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 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.
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).
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 does not 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).
Agasicles hygrophila is sensitive to both high and low temperatures. Experimental work by Stewart et al. (1999) revealed a narrow optimum development temperature range of 27-30°C and an optimum survival temperature range of 25-27°C. A low temperature development threshold temperature of 13.3°C was reported. 70% adult mortality after exposure to 15°C for 12 weeks and fecundity and egg viability also declined at low temperature. Egg viability was very low after exposure to freezing temperature. The authors suggest a much reduced overwintering capacity at temperatures lower than 15°C.
In addition to cold intolerance, alligatorweed flea beetles are also sensitive to hot, dry summers. The narrow tolerance of A. hygrophila makes their establishment in certain areas difficult. (UF/IFAS CAIP undated, TAMU undated).
Trophic Mode: Agasicles hygrophila is an extreme specialist herbivore, feeding exclusively or nearly exclusively on the stems and, in particular, the leaves of alligatorweed. Both adult and larval forms feed on alligatorweed, attacking the emergent (above the waterline) portions of the plant (UF/IFAS CAIP undated, TAMU undated, USACE 2002).
Associated Species: Agasicles hygrophila exhibit an obligate association with alligatorweed, Alternanthera philoxeroides (TAMU undated), upon which they live and are essentially monophagous (feeding exclusively on one species).
Invasion History: Agasicles hygrophila, native to Brazil, was purposefully introduced in California and South Carolina in 1964 by the U.S. Army Corps of Engineers to control the invasive alligatorweed (Center et. al 1998). It was subsequently released throughout Florida and all southeastern coastal states to Texas beginning in that same year to control the spread of the plant.Alligatorweed was targeted for biological control because herbicide-based weed management had proved difficult and ineffective. In addition to A. hygrophila, two additional host-specific South American insects were identified, evaluated, and intentionally released for the purpose of biocontrol: the alligatorweed thrips, Amynothrips andersoni, and the alligatorweed stem borer, Vogtia malloi (Center et al., 1998).Since its release, A. hygrophila has been highly successful at controlling the spread of aquatic alligatorweed but has had little effect against the terrestrial form. The insect has been purposefully introduced elsewhere in the world where invasive alligatorweed has become a problem, including in Australia, New Zealand, China and Thailand. The success of these introductions as a means of controlling Alternanthera philoxeroides has generally been good to excellent (UF/IFAS CAIP undated).
Potential to Compete With Natives: The lack of appropriate native herbivorous insects capable of controlling the spread of alligatorweed is the underlying issue necessitating the release of non-native biocontrol agents like Agasicles hygrophila. As such, resource competition between A. hygrophila and native insect species is likely to be minimal.Impacts on native, non-target vegetation are likely to be similarly minimal. Research on A. hygrophila was conducted in Argentina before introduction into the U.S. At least 14 different plant species were examined for damage caused by flea beetles. Results showed no damage and no beetles present on nearby non-target plants (Buckingham, 2002).
Possible Economic Consequences of Invasion: The introduction of Agasicles hygrophila to Florida and elsewhere has provided positive economic benefits by reducing the infestation effects of invasive alligatorweed. The success of this introduction continues to serve as a model for future biological control projects (Julien et al. 1995).
Buckingham G.R. 2002. Alligatorweed. In: Van Driesche R., Lyon S., Blossey B., Hoddle M., and R. Reardon (eds.). Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p.
Carley M., and S. Brown. 2006. Invasive plants; Established and potential exotics, Gulf of Mexico Region. Gulf Coast Research laboratory, University of Southern Mississippi. 8 p. Available online.
Center T.D., Sutton D.L., Ramey V.A., and K.A. Langeland. 1998. Other methods of aquatic plant management In: Langeland K.A., (ed.): Training Manual for Aquatic Herbicide Applicators. University of Florida Institute of Food and Agricultural Science, Center for Aquatic and Invasive Plants.
Julien M.H., Skarratt, B., and G.F. Maywald. 1995. Potential Geographical Distribution of Alligator Weed and its Biological Control by Agasicles hygrophila Journal of Aquatic Plant Management 33:55-60.
Texas A&M University (TAMU). Undated. Alligatorweed, Alternanthera philoxeroides (Martius) Grisebach. Fact sheet in: Biological Control of Weeds in Texas. Available online.
UF/IFAS Center for Aquatic and Invasive Plants. Undated. Biological control insects for aquatic and wetland weeds. University of Florida Aquatic and Wetland Plant Information Retrieval System. Available online.
Stewart C.A., Chapman R.B., Emberson R.M., Syrett P., and C.M Frampton. 1999. The effect of temperature on the development and survival of Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae), a biological control agent for alligator weed Alternanthera philoxeroides. New Zealand Journal of Zoology 26:11-20.
U.S. Army Corps of Engineers (USACE). Aquatic Plant Information System. Available online.
Weeden C.R., Shelton A.M., and M.P. Hoffman. Undated. Biological Control: A Guide to Natural Enemies in North America. Cornell University College of Agriculture and Life Sciences. Available online.
Wunderlin R.P. and B.F. Hansen. 2004. Atlas of Florida Vascular Plants. Institute for Systematic Botany, University of South Florida, Tampa. Available online.