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Home , Argyractis, Athetis, Auleutes (beetle), Lemna valdiviana, Mansonia dyari, Namangana

5 WATERLETTUCE

F. Allen Dray, Jr. and Ted D. Center

U.S. Department of Agriculture, Agricultural Research Service, Invasive Research Laboratory, Fort Lauderdale, Florida, USA

PEST STATUS OF WEED

Waterlettuce, stratiotes L., (Fig. 1) is a floating, herbaceous hydrophyte first recorded from Florida during the 18th century (Stuckey and Les, 1984). It forms extensive mats (Fig. 2) capable of blocking navi- gational channels, impeding water flow in irrigation and flood control canals, and disrupting submersed and plant communities (Sculthorpe, 1967; Attionu, 1976; Bruner, 1982; Sharma, 1984). Waterlettuce is among the world’s worst weeds (Holm et al., 1977). It has been placed on prohibited Figure 2. A severe waterlettuce (Pistia stratiotes plant lists in Florida (FDEP, 2000), Louisiana L.) infestation on Lake Okeechobee in (LDWF, 2000), Mississippi (MDAC, 1997), and Texas southern Florida. (Photograph courtesy of (TPWD, 2000), and is considered a noxious USDA, ARS Invasive Plant Research Laboratory.) (an of concern, but not regulated) in South Carolina (SCDNR, 2000) and Delaware Nature of Damage (DDFW, 2000). Economic damage. Waterlettuce is a serious weed of rice crops in other countries (Suasa-Ard, 1976), but has not been reported as interfering with production in the . It also can interfere with hy- droelectric operations (Napompeth, 1990), but has not done so in the United States. Consequently, di- rect losses attributable to waterlettuce result prima- rily from restricted water flow in irrigation and flood control canals in Florida. Unfortunately, the eco- nomic costs associated with such damage have not been quantified, but federal and state waterlettuce control operations in Florida cost nearly $650,000 annually (Center, 1994). Other states treat intermit- tently as nuisance populations arise, but seldom more than a few hundred acres each year. Estimates of expenditures by local agencies and private agricul- tural interests are unavailable. Indirect losses accrue when large floating mats Figure 1. The waterlettuce, Pistia stratiotes L. (Photograph interfere with recreational activities such as boating courtesy of USDA, ARS and fishing, but these have not been quantified. Also, Invasive Plant Research several species of mosquitoes that breed on Laboratory.) waterlettuce are important vectors of malaria, 65 Biological Control of Invasive in the Eastern United States encephalitis, and filariae (Dunn, 1934; Bennett, 1975; However, the implications of this finding for hydro- Lounibos and Dewald, 1989; Lounibos et al., 1990). logic cycles in U.S. waterways has not been deter- Outbreaks of St. Louis encephalitis are generally rare; mined. there were 223 reported cases with 13 deaths in Extent of losses. In Florida, waterlettuce infests Florida in 1990 and nine cases (one death) in 1997 about 2,500 acres of public waterways (after control (FDOH, 2000a). Equine encephalitis, also vectored operations), and a large, but uncounted number of by mosquitoes associated with waterlettuce, affects acres of irrigation and flood control canals (Schardt, about 50 horses in Florida each year as well (FDOH, 1992). Based on the annual costs associated with con- 2000b). Costs associated with these diseases are un- trolling waterlettuce on at least 10,000 acres of pub- known, and the portion of control opera- lic waterways (Schardt, 1992; Center, 1994), it is rea- tions directed toward waterlettuce-borne mosquitoes sonable to estimate that total expenditures exceed $1 has not been reported. million annually in Florida. Other states in the east- Ecological damage. There are few reports of del- ern United States spend a combined total of less than eterious ecological impacts associated with P. $100,000/yr on waterlettuce control. stratiotes infestations and these studies have gener- ally been limited in scope. Sculthorpe (1967), for in- Geographical Distribution stance, noted that the intertwined root systems (com- Paleobotanical evidence suggests that prior to the posed of long adventitious roots arrayed with copi- the Pistia extended well beyond its ous lateral rootlets) of extensive infestations acceler- present range into what is now temperate Asia, Eu- ate siltation rates as they slow water velocities in riv- rope, and North America (Dorofeev, 1955, 1958, ers and streams (see also Anonymous, 1971). The 1963; Friis, 1985; Mai and Walther, 1983; Stuckey and resultant degradation of benthic substrates under Les, 1984; Stoddard, 1989). Today, waterlettuce is these infestations has never been studied directly, but primarily pan-tropical (Sculthorpe, 1967; Holm et al., accelerated siltation often renders the affected 1977), although it also occurs in the Netherlands benthos unsuitable as nesting sites for various where it dies back in winter and then reinfests from species (Beumer, 1980) and as macroinvertebrate each spring (Pieterse et al., 1981). This habit habitat (Roback, 1974). The accumulation of could permit populations to persist in states with cold waterlettuce-generated detritus under large infesta- temperate climates. Populations have been recorded tions only adds to this problem, and likely increases from as far north as the Erie Canal in upstate New sediment and nutrient loadings much as it does un- York and Lake Erie in northern Ohio (Mike Weimer, der waterhyacinth mats (Schmitz et al., 1993). Fur- US Fish and Wildlife Service, Buffalo, New York, and thermore, Sridhar (1986) reports that waterlettuce can Doug Wilcox, US Geological Survey, Great Lakes bioaccumulate considerable amounts of heavy met- Science Center, Ann Arbor, Michigan, pers. comm.). als, so the detritus under some mats could be toxic. Subtropical Florida harbors the most abundant The waters under dense waterlettuce popula- waterlettuce populations in the eastern United States tions in lakes can become thermally stratified (Fig. 3). Other principal infestations occur in the (Sculthorpe, 1967; Attionu, 1976), with reduced dis- warm temperate regions of the Gulf Coast states solved oxygen levels and increased alkalinity (Yount, (Godfrey and Wooten, 1979), with the exception of 1963; Attionu, 1976; Sridhar and Sharma, 1985). Pro- Alabama (Kartesz, 1999). Scattered ephemeral popu- longed oxygen deficits reduce plankton abundance lations – those that occur outside the naturalized (Hutchinson, 1975), and cause increased mortality of range of waterlettuce and are of relatively recent ori- fish (Ayles and Barica, 1977; Clady, 1977) and gin, but which have been present for several years – macroinvertebrates (Roback, 1974; Cole, 1979). Al- have been recorded from Virginia, North Carolina, though these effects likely occur in waterlettuce- South Carolina, Mississippi, and northern Louisiana dominated systems, they have not been investigated. (USGS 2001). Some of these populations may persist Finally, Sharma (1984) reported that the evapotrans- over winter in the form of seeds, but others are likely piration rate over a waterlettuce mat in one African being re-introduced each year. A few occasionally lake was ten-fold greater than the evaporation rate achieve nuisance proportions. Waterlettuce also oc- over open water (but see the discussion on this topic curs in California, Arizona, Puerto Rico, the Virgin and common misconceptions in Allen et al., 1997). Islands, and Hawaii (Degener, 1938; Kartesz, 1999;

