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.
Nature of Damage
Economic damage. In areas other than closed-canopy forests or plantations, where cogon grass survives poorly due to shading, and heavily cultivated lands, where it is kept in check mechanically, infestations are treated by relatively costly, laborious, and repetitive control measures. Currently the most effective management strategies in the United States have involved integrating mechanical (e.g., discing, mowing), cultural (e.g., burning), chemical (e.g., herbicide applications of glyphosate and imazapyr), and revegetation methods (Shilling and Gaffney, 1995; Dozier et al., 1998). However, a single herbicide application can cost as much as $400/ha. Impacts on non-target species from herbicide application are often severe, creating disturbances that allow for the re-invasion by cogon grass or secondary invasion by other weedy species (Gaffney and Shilling, 1996). For both economical and environmental reasons, the currently recommended control strategies often are unacceptable, necessitating consideration of some form of classical biological control (Shilling and Gaffney, 1995; Dozier et al., 1998). There are only a few localized benefits of cogon grass. These include use for thatch, forage, erosion control, paper making, and bedding material for livestock. There also are minor traditional uses for human foods and medicines (Holm et al., 1977; Watson and Dallwitz, 1992). Silica bodies in the leaves, razor-like leaf margins, relatively low yields, and very low nutritive and energy values make cogon grass a poor forage (Coile and Shilling, 1993; Colvin et al., 1993).
Outside of the United States, cogon grass has been reported as a problem in more than 35 annual and perennial crops, including rubber, coconut, oil palm, coffee, date, tea, citrus, forests, field crops (rice), and row crops (corn) (Holm et al., 1977; Brook, 1989; Waterhouse, 1999). Problems with cogon grass often have arisen on lands cleared of natural forest, which are then quickly colonized by cogon grass before cultivation, during plantation establishment and growth, or soon after the abandonment of land used for short-duration shifting agriculture (Brook, 1989). Left unchecked, colonized areas become densely infested with cogon grass, are difficult to convert to other vegetation, and are fire-prone climax communities (Seth, 1970). Cogon grass infestations damage crops through competition, causing suppressed growth, reduced yields, and delayed harvests. In addition to being highly competitive, the rhizomes of cogon grass may physically injure other plants and appear to be allelopathic in certain situations (Brook, 1989; Bryson and Carter, 1993).
recruitment of other plants and changes the properties of the litter and upper soil layers (Lippincott, 1997). In Florida sandhill communities, cogon grass stands can destroy the habitat of rare species such as gopher tortoises (Gopherus polyphemus [Holbrook]) and indigo snakes (Drymarchon corais couperi [Daudin]) (Shilling et al., 1995; Lippincott, 1997). Cogon grass also is flammable and increases fine-fuel loads. Resultant fires tend to be hotter and taller, and potentially more frequent, even in communities adapted to frequent fire (e.g., longleaf pine [Pinus palustris Mill.] and wiregrass [Aristida beyrichiana Trin. and Rupr.] cover type). Extensive rhizome reserves of cogon grass enable it to quickly regrow. Also, fires induce flowering and seeding, reduce competition from other plants, and create openings for seedling establishment (Bryson and Carter, 1993; Dozier et al., 1998; Shilling et al., 1995).
Extent of losses. More than 500 million ha of cogon grass have been estimated to occur worldwide (Holm et al., 1977). In Asia, where an estimated 200 million ha are dominated by cogon grass, infested areas are increasing at a rate of 150,000 ha annually (Soerjani, 1970). At least 100,000 ha. are estimated to be infested in Alabama, Florida, and Mississippi (Dickens, 1974; Schmitz and Brown, 1994).
Cogon grass has been reported as a weed in 73 countries and on all six continents. It is widely distributed in Africa, Australia, southern Asia, and the Pacific Islands, and less extensively distributed, or a less serious problem, in southern Europe, the Mediterranean, the Middle East, Argentina, Chile, Colombia, the Caribbean, and the southeastern United States (Fig. 3). It has been found at latitiudes from 45°N (Japan) to 45°S (New Zealand), and from sea level to over 2,000 m elevation (Holm et al., 1977).
