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Biological Control : A Guide to Natural Enemies in North America Anthony Shelton, Ph.D., Professor of Entomology, Cornell University

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Agasicles hygrophila
(Coleoptera: Chrysomelidae)
Alligatorweed Flea Beetle

Alligatorweed has been a waterway pest in the United States since it was introduced from South America in the 1890s. It forms dense, impenetrable mats that float on the surface and can also exist during drought conditions in a terrestrial form. The flea beetle, Agasicles hygrophila, was introduced in 1964 to combat alligatorweed which invades United States waterways from Virginia to southern Florida and along coast waterways to Texas and in California. The success of this program was so remarkable that the U.S. Army Corps of Engineers canceled all herbicide spraying against alligatorweed in Florida in 1968, three years after the beetles were introduced there!


Adults are black, about 4-6 mm long, with two longitudinal yellow stripes. The larvae are black, up to 6 mm long when mature third instars; eggs are yellowish, 1.25 mm long and are laid in clusters on the undersides of leaves. Adults have greatly enlarged hind femora for jumping, hence the name flea beetle.


Host-range studies in Argentina and Uruguay limited to 14 plant species in 8 families determined that only alligatorweed was suitable as a host plant for A. hygrophila. No additional testing was done in North America.

Pests Attacked

Alligatorweed (Alternanthera philoxeroides).

Life Cycle

Adults overwinter among the roots and stems of alligatorweed along the margins of waterways. Eggs are laid on the undersides of leaves. Larvae eat the leaves during their development, often leaving transparent windows of uneaten epidermis. Pupation takes place inside the hollow plant stems. The life cycle takes less than one month, and adults live about 48 days.

Relative Effectiveness

The alligatorweed flea beetle is so successful that it is often used as the symbol of biological weed control. Within 4 years, alligatorweed was practically eliminated at the two northern Florida sites where the flea beetle was first introduced. The beetle is less effective in southern Florida and in northern areas of Alabama, Louisiana, Mississippi, and in North and South Carolina. Additional beetles from a colder region in Argentina were released in the Carolinas in 1979, apparently without success.

Pesticide Susceptibility

Susceptible to topically applied toxaphene and methyl parathion, but not to plants grown in water with those pesticides.

Commercial Availability

Not available commercially.


Thanks to Gary Buckingham for reviewing and correcting this section and for adding valuable information.

Taken from

Buckingham, G.R. (1994) Biological control of aquatic weeds. In Pest Management in the Subtropics: Biological Control - a Florida Perspective. Rosen, D., Bennett, F.D., Capinera, J.L. (Eds.) Intercept Ltd, Andover, UK. 737 pp.

Additional References

Maddox, D.M. (1968) Bionomics of an alligatorweed flea beetle, Agasicles sp., in Argentina. Ann. Entomol. Soc. America. 61:1299-1305.

Vogt, G.B., Quimby Jr., P.C., and Kay, S.H. (1992) Effects of weather on the biological control of alligatorweed in the lower Mississippi Valley region, 1973-83. USDA Tech. Bull 1766. 143 p.

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Top: A. hygrophila adult.

 Bottom:A. hygrophila larvae. Note the window-like appearance of the holes on the leaves.

Top: A. hygrophila adult.

Bottom:A. hygrophila larvae. Note the window-like appearance of the holes on the leaves.

Photos: G.Buckingham

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