66 Waterlettuce

Figure 3. The distribution of waterlettuce, Pistia stratiotes L., in drainage basins (USGS Hydrologic Unit 8) in the United States (data from USGS, 2001).

USGS, 2001), but we have been unable to confirm male above and a single female below reports (Kartesz, 1999) of isolated occurrences in the constriction. Fruits are many-seeded green ber- Georgia, Maryland, and New Jersey. ries, and the mature coat is thick, golden-brown, and wrinkled. Pistia is a monotypic genus in the subfamily BACKGROUND INFORMATION (Grayum, 1990). There are at least two ex- ON PEST PLANT tinct species: Pistia siberica Dorofeev (Dorofeev, 1955, 1958, 1963) and Pistia corrugata Lesquereux (Stockey et al., 1997). The genus also is closely associated with Waterlettuce is a perennial herb in the aroid family the genus Limnobiophyllum Krassilov, through (). The plant consists of free-floating rosettes which it is related to the Lemnaceae (Kvacek, 1995; of many leaves. The rosettes occur singly or con- Stockey et al., 1997). nected by short stolons. Leaves are gray-green, Biology densely pubescent, and wedge shaped (obovate-cu- neate). They have conspicuous parallel veins, fre- Waterlettuce inhabits lakes, ponds, canals, and slow- quently have thick spongy parenchymous tissue at flowing streams. The rosettes are perennial along the the base, and vary from being slightly broader (at Gulf Coast, but act as annuals in more temperate apex) than long to much longer than broad. Leaves zones. Waterlettuce exhibits seasonal growth in range from 2 to 35 cm long. Roots are numerous and Florida with high rosette densities during winter and feathery. The inflorescences are inconspicuous pale- spring, and low densities during late summer and green spathes near the center of the rosette. Each early autumn (Dewald and Lounibos, 1990; Dray and spathe is constricted near the middle, with a whorl of Center, 1992). Conversely, leaf size, leaf density per 67 Biological Control of Invasive Plants in the Eastern United States