In the United States, cogon grass occurs in Florida, Georgia, Alabama, Mississippi, Louisiana, South Carolina, and Texas (Fig. 4). It is distributed throughout Florida (Langeland and Burks, 1998) and is widely distributed in Mississippi (Patterson and McWhorter, 1983; C. Bryson, pers. comm.), and southern Alabama (Dickens, 1974). It is established at some locations in Louisiana, South Carolina (Allen et al., 1991; Bryson and Carter, 1993), southern Georgia (Byrd and Bryson, 1999; Coile, pers. comm.), and in Tyler County, Texas (USGS, 1999).
A red-tipped ornamental cultivar, Imperata cylindrica ‘Rubra,’ is extensively promoted as Red Baron, or Japanese Blood Grass, in many other states (Hall, 1998; C. Bryson, pers. comm.). This red-colored ornamental cultivar can revert to the green form, which is the invasive variety (Greenlee, 1992; Dozier et al., 1998; Hall, 1998; C. Bryson, pers. comm.). Plants propagated by tissue culture, rather than division, seem particularly prone to revert to the aggressively spreading green form (Greenlee, 1992).
Imperata cylindrica ‘Rubra’ is very cold tolerant (Shilling et al., 1997), and has persisted in Michigan for several years in an ornamental garden (C. Bryson, pers. comm.). Should this cold-tolerant cultivar be introduced into the southeast and hybridize with I. cylindrica var. major, hybrids might exhibit both invasiveness and cold tolerance, allowing for significant range extension to the north and west (Shilling et al., 1997).
Background Information On The Pest Plant
The descriptions of the genus (Imperata Cirillo. Pl. Rar. Neap. 2:26 1792) and species (Imperata cylindrica [L.] Beauv., Ess. Nouv. Agrost. 7. 1812) occurred in 1792 and 1812, respectively. Gabel (1982) identified 29 synonyms for I. cylindrica.
The genus Imperata, family Poaceae, subfamily Panicoideae, supertribe Andropogonodae, tribe Andropogoneae (Gabel, 1982; Watson and Dallwitz, 1992), subtribe Saccharinae (Clayton, 1972; Campbell, 1985), includes nine species worldwide (Gabel, 1982). Hubbard et al. (1944) recognized five varieties of I. cylindrica worldwide: major, africana, europaea, latifolia, and condensata, with the most widely distributed variety, major, occurring in the United States.
The global cultural impact and importance of I. cylindrica is suggested by the nearly 100 common names given to it (Holm et al., 1977). Some of the most widely recognized of these are blady grass, alang-alang, lalang, cogon grass, and speargrass.
et al., 1977; Langeland and Burks, 1998). Panicles (6 to 22 cm long by 3.5 cm wide) are plume-like, cylindrical, dense, and silvery (Holm et al., 1977; Lippincott, 1997; Langeland and Burks, 1998). Spikelets are 3 to 6 cm long, crowded and paired on unequal stalks, with each spikelet surrounded by white hairs up to 1.8 mm in length (Bryson and Carter, 1993; Langeland and Burks, 1998)
Cogon grass is a C4 grass found mainly in tropical and subtropical areas with 75 to 500 cm of annual rainfall (Bryson, 1999). Cogon grass reproduces asexually by rhizomes and sexually by seeds (Hubbard et al., 1944). Imperata cylindrica is the most morphologically variable species in the genus Imperata (Gabel, 1982). Rhizomes are very resistant to heat and breakage, and may penetrate soil up to 1.2 m deep, but generally occur in the top 0.15 m in heavy clay soils, and 0.4 m of sandy soils (Holm et al., 1977; Bryson and Carter, 1993). Rhizome biomass can reach 40 tons of fresh weight per hectare (Terry et al., 1997; English, 1998), and regrowth potential of roots is a critical issue in development of control methodologies, including biological control. Regeneration from rhizome segments as small as 2 mm has been observed. Success of segment regeneration is determined by the original location of the segment on the rhizome, including proximity to, or inclusion of, axillary and apical buds, as well as environmental conditions (Holm et al., 1977; Wilcut et al., 1988a; Gaffney, 1996; English, 1998). Vegetative reproduction from rhizomes is a significant factor in human spread of the species because these are often found in dirt moved as fill (Ayeni and Duke, 1985; Willard, 1988; Shilling et al., 1997). Cogon grass rhizomes exhibit apical dominance (English, 1998), which may be an important factor both in limiting the local spread of cogon grass via rhizomes (Wilcut et al., 1988a), and reducing the efficacy of herbicidal control due to sub-lethal herbicide sink activity in dormant axillary buds (Shilling et al., 1997; English, 1998).
