the same time it did not pose a threat to salmon and other game fisheries in the colder waters of central California. It was the superior game species and most desirable as human food

       Nevertheless, the agencies supporting the research (mosquito abatement and county irrigation districts) acquired and distributed all three species simultaneously throughout thousands of kilometers of irrigation system, storm drainage channels and recreational lakes. The outcome was the permanent and semipermanent establishment of the two less desirable species, S. mossambica and S. hornorum over a broader portion of the distribution range. This was achieved apparently by a competitive superiority rendered by an ability to mouth-brood their fry, while T. zillii did not have this attribute strongly developed. It serves as an example of competitive exclusion such as conjectured by Ehler & Hall (1982). In the clear waters of some lakes in coastal and southwestern California, the intense predatory behavior of S. mossambica males on the fry of T. zillii could be easily observed, even though adults of the latter species gave a strong effort to fend off these attacks. The nests of all species are conspicuous in the benthos of waterways <PHOTO>.

       This outcome was not too serious for chironomid control because the Sarotherodon species were quite capable of permanently suppressing chironomid densities to below annoyance levels (Legner et al. 1980). However, for control of higher aquatic weeds, namely Potamogeton pectinatus L., Myriophyllum spicatum var. exalbescens (Fernald) Jepson, Hydrilla verticillata Royle and Typha species, they showed no capability whatsoever (Legner & Medved 1973). Thus, competition excluded T. zillii from expressing its maximum potential in the irrigation channels of the lower Sonoran Desert of California and in recreational lakes of southwestern California. Furthermore, as the Sarotherodon species were of a more tropical nature, they died out annually in the colder waters of the irrigation canals and recreational lakes. Although T. zillii populations could have been restocked, attention was later focused on a potentially more environmentally dangerous species, the white amur Ctenopharyngodon idella (Valenciennes), and other carps. The substitution of T. zillii in storm drainage channels of southwestern California is presently impossible because the competitively advantaged Sarotherodon species are permanently established over a broad geographic area.

Arthropods

       Alligator Weed, Alternanthera phylloxeroides (Martius) Grisebach -- Amaranthaceae is an emersed, perennial, aquatic plant from South America whose hollow, segmented stems allow it to form dense floating mats on the surface of rivers and other bodies of water. The floating mats block navigation, inhibit water use and limit water flow. The rooted, segmented stems often break and allow the mats to float freely, spread and root at new sites. These freely rooting stems renders mechanical removal of the mats ineffective, as the remaining fragments grow vegetatively. Stem segmentation also encumbers herbicide translocation and effectiveness. Many attempts to control alligatorweed with herbicides have worsened the problem by killing neighboring plants and allowing the alligatorweed to grow unimpeded (Maddox et al. 1971, Coulson 1977).

       Because of problems encountered with alligatorweed control, and as part of an expanded aquatic weed control program, the United States Army Corps of Engineers sought the assistance of the U. S. Department of Agriculture, Agricultural Research Service to assess the potential for biological control of this noxious plant. In 1960, G. B. Vogt explored in Argentina and adjacent countries to the north in search of phytophagous arthropods and plant pathogens of alligatorweed. He reported over 40 species of natural enemies attacking alligatorweed, three of which he considered particularly important: Amynothrips andersoni O\'Neill (Thysanoptera: Phlaeothripidae), Agasiceles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae), and Vogtia malloi Pastrana (Lepidoptera: Phycitinae). In 1962, the U. S. Department of Agriculture established a laboratory near Buenos Aires, Argentina, to study the biologies and host plant relationships of these biological control agents (Coulson 1977) (also see Fuller 1961, Anonymous 1962, Hawkes et al. 1967, Zeiger 1967, Maddox & Resnik 1968).

       Following are some of the attributes of the several species found by C. F. Vogt:

Agasiceles hygrophila adults feed on the submerged leaves and stems of alligatorweed. The eggs are laid in clusters on the undersides of the young leaves of this plant. Developing larvae feed on the leaves and stems, and third or final instar larvae tunnel into the hollow stems to pupate. Adults later chew through the stem wall and the life cycle is repeated. As many as five generations per year occur in Argentina (Maddox (1968). Feeding by beetles destroys both leaves and stems, the latter becoming waterlogged after repeated perforations with adult emergence holes, causing the mats to sink.

Vogtia malloi is a nocturnal moth that oviposits on terminal leaves. Larvae tunnel into stems, and may later exit at irregular intervals, reenter and thereby damage a number of stems as they pass through five instars. Pupation is inside the hollow stem, and there are 3-5 generations per year. Extensive stem collapse results from the feeding of V. malloi and it develops satisfactorily on both rooted and free floating plants (O\'Neill 1968, Maddox et al. 1971).