rosette, and total biomass increase during spring and (Grayum, 1990). Lasioideae is the subfamily most summer then begin to decline during late autumn closely allied with the Aroideae, and it contains two (Dewald and Lounibos, 1990; Dray and Center, genera with native representatives in the east: 1992). Population expansion is primarily by vegeta- Orontium and Symplocarpus (one species each) tive propagation. Up to 15 secondary rosettes may (Grayum, 1990; USDA, 1999). The other aroid sub- be attached to a single primary plant, and up to four family with native genera is the Calloideae, which is generations of rosettes may be interconnected by sto- represented by Calla (one species) and (two lons (Dray and Center, 1992). Standing crops may species). An examination of the conservation status reach 2,000 g/m2 by the end of the growing season of the Araceae shows that half of the 26 species in the (Dray and Center, 1992). Flowering occurs year- eastern United States are considered imperiled in at round in southern Florida, but peaks during sum- least one state where Pistia occurs: Spirodela mer and early autumn (Dray and Center, 1992). Dray polyrrhiza (L.) Schleiden, brasiliensis Weddell, and Center (1989) reported a crop of 726 seeds/m2 Wolffia columbiana Karst., gibba L., Lemna on the rosettes at one site. The hydrosoil under that perpusilla Torr., Lemna trisulca L., Lemna valdiviana waterlettuce infestation held 4,196 seeds/m2. Mature Phil., Wolffiella oblongata (Phil.) Hegelm., seeds in fruits had an 84% germination rate, as did Symplocarpus foetidus (L.) Nutt., Orontium seeds in the upper 15 cm of the hydrosoil (Dray and aquaticum L., Arisaema dracontium (L.) Schott., Calla Center, 1989). Historically, waterlettuce has been palustris L., and Peltandra sagittifolia (Michx.) known to form large floating islands, nearly block- Morong. (ABI, 2000). The latter five species do not ing upper reaches of the St. Johns River (Stuckey and occur in the same habitat as Pistia, however. Les, 1984), but these are uncommon today. Sculthorpe (1967) attributes this to suppression of waterlettuce by waterhyacinth after the latter was HISTORY OF BIOLOGICAL CONTROL introduced into Florida during the late 19th century EFFORTS IN THE EASTERN (see also Stoddard, 1989). Competition experiments UNITED STATES between the two species support this conclusion (Tag el Seed, 1978; Agami and Reddy, 1990). Area of Origin of Weed Analysis of Related Native Plants in the Eastern Grayum (1990) suggested that Pistia is an ancient genus with subtropical Laurasian origins, which then United States migrated into tropical West Gondwanaland. This Recent molecular phylogenetic analyses have unified view is supported by recoveries of fossil Pistia spe- the Lemnaceae within the Araceae, and shifted the cies in strata from the Upper Cretaceous Period (103 aroids into the order (Bremer et al., 1999; to 65 million years ago [MYA]) in the United States Chase et al., 2000). The resulting family contains (Wyoming and North Carolina) and southern France, more than 2500 species in about 150 genera and in strata from the Tertiary Period (65 to 2.5 MYA) (Zomlefer, 1994) and is distributed primarily in the southern United States and western Siberia throughout the tropics. Kartesz (1999) lists 40 na- (Stoddard, 1989). The colder climates associated with tive aroid species in 16 genera for the United States, the Pleistocene Epoch (2.5 to 0.01 MYA) undoubt- many (12 species in six genera) of which are limited edly forced a sharp contraction of the genus’ distri- to Puerto Rico and the Virgin Islands. Nine of the bution worldwide. Stoddard (1989) argues that remaining genera, containing a total of 26 species, Florida served as a refugium for Pistia during this occur in the eastern continental United States (USDA, period, and that the genus is therefore native to the 1999). Among these, Pistia forms a monophyletic United States. However, July temperatures in the group with the duckweed (Lemnaceae) genera southeastern United States averaged 12°C lower dur- (Stockey et al., 1997), all of which have species native ing the Pleistocene than today (Watts, 1980) and win- to the eastern United States (Spirodela, two species; ters were almost certainly punctuated by severe Lemna, nine species; Wolffia, four species; Wolffiella, freezes, so it is likely that the genus became extinct three species) (USDA, 1999). Pistia’s next closest af- in the United States (Stuckey and Les, 1984). Sup- finities are with Arisaema (three species), which, like port for this hypothesis is found in the paucity of waterlettuce, belongs to the subfamily Aroideae specialist herbivores found on waterlettuce in Florida 68 Waterlettuce as compared to other regions