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A. hygrophila on an A. philoxeroides ) leaf. Photograph courtesy USDA. Photographer M. Julien.

Species Name: 
Agasicles hygrophila Selman and Vogt, 1971

Common Name:
Alligatorweed Flea Beetle

Species Description:
The alligatorweed flea beetle, Agasicles hygrophila, is a beetle non-native to the U.S. but released here intentionally as a potential biological control agent for the exotic invasive plant alligatorweed (Alternanthera philoxeroides).

Adult A. hygrophila range in size from 4 to 7 mm in length and approximately 2 mm wide.. Adult alligatorweed flea beetles have a black head and thorax and a yellow and black striped elytra (wing covers) covered with a waxy cuticle that gives it a shiny appearance (Buckingham 2002, USACE 2002).

A key distinguishing characteristic of adult members of the flea beetle family (Chrysomelidae) is the presence of greatly enlarged femora (the upper leg sectons) of the hind legs. Chrysomelids use these powerful hind legs to leap considerable distances if disturbed, thus earning the common name "flea beetle" (Weeden et al. undated).

Larvae are black and worm-like, attaining as much as 6 mm length as mature third instars (Weeden et al. undated). Instars are the intermolt (between molts) developmental stages insects and other arthropods pass through before reaching sexual maturity.

Potentially Misidentified Species:
The size, coloration, and presence of the enlarged hind femora are useful characteristics to allow identification of adult forms of this species.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Agasicles hygrophila, native to Argentina, was purposely introduced in the United States in 1964 to help eradicate invasive alligatorweed, the first such introduction of an insect for biocontrol purposes in the U.S. (Carley and Brown 2006). The beetle was released from Virginia to southern Florida and along coastal waterways to Texas and in California. The success of this introduction virtually eliminated the need for pesticide spraying and is considered one of the most successful biological control programs to date (Weeden et al. undated).

IRL Distribution:
The Florida distribution of Agasicles hygrophila reflects that of its host plant. In the IRL region of Florida, Alternanthera philoxeroides is largely confined to the northern IRL watershed counties (Volusia, Brevard).

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Agasicles hygrophila is a small beetle, with adults typically reaching a length of up to 7 mm and a width of 2 mm. The life cycle is short, lasting for approximately 30 days. (USACE 2002, UF/IFAS CAIP undated).

Abundance:
Agasicles hygrophila populations increase and decrease with changes in the abundance of host alligatorweed plants. Florida populations of A. hygrophila are large enough that they continue to keep invasive A. philoxeroides in check. USACE (2002) notes that it is rare to encounter alligatorweed anywhere in the U.S. that doesn't bear evidence of the presence of A. hygrophila.

Reproduction:
Reproduction in Agasicles hygrophila is sexual. Females deposit egg masses containing 12-54 eggs on the underside of alligatorweed leaves, thus insuring an abundant food supply for the larvae after they hatch. The eggs are yellowish, and approximately 1.25 mm long. A. hygrophila females lay up to 300 eggs during their short lives (Weeden et al. undated, TAMU undated).

Embryology:
Eggs hatch approximately 4 days after they are laid at temperatures ranging from 20-30°C. The larvae are initially pale gray, with the legs turning brown within a few hours of hatching. Succeeding instar larvae are pale gray with brown heads and legs, and the third (final) instar is almost black in color. These mature larvae burrow into and pupate in the hollow stems of the alligatorweed plant and emerge as adults through the holes in the stems (USACE 2002).