In: Van Driesche, R., et al., 2002, Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p.
Pest Status of Weed
Alligatorweed (Alternanthera philoxeroides [Mart.] Griseb.) is a South American immigrant that has invaded waterways in the United States, primarily in the southeastern states. It also is a weed in tropical and mild temperate regions around the world. Alligatorweed roots readily along waterways and then grows over the water surface as an anchored floating plant. It also grows terrestrially during dry periods. Alligatorweed is a federal noxious weed and a prohibited or noxious plant in Arizona, California, Florida, and South Carolina (USDA, NRCS, 1999).
probably benefits fishing. A perusal of various commercial Internet sites in April, 2001 indicated that costs would be approximately $170 to $370/ha for control of alligatorweed
with the herbicides glyphosate and fluoridone.
Ecological damage. Alligatorweed, like many other invasive aquatic plants, displaces native plants in ditches, along banks, and in shallow water (Holm et al., 1997). Vogt et al., (1992) discussed competition with native plants before and after insect biological control agents were released. Alligatorweed disrupts water flow causing increased sedimentation, and it shades submersed plants and animals causing reduced oxygen levels beneath the mat (Quimby and Kay, 1976).
Extent of losses. Current data on the extent of infestation and control costs are lacking. At the beginning of the biological control program in 1963, there were an estimated 65,723 ha of water infested in eight southern states and 26,933 ha of plants in 1970 (Coulson, 1977). The largest infestation, 22,700 ha, was in Louisiana and the smallest, 21 ha, in Mississippi. In 1981, the infestation in the southern states was estimated to have increased, but only because of increases in Texas and Louisiana (Cofrancesco, 1988). All other states reported a decrease. Even though the infestation estimate had tripled in Louisiana, state officials considered biological control to be satisfactory. Much of the increase was due to terrestrial invasion by alligatorweed.
Alligatorweed, a South American native, grows in the coastal plain from Virginia, ca. 37º N, to southern Florida, ca. 25º N, and westward along coastal areas to Texas. It is also found in southern California (Reed, 1970). A distribution map provided by Reed (1970) indicates that the northern limit inland is at about the middle of Alabama, Georgia, and South Carolina, ca. 33.5º N, with an extension slightly further north in the warmer Mississippi Valley, ca. 35º N. However, both southwestern Kentucky, ca. 36.5º N, and Tennessee are included within its range on the USDA PLANTS Database on the Internet (USDA, NRCS, 1999).
Background Information on Pest Plant
Alligatorweed is in the tribe Gomphreneae, subfamily Gomphrenoideae, family Amaranthaceae (Mabberley, 1997), order Caryophyllales, subclass Caryophyllidae (Cronquist, 1988). There are an estimated 170 species of Alternanthera in the Western Hemisphere with 120 species in South America alone (Vogt et al., 1979). Less than 5% of the species in South America are amphibious with most being mesophytic or xerophytic. Kartesz (1994) listed 15 species of Alternanthera, including ornamentals and immigrants, in the United States and Canada. There are a few species in Asia, mostly introduced from South America. Engler (1934) included A. philoxeroides in the subgenus Telanthera, section 1. Alternanthera can be differentiated from related aquatic species by the opposite, non-succulent leaves; white flowers in short, headlike spikes; and by the presence of a style (Figs. 2, 3). Wain et al. (1984) reported two diverse forms of alligatorweed – one with slender stems and short, rounded leaves, and the other with broad stems and long, slender leaves. Their isozyme study indicated that the genetic difference between the forms was similar to the distances reported between subspecies in other plant studies. The importance of these forms in plant invasion and in control efforts has not been investigated. Julien and Broadbent (1980) listed the synonymy for A. philoxeroides.