of the world (Dray et whether this is a herbivore. Eleven phytopha- al., 1993). For example, hosts at least gous , including eight species (one of thirteen specialist phytophagous insects (Dray et al., which is a specialist – pectinicornis 1993) and at least two mosquitoes that are oviposi- [Hampson]), have been observed feeding on tional specialists (Lounibos et al., 1992), which sug- waterlettuce in Asia. Nine insects feed on gests a lengthy tenure on that continent (Bennett, waterlettuce in Florida, including a moth (Argyractis 1975). Also, ancient folk medicines using Pistia are [=Petrophila] drumalis [Dyar]) whose larvae only known from and Asia (Stoddard, 1989), argu- have been found on waterlettuce roots (Dray et al., ing for their antiquity in these regions. The extent of 1989; Dray et al., 1993; Habeck and Solis, 1994). P. stratiotes’ distribution in Florida by the mid-18th century suggests that re-introduction into the United Host Range Tests and Results States occurred soon after European settlements were Host range trials have been conducted on several of established (Stuckey and Les, 1984). the Neotropical species, a Neotropical grass- Areas Surveyed for Natural Enemies hopper, and two moth species (one Asian and one Neotropical). DeLoach et al. (1976), Harley et al. Few surveys for natural enemies have specifically tar- (1984), and Thompson and Habeck (1989) studied geted P. stratiotes, aside from searches in Florida the host range of the weevil Neohydronomus affinis (Dray et al., 1988, 1993). However, several general Hustache (as N. pulchellus Hustache), testing a total surveys in India (Rao, 1964, of 89 species in 66 genera and 37 families. Aside from 1970; Sankaran and Rao, 1974), waterlettuce, only duckweed (Spirodela and Lemna) Indonesia (Mangoendihardjo and Nasroh, and frogbit (Limnobium) species sustained any ovi- 1976; Mangoendihardjo and Soerjani, 1978; position or meaningful feeding. As noted above, the Mangoendihardjo et al., 1979), and Thailand duckweeds and Pistia group together in a single clade (Napompeth, 1990) noted the occurrence of herbi- within the Araceae, and Limnobium has spongy tis- vores on waterlettuce. Similarly, biological control sues similar to Pistia (Thompson and Habeck, 1989) scientists conducting surveys on Salvinia spp. and as well as being in a family (Limnocharitaceae) closely waterhyacinth in South and Central America re- related to the aroids (Chase et al., 1995). Cordo et al. corded natural enemies of waterlettuce (Bennett, (1978) reported that adults of the weevil 1975; DeLoach et al., 1976, 1979; Cordo et al., 1978, Argentinorhynchus bruchi (Hustache) fed and ovipos- 1981; Cordo and DeLoach, 1982). Further, ecologi- ited, and larvae completed development, only on cal studies of the Argentine waterlettuce fauna pro- waterlettuce (with very slight feeding on Spirodela duced a few observations on herbivorous species (Poi intermedia W. D. J. Koch) among the 31 plant spe- de Neiff and Neiff, 1977; Poi de Neiff, 1983). Natu- cies (21 genera, 12 families) they tested. Host range ral enemies have seldom been reported from Africa trials conducted by Cordo et al. (1981) demonstrated despite the presence of waterlettuce there for several that the weevil Pistiacola (as Onychylis) cretatus millennia (Stoddard, 1989). (Champion) has a diet similar to N. affinis. These authors also reported that the weevil Ochetina bruchi Natural Enemies Found Hustache has a broad food-host range, but failed to Dray et al. (1993) and Center (1994) discuss the her- identify its developmental host. bivorous entomofauna reported from P. stratiotes Larvae of the pyralid moth multiplicalis worldwide. Among the species known or suspected (Guenée) fed on eight of 17 species (15 genera, 11 to be plant-feeders, 44 include waterlettuce in their families) included in two separate host range studies, diets at least occasionally. The Neotropics harbor but adults oviposited almost exclusively on 21 waterlettuce-feeding insects, including at least 14 waterlettuce (Knopf and Habeck, 1976; DeLoach et species of – many of which are known only al., 1979). Food hosts are summarized by Center from this plant. Five waterlettuce herbivores have (1994). The noctuid moth S. pectinicornis (= been reported from Africa, including a weevil (Bagous hennia, variously placed in the genera Xanthopter, pistiae Marshall) known exclusively from P. stratiotes. Athetis, Namangana, and Episammia) was tested The African fauna also contains a collembolan known against a total of 125 plant species (in 103 genera and exclusively from waterlettuce, but it is unclear 49 families), but completed development only on 69 Biological Control of Invasive Plants in the Eastern United States

