Biological Control of Alligatorweed, Alternanthera philoxeroides, the World’s First Aquatic Weed Success Story

ABSTRACT
Alligatorweed, Alternanthera philoxeroides (Mart.) Griseb., a South American immigrant, is an aquatic weed in the southern United States where it roots in shallow water or on shore. Floating stems grow across the surface of the waterway forming a dense interwoven mat. This mat clogs the waterway and outcompetes native plants along the shore. Three species of South American insects were released from 1964 to 1971 for biological control of this aggressive invader. The flea beetle Agasicles hygophila Selman & Vogt strips the leaves from the stems and the moth Vogtia malloi Pastrana bores inside the stems. Heavy damage by either species kills the stems thereby causing the mat to break up, clearing the waterway. The thrips Amynothrips andersoni O’Neill feeds on the young apical leaves. Heavily damaged plants are often stunted. Alligatorweed has been controlled or the populations reduced throughout much of its range, especially in Florida and along the Gulf Coast. Colder climates, though, from Arkansas to North Carolina along the margin of the range preclude establishment of the insects. In some colder areas, for example the Lower Mississippi River Valley, the moths and flea beetles immigrate from the warm coastal areas and provide local control.