Incapable of self-pollination (Gabel, 1982), I. cylindrica produces viable seed only when cross-pollinated (McDonald et al., 1996), and the success rate of outcrossing is low (Shilling et al., 1997). Cogon grass produces as many as 3,000 seeds per plant (Holm et al., 1977). Having no dormancy, seeds are highly germinable (90% or higher), but often with low spikelet fill (less than 40%) in natural populations. Seed viability is highest for seeds less than three months old (Shilling et al., 1997). Sexually produced seeds are capable of long distance dispersal, ranging from an average of 15 m (Holm et al., 1977) to 100 m (Shilling et al., 1997). Flowering is variable between individual plants and stands, but generally occurs in spring or fall, and often in response to a range of disturbances (e.g., burning, mowing, soil disturbance) throughout the year. Flowering has been observed throughout the year in most of Florida. (Holm et al., 1977; Willard, 1988). Cogon grass seedling survivorship is low with less than 20% of emergent seedlings surviving to one year.
Cogon grass has invaded a variety of habitats, from highly xeric uplands to fully shaded mesic sites. Sandhills, flatwoods, hardwood hammocks, sand dunes, grasslands, river margins, swamps, scrub, and wet pine savanna communities all are invaded by cogon grass. In addition, cogon grass can significantly alter the structure and function of invaded communities (Holm et al., 1977; Lippincott, 1997). While cogon grass is tolerant of wide variations in soil fertility, organic matter, and moisture, it grows best in relatively acidic soils (pH 4.7) (Hubbard et al., 1944; Wilcut et al., 1988a). Moosavi-nia and Dore (1979) found that increasing shade levels of more than 50% reduces shoot dry weight and both rhizome length and dry weight; causing an increase in the shoot/rhizome ratio. Temperature markedly affects shoot and rhizome growth, with increased growth occurring at 29º/23ºC (day/night), compared to lower temperatures (Patterson et al., 1980). In general, rhizomes do not exhibit extreme cold hardiness, but stands of cogon grass have survived temperatures as low as –14ºC (Wilcut et al., 1988b). In greenhouse studies, King and Grace (2000a) found cogon grass to be most sensitive to soil saturation during early establishment (following seed germination). Cogon grass invasion by seed may therefore be limited by excessive moisture in the spring, during early seedling development. Once established, cogon grass becomes increasingly tolerant of flooding. Cogon grass seed germination rates and survival rates of newly germinated seedlings were not significantly affected by gap size or disturbance type (King and Grace, 2000b). Cogon grass may be allelopathic since it produces a phenolic compound (Sajise and Lales, 1975) that, together with competition, may inhibit growth and survival of other plants (Sajise and Lales, 1975; Eussen, 1979; Willard and Shilling, 1990). However, Lippincott (1997) suggests that other explanations may exist for the competitive success of cogon grass and that the existence of allelopathy is not certain.