waterlettuce (Mangoendihardjo and Nasroh, 1976; Suasa-Ard, 1976; Habeck and Thompson, 1994). Feeding and oviposition also were largely confined to this plant. The acridid grasshopper Paulinia acuminata (De Geer) feeds and develops on the waterferns (Salvinia spp. and Azolla sp.) as well as waterlettuce (Bennett, 1966; Vieira, 1989). Releases Made Only two insects have been released into the United States as biological control agents against this weed, the South American weevil N. affinis and the Asian Figure 4a. moth S. pectinicornis (Dray et al., 1990, 2001).

BIOLOGY AND ECOLOGY OF KEY NATURAL ENEMIES Neohydronomus affinis Hustache (Coleoptera: ) Adult N. affinis (Fig. 4a) weevils are small (3 mm long) and have a nearly straight rostrum that is strongly constricted ventrally at the base. Neohydronomus affinis ranges in color from uniform bluish gray to reddish brown with a tan, chevron-like band across the elytra. Further information on the identification Figure 4b. of this species may be found in DeLoach et al. (1976) or O’Brien and Wibmer (1989). The following sum- mary of this weevil’s biology is based on DeLoach et al. (1976) and Thompson and Habeck (1989). Eggs are cream colored and subspherical (0.33 x 0.4 mm). Females chew a hole about 0.5 mm diam- eter in the leaf (usually on the upper surface near the leaf edge), deposit a single egg inside this puncture, and close the hole with a black substance. Eggs hatch within four days (at temperatures above 24oC). Young larvae (Fig. 4b), which are very small (head diameter of 0.2 mm), burrow under the epidermis and work their way toward the spongy portions of the leaf at a rate of about 1.5 to 2.0 cm/day. Figure 4c. Larval mines (Fig. 4c) often are visible in the Figure 4. Adult (a) and larvae (left) and outer third of the leaf where tissues are thin, but are (right) (b) of the waterlettuce weevil less apparent in the central and basal portions of the Neohydronomus affinis Hustache. Mining by leaf. The first molt occurs when larvae are about three larvae produces characteristic tunnels (c). days old, the second molt occurs 3 to 4 days later. (Photograph courtesy of USDA, ARS The three larval stages last 11 to 14 days in total. Invasive Plant Research Laboratory.)

