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
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.
HABITAT AND DISTRIBUTION
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).
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).
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).
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 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).
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
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 flee 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).
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).
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).
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 damaged
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.
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