Alligatorweed initially roots in wet soil on banks or in shallow water along shorelines and then grows out into waterways. Penfound (1940) reported that beginning in March in Alabama, shoots grew to 38 cm in 1.5 months, to 145 cm in 2.5 months, and to 508 cm in 5.5 months. By September, the mat extended up to 4.6 m away from shore. Alligatorweed is a perennial with hollow stems that buoy the shoots. Floating mats expand over surfaces of all types of waterways and are practically impenetrable. If a waterway dries, alligatorweed changes to a terrestrial form with smaller, tougher leaves and stems. Only vegetative growth has been reported in the United States, although viable seeds have been found in the United States (Holm et al., 1997). Roots develop at closely spaced nodes along stems. When the stems break, floating sections are able to establish readily on moist soil. Alligatorweed has been reported to reproduce by seeds in South America (Holm et al., 1997).
Analysis of Related Native Plants in the Eastern United States
There are no native Alternanthera in the aquatic habitats of the United States. A second introduced species, Alternanthera sessilis (L.) R. Br. ex DC., which is pantropical, is reported to be naturalized in the Florida panhandle (Godfrey and Wooten, 1981). As the name implies, the flowers of A. sessilis are sessile compared with flowers of alligatorweed, which are stalked. According to Vogt et al. (1979), the South American Alternanthera pungens Kunth also is established in the United States. Three additional genera in the Amaranthaceae are associated with aquatic habitats. Amaranthus has six species in the range of alligatorweed in the southeastern United States, Iresine has one species, and Blutaparon has one species, Blutaparon (=Philoxerus) vermiculare (L.) Mears or silverhead, which occurs in maritime habitats. Corell and Corell (1972) placed some of the wetland species of Amaranthus in the genus Acnida. One Amaranthus in the eastern United States, Amaranthus pumilus Raf. (dwarf or seabeach amaranth) is federally listed as threatened. Its range might overlap with the distribution of alligatorweed biological control agents along the coast of South Carolina, but it is found mostly north of their ranges (Godfrey and Wooten, 1981).
History of Biological Control Efforts in the Eastern United States
In 1959, the U.S. Army Corps of Engineers requested that the U.S. Department of Agriculture, Agricultural Research Service evaluate the potential for biological control of alligatorweed (Zeiger, 1967; Buckingham, 1994). Consequently, field surveys and studies of biologies and host ranges of potential biological control agents were conducted in South America (Buckingham, 1996; Coulson et al., 2000). Ultimately, three insect species, Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae), Amynothrips andersoni O’Neill (Thysanaptera: Phlaeothripidae), and Arcola (as Vogtia) malloi (Pastrana) (Lepidoptera: Pyralidae, Phycitinae), were introduced into the United States (Coulson, 1977).
Area of Origin of Weed
Alligatorweed is native along the coast of South America from Venezuela to Buenos Aires Province in Argentina (Vogt et al., 1979). It also was reported from the upper and middle Amazon River basin and the Paraná River basin (Vogt et al., 1979). Sites with alligatorweed were most common in Paraguay, Uruguay, and northern Argentina, but this might reflect more intensive surveys for biological control agents in those areas.
Areas Surveyed for Natural Enemies
Annual surveys from 1960 to 1962 covered eastern and northern South America from Argentina to Venezuela, including Trinidad (Coulson, 1977). Additional surveys were conducted in Argentina and Uruguay (Coulson et al., 2000). Surveys also were conducted in the southeastern United States (Coulson, 1977).
Natural Enemies Found
As many as 40 insect species (not all could be identified to species level) were recorded on alligatorweed (Vogt, 1973). Five of the 40 species were considered to suppress alligatorweed (Vogt, 1973). These five were: A. hygrophila, A. andersoni, A. malloi, Herpetogramma bipunctalis (F.) (Lepidoptera: Pyralidae), and Prodenia sp. (Lepidoptera: Noctuidae) (Maddox et al., 1971; Vogt, 1973). The flea beetle Disonycha argentinensis Jacoby (Coleoptera: Chrysomelidae) was later considered a potential agent for control of terrestrial alligatorweed (Cordo et al., 1984). No species able to suppress the weed were discovered in the United States, but native species of Disonycha and the moth H. bipunctalis were commonly found on alligatorweed in the United States (Vogt et al., 1992).