70 Waterlettuce

Third instars are generally found excavating the spongy portions of the leaf where they pupate. Un- der optimal temperatures, 4 to 6 weeks are required for N. affinis to develop from egg to adult. Adults chew holes (about 1.4 mm in diameter) in the leaf surface and burrow in the spongy tissues of the leaf. The characteristic round feeding holes are easily ob- served when weevil populations are large (several hundred insects per m2), but may be concentrated near leaf edges and more difficult to observe when populations are small. (Guenée) (: Figure 5a. Pyralidae) The following section summarizes the biology of the pyralid moth S. multiplicalis based on observations of several authors (Knopf and Habeck, 1976; Deloach et al., 1979; Center et al., 1982; Sands and Kassulke, 1984). Adults are small (wingspread about 17 mm), tan with dark markings on fore and hind wings (Fig. 5a). Females each lay about 150 eggs during their brief life span (four to seven days). Eggs most often are laid singly among the epidermal host plant hairs on the lower surfaces of waterlettuce leaves or the upper surface of Salvinia leaves, or lodged be- Figure 5b. tween the scale-like leaves of Azolla. Eggs hatch in Figure 5. Adult (a) and larvae (b) of the about four days (at 28o C). Larvae (Fig. 5b) may feed waterlettuce moth Samea multiplicalis from within a refugium made of silk and hairs of the (Guenée). (Photograph courtesy of USDA, host plant attached to the external leaf surface, or ARS Invasive Plant Research Laboratory.) within galleries in the leaves (waterlettuce). The ref- ugium, when present, consists of a silk canopy Populations of S. multiplicalis tend to be spo- stretched across the surface of the leaf. Larvae peri- radic, possibly due to high rates. None- odically extend the area covered to reach fresh leaf theless, densities can become exceedingly high dur- material. Larger larvae feed on the buds of plants, ing intervals of peak abundance. If this coincides with often killing the growing apex. Larvae also will eat cooler periods and correspondingly slow waterlettuce mature waterlettuce fruits and consequently destroy growth, massive destruction of the mat results. None- the enclosed seeds. The larval stage is composed of theless, because of lack of persistence by this species, five to seven instars, which require 15 to 16 days for the waterlettuce mats normally recover later during development at 28oC when fed waterlettuce or S. the growing season. o minima and 21 to 35 days at 26 C when fed S. molesta. (Hampson) Pupation occurs within a silken cocoon. On (Lepidoptera: ) waterlettuce, this cocoon usually is formed within the spongy portion of a leaf, but on S. molesta it is con- Several authors have reported on the noctuid moth structed among old leaves. Pupal development re- S. pectinicornis (Fig. 6a) (George, 1963; Suasa-Ard, quires four to seven days at 28oC on waterlettuce and 1976; Mangoendihardjo and Soerjani, 1978; Suasa- eight to nine days at 26oC on S. molesta. The total Ard and Napompeth, 1982; Habeck and Thompson developmental times (egg to adult) are 25 and 42 days 1994). The following section summarizes their ob- under the two respective temperature/host plant regi- servations. Female S. pectinicornis oviposit on both mens. surfaces of waterlettuce leaves. Eggs are laid in masses

71 Biological Control of Invasive Plants in the Eastern United States

(Fig. 6b) of up to 150 eggs each (average 94 eggs per mass) and covered by a substance produced by the female, perhaps scales from her abdomen. Oviposi- tion lasts two to six days and each female lays up to 990 eggs (average 666 eggs per female). The incuba- tion period ranges from three to six days (average 4.4 days). Eggs are subspherical, about 0.03 mm in di- ameter, greenish when newly deposited, and turn yellow as they develop. First instars are creamy white and feed within the leaf on the spongy tissues. Larval development progresses through seven instars and requires 17 to Figure 6a. 20 days (average 18 days). Fully-grown larvae attain lengths of up to 25 mm. They pupate in a leaf base or between the leaves, or between the thick ribs on the underside of the leaf. The pre-pupal period lasts one to two days and the pupal stage lasts 3.5 to 5.5 days. Total generation time is about 30 days. feeding causes plant destruction. Although considerable damage accrues on leaves (Fig. 6c), this alone probably would not kill plants. How- ever, larvae also destroy meristematic tissue, which prevents leaf replacement and impedes asexual repro- duction. George (1963) estimated that one hundred from one average-sized egg mass could Figure 6b. destroy the waterlettuce within a 1 m2 area. He also calculated that a single caterpillar, during its larval development, eats two sizable waterlettuce rosettes at a rate of one leaf per day. In India, periods of peak S. pectinicornis occur- rence coincide with monsoons and with periods of rapid waterlettuce growth. During these periods, moth infestations occur at most sites and the destruc- tion to waterlettuce mats frequently exceeds 75%. During dry periods, fewer sites are infested and smaller proportions of the waterlettuce populations are affected. However, moth populations are report- edly present all year and produce continuous, over- lapping generations. Synclita obliteralis (Walker) (Lepidoptera: Pyralidae) The following information is derived from Lange Figure 6c. (1956), Kinser and Neunzig (1981), and Habeck Figure 6. Adult (a) and egg mass (b) of the (1991). Adults (Fig. 7a) are small moths; males are waterlettuce moth Spodoptera pectinicornis distinctly smaller (wingspread 11 to 13 mm) than the (Hampson). Damage from larval feeding (c) females (wingspread 15 to 19 mm). The wings of can destroy leaves. (Photograph courtesy of males are dark in coloration, but interspersed with USDA, ARS Invasive Plant Research brown and white markings. The wings of females Laboratory.)