Analysis of Related Native Plants in the Eastern United States
The genus Imperata belongs to the tribe Andropogoneae, in the subtribe Saccharinae Griseb. (Clayton, 1972). Of the nine species of Imperata worldwide, two occur in the eastern United States – I. cylindrica and Imperata brasiliensis Trin. A third species, Imperata brevifolia Vasey, occurs in the western United States (Gabel, 1982). Though considered by Gabel (1982) to be native to Florida, South America, Central America, southern Mexico, and Cuba, I. brasiliensis is listed as an introduced Federal Noxious Weed in the United States, as well as a state-level noxious weed in Florida and North Carolina (USDA, 1999). Imperata cylindrica has been distinguished from I. brasiliensis based on number of stamens. Imperata cylindrica has two stamens and I. brasiliensis has one stamen (Gabel, 1982). However, overlapping variability often occurs in this character, and Hall (1998) has suggested the possibility that the two species may be the same. Imperata brasiliensis and I. cylindrica have undergone human-disturbance-associated range extension (Brook, 1989; Hall, 1998). Evidence of frequent hybridization between the two species has been observed (Gabel, 1982), and all seed produced by crossing the two species in a study by McDonald germinated (Shilling et al., 1997).
Campbell (1985) agrees with Clayton (1972, 1981), placing Imperata Cirillo in the subtribe Saccharinae Griseb. Other genera in Saccharinae include Erianthus Michx. and Microstegium Nees. Dahlgren et al. (1985) placed Imperata in the Saccharum Group, including Saccharum L., Imperata, and Eulaliopsis Honda. Hitchcock and Chase (1951) consider Miscanthus Andersson, Saccharum, Erianthus, and Microstegium Nees closely related to Imperata.
Outside the genus Imperata, the species most closely related to I. cylindrica that are native to the eastern United States are five species in the genus Erianthus (Hitchcock and Chase, 1951; Clayton, 1972; Campbell, 1985). They are Erianthus strictus Baldwin (narrow plumegrass), Erianthus contortus Baldwin ex Elliot (bent-awn plumegrass), Erianthus alopecuroides (L.) Elliot (silver plumegrass), Erianthus coarctatus Fernald, and Erianthus giganteus (Walt.) Muhl. (sugarcane plumegrass). All five species occur in soil types and habitats that overlap with those of cogon grass.
While comparatively few native species are closely related to I. cylindrica, several notable non-native species should be mentioned. Mangoendihardjo and Soerjani (1978) felt that the biological control potential of cogon grass in Indonesia was limited by its close relationship to many graminaceous food plants. In the United States, the closeness of this relationship is of greatest concern with sugarcane (Saccharum spp.), with approximately 88% of domestic cane sugar production in the United States occurring in Florida, Louisiana, and Texas (Haley, 2000). Fertile, intergeneric hybrids have been procured by crossing species of Imperata with those of Saccharum (Gabel, 1982; Watson and Dallwitz, 1992).
Additionally, several species of the genera Miscanthus and Microstegium have been introduced into the eastern United States. Miscanthus sinensis (Andersson) has been identified as invading clearings in wooded areas throughout the eastern United States (Randall and Marinelli, 1996). Microstegium vimineum (Trin.) A. Camus, Japanese stilt grass, is an invasive, exotic grass currently established in sixteen eastern states (Swearington, 1997).
History of Biological Control Efforts in the Eastern United States
Area of Origin of Weed
The exact center of origin of I. cylindrica is in doubt, but is believed to be East Africa (Evans, 1987, 1991). Imperata cylindrica var. major originated in Southeast Asia and occurs throughout the tropical and warmer regions of the world, from Japan to southern China, through the Pacific islands, Australia, India, East Africa, and the southeastern United States (Holm et al., 1977). Differences in the areas of origin of the various introductions of I. cylindrica var. major in the United States are a likely source of genetic variation in the growth potential and range of different populations and ecotypes present in the United States (Patterson et al., 1980; Gabel, 1982).