A fungus native to the United States, Nimbya (=Alternaria) alternantherae (Holcomb and Antonopoulus) Simmons and Alcorn (Hyphomycetes), causes purplish leaf spots and can defoliate plants. However, damage is rarely severe (Holcomb, 1978). In Brazil where it also is native, N. Alternantherae was not particularly damaging, but preliminary studies indicated possible potential as a mycoherbicide (Barreto et al., 2000). Damage by a second Brazilian fungus, Cercospora Alternantherae Ellis and Langlois, was occasionally severe and this species might have potential as a biocontrol agent. (Barreto et al., 2000). A virus-like disease that stunts alligatorweed in Florida has not been studied (Hill and Zettler, 1973).
Host Range Tests and Results
Four insect species were tested in host range experiments in Argentina and Uruguay and in quarantine in Albany, California. Maddox et al. (1971) reported that 14 plant species were tested with the alligatorweed flea beetle, but they did not list the species. Buckingham (1996) reported that the 14 species were apparently in 12 genera of eight families. These families were Amaranthaceae, Brassicaceae, Chenopodiaceae, Cucurbitaceae, Malvaceae, Nymphaeaceae, Poaceae, and Polygonaceae. For the flea beetle, A. hygrophila, slight adult feeding was found on apical leaves of Chenopodium ambrosioides L. and larval feeding and development on Atriplex patula var. hastata (L.) Gray, but only one malformed adult emerged (Maddox and Resnik, 1969; Maddox et al., 1971). Field observations in South America also played a role in obtaining approval of the flea beetle for introduction into the United States (Anderson, 1963). At least 14 species of aquatic or related plant species in proximity to damaged alligatorweed in Argentina were examined for flea beetles, as were crop plants in the vicinity. No beetles or damage were found.
The alligatorweed thrips, A. andersoni, was tested on 21 species in 13 genera of six families (Maddox, 1973). Families were the same as those tested with the flea beetle except Cucurbitaceae and Malvaceae were not tested with the thrips. No development took place in the no-choice and choice experiments except on alligatorweed. Vogt found thrips on the native Alternanthera hassleriana Chod. in Argentina (Maddox et al., 1971), but that species has not been reported as naturalized in the United States. Field examinations in Argentina of 46 other plant species in 26 genera of 11 families yielded no A. andersoni or its damage (Maddox, 1973).
The alligatorweed stem borer, A. malloi, was tested in choice and no-choice tests on 30 plant species in 17 genera of the six families tested with the thrips (Maddox and Hennessey, 1970). Although there was some feeding on test plants in no-choice tests, development of third or younger instars was restricted to alligatorweed. A few older larvae finished their development on five species in the same amaranth tribe as alligatorweed, Gomphrenae. Field examinations of 51 plant species in Argentina from 1962 through 1967 discovered this moth only on alligatorweed (Maddox and Hennessey, 1970). In South American surveys, moths were reared from A. hassleriana and from the closely related Philoxerus portulacoides St. Hil. (Vogt et al., 1992). After the moth was released in the United States, it was reared from the native species Blutaparon vermiculare, collected in Louisiana and Texas and from subsequent cage tests (Vogt et al., 1992). However, the numbers reared from Philoxerus and Blutaparon, which are closely related, were small. Pemberton (2000) reported rearing this species from Alternanthera flavescens Kunth., a native of coastal hammocks in Florida.