72 Waterlettuce are paler grayish brown with orange and dark mark- vae obtain oxygen through their skin. Cases of older ings. larvae are airfilled. Larvae extend the anterior por- The whitish eggs are oval and flattened, appear- tion of their bodies out of the case to feed on sur- ing domelike. They are laid near edges of submersed rounding plants. They abandon smaller cases as they leaf-surfaces of aquatic plants and are placed singly grow larger, and then cut pieces from new leaves to or slightly overlapping, often in ribbon like masses. construct larger cases. Larvae (Fig. 7b) reside between two roundish pieces Unlike most nymphulines, larvae of S. obliteralis of leaves that form a sandwich-like portable case. lack tracheal gills. The general body color is creamy When feeding on small plants, these cases can consist white grading into brownish anteriorly (towards the of whole leaves or even whole plants. Cases are usu- segments that protrude from the case). The epider- ally, though not exclusively, constructed from the mal surface is textured with minute papillae that cre- plant species on which the larva is feeding. The cases ate a distinctive satiny appearance. The head is yel- made by young larvae are waterfilled, and these lar- lowish or brownish with patches of slightly darker coloration. Before pupation, larvae attach their cases to leaves of aquatic plants either above or below the water surface. They then spin cocoons within the cases in which to pupate.

EVALUATION OF PROJECT OUTCOMES Establishment and Spread of Agents Dray et al. (1990) describe the release and establish- ment of the weevil N. affinis, which was initially re- leased at seven sites in southern Florida in 1987 and 1988. Populations established at four of these sites within a year. By fall of 1990, the weevils had dis- persed to waterlettuce-infested canals and ponds up to 25 km from initial release sites on Lake Okeechobee (Dray and Center, 1992). Collabora- Figure 7a. tors with the South Florida Water Management Dis- trict and the Florida Department of Environmental Protection collected infested waterlettuce plants and transplanted them into about 30 additional waterways throughout the state in the spring of 1989 (Dray and Center, 1992). The weevil also was recovered from several sites in southern Louisiana during surveys in spring and summer 1990, although how it arrived there remains unclear (Grodowitz et al., 1992). Sur- veys during the fall of 1991 showed N. affinis popu- lations had become established at 45 sites in Florida and six sites in Louisiana (Dray and Center, 1993). The weevil also was released at one site in Texas in Figure 7b. the fall of 1991 (Grodowitz et al., 1992). The moth S. pecitnicornis was released at 22 sites Figure 7. Adult (a) and larvae (b) of the waterlily leafcutter Synclita obliteralis in southern Florida from 1990 to 1997 (Dray et al., (Walker) which feeds on waterlettuce. 2001). Several provisionally established populations (Photograph courtesy of USDA, ARS developed, but ultimately failed to persist (Dray et Invasive Plant Research Laboratory.) al., 2001).

73 Biological Control of Invasive Plants in the Eastern United States

Suppression of Target Weed Recovery of Native Plant Communities Neohydronomus affinis has produced dramatic de- There have been no studies investigating the re-emer- clines (up to 90%) in waterlettuce abundance at five gence of native plant communities at sites where sites in Florida (Fig. 8) and two in Louisiana (Dray waterlettuce control has occurred. and Center, 1992, 1993). Long-term suppression of this weed has not occurred, however, although in at Economic Benefits least one site in Florida there were annual cycles from There are no known economic benefits accruing from 1990 to 1994 in which spring increases in waterlettuce this project. abundance were followed by sharp declines attribut- able to the weevil (Dray, unpub.). Plants under stress from weevil feeding are typically smaller, have fewer RECOMMENDATIONS leaves, and grow less rapidly than un-infested plants FOR FUTURE WORK (Dray and Center, 1992). Future Needs for Importation or Evaluation Although the weevil N. affinis has been used success- fully in other countries, it has had only limited effect in Florida (Dray and Center 1992). Furthermore, the moth S. pectinicornis has failed to establish (Dray et al. 2001). Hence, new biocontrol agents are needed. Many additional natural enemies are known from South America that should be studied further to as- sess their value. Waterlettuce has never been thor- oughly surveyed for natural enemies, having gener- ally been a side project of research focused on Figure 8a. waterhyacinth or . Hence, it is an- ticipated that intensive faunal surveys would reveal many more potential biological control agents.

REFERENCES

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