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ALLIGATORWEED, Alternanthera phylloxeroides (Martius) Grisebach -- Amaranthaceae

(Contacts)

Alligatorweed 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 (Goeden & Andrés 1999). 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, Goeden & Andrés 1999).

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: [ Additional references may be found at: MELVYL Library ]

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.

Goeden, R. D. & L. A. Andrés. 1999. Biological control of weeds in terrestrial and aquatic environments. In: Bellows, T. S. & T. W. Fisher (eds.), Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, New York. 1046 p

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.

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.