The flea beetle, D. argentinensis, was tested on 54 species in 38 genera of 19 families in no-choice larval tests (Cordo et al., 1984). All of the eight families used in tests with A. hygrophila were included along with additional families containing crop and ornamental species. Flea beetle larvae fed moderately on four species of Amaranthaceae and two species of Chenopodiaceae, but development to adults was restricted to Alternanthera paronychioides St. Hilaire (6.4%) and Beta vulgaris L. (3.0%), while 44% completed development on alligatorweed. The development on beets, B. vulgaris, prevented release of this species in the United States. Interestingly, Australia tested this species with 36 species in 31 genera of 18 families (Sands et al., 1982). In those tests, no development was observed on beets, but the smaller number of larvae used in the tests (52 versus 234 in the American tests) could have accounted for the slight difference between the two studies. Based upon the American test results (3% development), only one adult would have been expected in the Australian tests and, indeed, one of the 52 larvae did develop to the last instar. Both Australia and New Zealand released this species, but it did not establish (Julien and Griffiths, 1998).
No host range tests were conducted with the other two major biotic suppressants listed by Vogt (1973), H. bipunctalis and Prodenia sp. The former, the southern beet webworm, also is native to North America. The latter pupates in the soil, which prevented its use for control of aquatic alligatorweed that was the target of the biological control program (Maddox et al., 1971). If there is future interest in controlling terrestrial alligatorweed, which is commonly eaten by cattle, the Prodenia sp. could be studied further although it also attacked Amaranthus sp.
Field-collected alligatorweed flea beetles from Argentina were processed through quarantine and released in 1964 in California and South Carolina and in 1965 in Florida (Coulson, 1977). Beetles from Uruguay also were released in South Carolina and a mixed quarantine colony started with beetles from both Argentina and Uruguay was released in Mississippi (1965). Later, beetles collected at release sites, mostly in Florida, were redistributed in Alabama (1967), Arkansas (1969), Georgia (1966), Louisiana (1970), North Carolina (1967), Tennessee (1968), and Texas (1967). Beetles from a quarantine colony held in Gainesville, Florida (of insects originating from Necochea, Argentina) were released in 1979 in Alabama, Florida, North Carolina, and South Carolina (Buckingham et al., 1983).
A quarantine colony of alligatorweed thrips from Argentina was released in Alabama (1968), California (1967), Florida (1967), Georgia (1967), Mississippi (1968), South Carolina (1967), and Texas (1968) (Coulson, 1977).
Eggs from alligatorweed stem borer females collected as larvae in Argentina and held in quarantine were first released in Florida and Georgia in 1971 (Coulson, 1977). Eggs from females collected at Necochea, Argentina, and held in quarantine were released in Georgia and South Carolina in 1972 in an attempt to establish more cold-tolerant populations. Most of the other releases in 1971 and 1972 were from quarantine (Albany) or greenhouse (Gainesville) colonies started with part of the eggs collected in Necochea. Releases were made in the preceding states and in Alabama (1972) and North Carolina (1971).
A handbook that provides instructions for release of the alligatorweed agents was prepared by the U.S. Army Engineer Waterways Experiment Station, based upon the successes of these releases (Anonymous, 1981).
Biology and Ecololgy of Key Natural Enemies
Alligatorweed flea beetle, Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae)
usually deposited only one egg cluster per day for an average of 1,127 total eggs (Maddox, 1968). Beetles were multivoltine near Buenos Aires, Argentina, producing five generations per year (Maddox, 1968) and probably four to six generations in Florida and the lower Mississippi River Valley (Coulson, 1977; Vogt et al.,1992).
Most feeding and oviposition by A. hygrophila is on aquatic alligatorweed. Flea beetles, especially larvae, rarely attack plants rooted on shore or in moist ditches. What appears to be typical feeding damage is occasionally observed on terrestrial plants, but it is usually nocturnal feeding by native Disonycha flea beetles. In laboratory experiments in Argentina, females oviposited equally on aquatic and terrestrial alligatorweed (Maddox, 1968), but in my experience, females stopped ovipositing almost immediately when fed terrestrial plants. Beetles are specific to alligatorweed and have not been reported on other host plants in the United States even after almost 40 years. A flavone feeding stimulant, 7-a-L-rhamnosyl-6-methoxyluteolin (I), has been isolated from alligatorweed (Zielske et al., 1972) and may be the basis for this specificity.