       The small (2.2 mm) Amynothrips andersoni feed among the bracts of the young buds or in the leaf axils. Larvae complete their development in about 30 days, and their are 3-5 generations annually. These thrips overwinter primarily as adults, and their feeding scars the leaf surface and stunts stem growth (Maddox et al. 1971).

These three fleabeetles were imported to the United States during 1964-70 from Argentina (Coulson 1977). They are now established in the southeastern United States.

Agasiceles hygrophila gave moderately good initial control in many coastal areas of the SE USA, but it has subsequently exhibited intolerance to extremes in temperature and humidity. Early season supplemental releases of adult fleabeetles have enhanced their impact in the climatically extreme areas. IN the states of Florida, Louisiana and Texas, biological control of alligatorweed is successful. Vogtia malloi reduced the weed mats by 70-80% in coastal areas of Mississippi, but control there is not altogether satisfactory (Julien 1987).

       The introduction of A. hydrophila into the SE United States in 1964 was the first use of an insect as an aquatic noxious plant control agent. The success of this effort has reduced skepticism on the us of monophagous natural control agents (Andrés & Bennett 1975). The initial establishment of natural enemies on waterways associated with the St. Johns River in Florida occurred within 15 months of initial release, while it took much longer at other release sites. The different rates of control may relate to the carbohydrate reserves in the alligatorweed mat stems, the growth rate of the plant itself and the length of the growing season (Andrés & Bennett 1975, Coulson 1977).

       Australia, Thailand and New Zealand also received fleabeetles from colonies that became established in the United States, with the beetles having become established in all three countries. A fourth species, Disonycha argentinenesis Jacoby was introduced to Australia in 1980 and New Zealand in 1982 directly from Brazil, but failed to become established. Agasicles hygrophila spread quickly through the infestations of alligatorweed in Australia and provided substantial control of this aquatic pest within 14 months. Vogtia malloi impact on alligatorweed there is confounded with injury caused by Agasicles. This moth is completely ineffective in terrestrial terrain (Julien 1987).

References: (Please refer to the following for detailed references):

Anonymous. 1962. Alligatorweed controlled by insects? Agric. Res. 10: 8-9.

Andrés, L. A. & F. D. Bennett. 1975. Biological control of aquatic weeds. Ann. Rev. Ent. 20: 31-46.

Coulson, J. R. 1977. Biological control of alligatorweed, 1959-1972. A review and evaluation. U. S. Dept. Agric. Tech. Bull. No. 1547. 98 p.

Fuller, T. C. 1961. New weed problems. Calif. State Dept. Agric. Bull. 50: 20-8.

Garcia, R. & E. F. Legner. 1999. Biological control of medical and veterinary pests. In: T. W. Fisher & T. S. Bellows, Jr. (eds.), Chapter 15, p. 935-953, Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA

Hawkes, R. B., L. A. Andrés & W. H. Anderson. 1967. Release and progress of an introduced flea beetle, Agasicles n. sp., to control alligatorweed. J. Econ. Ent. 60: 1476-77.

Julien, M. H. (ed.). 1987. Biological control of weeds: a world catalogue of agents and their target weeds, 2nd ed. Commonw. Agric. Bur. Int., Wallingford, U.K. 150p.

Legner, E. F. 1995. Biological control of Diptera of medical and veterinary importance. J. Vector Ecology 20(1): 59-120.

Legner, E. F. 2000. Biological control of aquatic Diptera. p. 847-870. Contributions to a Manual of Palaearctic Diptera, Vol. 1, Science Hearld, Budapest. 978 p.

Maddox, D. M. 1968. Bionomics of an alligatorweed fleabeetle, Agasicles sp., in Argentina. Ann. Ent. Soc. Amer. 61: 1299-1305.

Maddox, D. M. & M. E. Resnik. 1968. Radioisotopes--a potential means of evaluating the host specificity of phytophagous insects. J. Econ. Ent. 61: 1499-1502.

Maddox, D. M., L. A. Andrés, R. D. Hennessey, R. D. Blackburn & N. R. Spencer. 1971. Insects to control alligatorweed, an invader of aquatic ecosystems in the United States. BioScience    21:  985-91.

O\'Neill, K. 1968. Amynothrips andersoni, a new genus and species injurious to alligatorweed. Proc. Ent. Soc. Wash. 70: 175-83.

Zeiger, C. F. 1967. Biological control of alligatorweed with Agasicles n. sp. in Florida. Hyacinth Control J. 6: 31-4.