Alligatorweed thrips, Amynothrips andersoni O’Neill (Thysanaptera: Phlaeothripidae)
Adults are small (ca. 2 mm long), black elongate insects with fringed wings (O’Neill, 1968). Larvae, in contrast, are bright orange (Fig. 5). Both feed in the tips of stems where they cause leaf deformation and stunting of the plant (Fig. 6). Often, the edges of leaves curl inwards which provides excellent shelter. Females had a four-day preoviposition period after which they deposited a mean of 201 eggs on hairs in the nodes of the apical leaves (Maddox and Mayfield, 1979). The elongated oval eggs were amber colored. Larval development took eight to 13 days at 24ºC and the whole life cycle from egg to egg required 28 days on average. There were two larval stages, followed by a resting pupal stage on the plant. Unmated females produced only males, but fertilized females produced equal numbers of males and females. Maddox and Mayfield (1972) reported a method for rearing and studying the thrips in the laboratory.
In Argentina, larvae were most abundant in the spring and declined through late summer (Maddox and Mayfield, 1979). Adults were the predominant overwintering stage, although small numbers of larvae and eggs also were present. There were four or five generations, with no reproductive diapause. Predation by spiders and hemipterous insects appeared to be an important regulating factor in Argentina, especially for pupae (Maddox et al., 1971). Dispersal is limited by wing length. Short-winged adults were present in Argentina at most times, but long-winged, dispersing adults were present in the spring (Maddox and Mayfield, 1979). Long-winged forms were believed to be absent in the United States (Coulson, 1977) but were later reported (Buckingham, 1989; Vogt et al., 1992). Unlike the flea beetle, the thrips attacks both aquatic and terrestrial plants, although Maddox et al. (1971) reported that it preferred terrestrial plants in Argentina.
Alligatorweed stem borer, Arcola malloi (Pastrana) (Lepidoptera: Pyralidae, Phycitinae)
Both aquatic and terrestrial alligatorweed plants are attacked by A. malloi. Stems collapse, turn yellow and die, and heavily damaged mats eventually rot and sink (Brown and Spencer, 1973). Leaves remain on damaged stems, distinguishing stem borer damage from that caused by flea beetles. Vogt et al. (1992) discussed the migratory behavior of A. malloi in the Mississippi River Valley, where it flew in spring and summer from winter refuges near the Louisiana coast north to Arkansas and northern Mississippi, up to 900 to 1000 km.
Evaluation of Project Outcomes
Establishment and Spread of Agents
Alligatorweed flea beetles did not establish in Arkansas, California, North Carolina, or Tennessee. It was hoped that the population from Necochea, Argentina, might be more cold tolerant than the first beetles released, but there have been no reports of an increase in the flea beetle’s range after those 1979 releases. Langeland (1986) reported that releases of the Necochea population were unsuccessful at two study sites in North Carolina. Flea beetles survive mostly in coastal areas or where the mean January temperature is 11.1ºC or higher (Coulson, 1977; Vogt et al., 1992).
Coulson (1977) reported the establishment of alligatorweed thrips in Florida, Georgia, and South Carolina. In 1981, thrips were still present in South Carolina (Buckingham, unpublished data) and in 1982 they were reported in Alabama, Florida, Louisiana, Mississippi, and Texas (Cofrancesco, 1988). The alligatorweed stem borer successfully established at release sites in all states except Alabama and North Carolina (Coulson, 1977). Later, Cofrancesco (1988) reported it in Alabama and North Carolina as well as in Louisiana, Mississippi, and Texas. Vogt et al. (1992) discussed this moth’s long distance dispersal ability and noted that in summer it is present in Arkansas.
Suppression of Target Weed
Alligatorweed flea beetle damage was spectacular in the early phases of the program. Vast areas were defoliated (Fig. 9). Mats attacked by the stem borer turned yellow and died (Fig. 10). These two agents have suppressed alligatorweed in much of the warmer parts of its range, so that control efforts are needed only sporadically. In the Carolinas, Tennessee, and the northern regions of the Gulf Coast states, the plant is usually not controlled biologically unless flea beetles are released early in the season from field collections made in Florida or other southern sites. In the Mississippi River Valley, moths and/or flea beetles often migrate north early enough to provide local control (Vogt et al., 1992). Fortunately, alligatorweed is not as invasive at the margin of its range as it is further south.
Cofrancesco (1988) surveyed aquatic plant managers in 1981 about the importance of alligatorweed in ten southern states. None reported that it was a major problem, although some reported locally serious problems, and none reported chemical control efforts directed specifically at it. Chemical controls were usually incidental to waterhyacinth control efforts.
Recovery of Native Plant Communities
There was relatively little evaluation of the biological control of alligatorweed program, mostly because of the speed of the control and the desire to quickly begin programs for biological control of waterhyacinth (Eichhornia crassipes [Martius] Solms-Laub.) and hydrilla (Hydrilla verticillata [L. f.] Royle). However, Vogt et al. (1992) did conduct long term studies in the lower Mississippi River Valley. They reported observations of native plant populations increasing after alligatorweed was suppressed and included tables of the species involved; however, they did not collect quantitative data.
Long-term economic benefits of alligatorweed control have not been estimated. The fact that most control efforts are now incidental to waterhyacinth control instead of directed at alligatorweed (Cofrancesco, 1988) suggests substantial benefit from reduced chemical control costs. Undoubtedly, there also are indirect cost savings from reduced ditch and canal clearing and from reduced local flooding. Andres (1977) discussed the costs and benefits of the alligatorweed program, including a 76% reduction in the hectares treated by the U.S. Army Corps of Engineers and a 92% reduction in weed control costs at one lake in Georgia. However, I am unaware of any newer studies on the costs or benefits of alligatorweed control.
Recommendations for Future Work
Alligatorweed has invaded regions in the United States with climates colder than the native regions in South America. Therefore, there might not be natural enemies suitable for use in the northern parts of alligatorweed’s range in the United States. However, more complete control in the warmer areas of the range might be possible by using some of the originally discovered agents that were not pursued or new, as yet, undiscovered agents. Recent surveys in the Amazon River drainage for waterhyacinth insects have located several species that had not been found during earlier surveys (DeQuattro, 2000). A similar situation might be true for alligatorweed. Also, pathogens, both in South America and in the United States, should be more carefully evaluated, especially for their potential to complement insect damage.
Vogt et al. (1992) suggested that Alternanthera species in Asia should be examined as sources of biological control agents for alligatorweed. Herbivorous insects and pathogens on Asian plants in this genus, if specific both to Alternanthera and to aquatic habitats, should be safe for use in the United States and might be more damaging than co-adapted agents from the target plant.
The terrestrial South American flea beetle D. argentinensis, which has been released in Australia, should be re-evaluated for its safety and potential use in the United States, if there are no conflicts over control of terrestrial alligatorweed, as there have been in the past (Coulson, 1977). The flea beetle might reduce the invasion of ditches, canals, and shallow ponds when water returns after a drought.
Additional attempts could be made to establish the alligatorweed flea beetle and the stem borer in California, where they did not establish. In the eastern United States, these two species have probably established in all areas where the long-term climate allows. Annual importation from overwintering sites in Florida or coastal areas will be necessary in northern areas of the range, as has been done with alligatorweed flea beetles by the U.S. Army Corps of Engineers (Zattau, 1989). A supply of these insects should be created for use by agencies and individuals other than the Corps of Engineers.
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