Areas Surveyed for Natural Enemies
Despite the importance of the problems caused by cogon grass throughout the tropical areas of the world, biological control efforts have been few and rather piecemeal (Caunter, 1996). This weak effort can be explained, in large part, by the historical emphasis in weed biological control projects on insects as biological control agents and lack of host specific insects associated with weedy grasses (Evans, 1991; Julien and Griffiths, 1998). This has resulted in a general absence of attempted, and thus of successful, biological control projects against grasses (Waterhouse, 1999). Other complicating factors include existence of closely related grasses of economic or ecologic value (Holm et al., 1977) and potential conflict of interest with groups that value cogon grass (Evans, 1991). Similarly, little information exists on the pathogens of cogon grass and their potential as biological control agents (Evans, 1991), even though pathogens often exhibit specific host associations (McFadyen, 1998). It is likely that fungi associated with cogon grass are more diverse and abundant than indicated by herbarium records (Evans, 1991; Charudattan, 1997; Minno and Minno, 2000).
Considerable scope exists for additional field surveys, given that I. cylindrica is distributed worldwide, has five major geographical varieties, and an undetermined center of origin. Locations of potential interest would include Southeast Asia, from which the common form major is believed to have come; East Africa, believed to be the center of origin; and the Mediterranean, where the plant is not a serious weed problem.
Ravenell (1985) lists twelve pathogenic fungi identified on cogon grass in Alabama. From 1994 to 1997, field surveys looking for diseased cogon grass or related grasses in Florida collected 70 fungal isolates. Recently, Minno and Minno (1999, 2000) surveyed cogon grass in Florida, Mississippi, and Alabama, looking for native or naturalized enemies.
Limited surveys also have been made in East Africa and Southeast Asia. Surveys of I. cylindrica in East Africa did not locate any suitably monophagous insects (Evans, 1991). In Egypt (Giza), Tawfik et al. (1976), and Ammar et al. (1977) periodically surveyed I. cylindrica, and respectively found one Hemiptera species, and three planthopper species.
A review of the literature on insects associated with I. cylindica in southeast Asia concluded that none of the recorded species were promising biological control agents (Syed, 1970). However, field surveys in Java from 1973 to 1976 identified 15 species of insects associated with I. cylindrica (Mangoendihardjo, 1980). Apart from the United States, field surveys for pathogens of I. cylindica have been made only in Malaysia (Caunter, 1996).
Natural Enemies Found
Literature records and on-line databases suggest an extensive number of potential natural enemies, including pathogens, arthropods, and other invertebrates found within and outside of North America. Outside the United States, 66 pathogens (primarily fungi), 42 insects, two nematodes, and one mite have been found on I. cylindrica. Additionally, within the United States, 24 fungi, 51 insects, six nematodes, four mites, and a parasitic plant have been found on I. cylindrica, primarily by Minno and Minno (1999, 2000).
Of the arthropods recorded on cogon grass worldwide, only one is repeatedly reported to be host specific to I. cylindrica – the gall midge Orseolia javanica Kieffer and van Leeuwen-Reijnvaan (syn. = Orseoliellia javanica). Introduction of this midge to infested areas outside of southeast Asia was recommended as early as 1975 (Mangoendihardjo, 1975), but no introductions were ever made. Other invertebrates from outside of the United States that may be host specific and damaging to cogon grass include the nematode Heterodera sinensis Chen, Zheng, and Peng (Chen et al., 1996), the mite Aceria imperata (Zaher and Abou-Awad) (Zaher and Abou-Awad, 1978), and two unidentified dipteran stem borers (Mangoendihardjo, 1980).
Evans (1987, 1991) suggested that some of the known pathogens of cogon grass should be considered for introduction to the United States as classical biological control agents. Promising species include the fungi Colletotrichum caudatum (Sacc.) Peck (Caunter, 1996), which recently was found on cogon grass in Florida (Minno and Minno, 2000); Puccinia fragosoana Beltrán (USDA, ARS, 2001); Puccinia imperatae Poirault (Evans, 1987); and Sphacelotheca schweinfurthiana (Thümen) Saccardo (Evans, 1987). Other fungi known as cogon grass pathogens pose greater difficulties because of conflicting or confusing taxonomy or insufficient information. Interestingly, the smut S. schweinfurthiana is common in the Mediterranean region where I. cylindrica is not a serious problem (Evans, 1991). This smut has recently been found on I. cylindrica in Florida (Minno and Minno, 1999).
Host Range Tests and Results
The only insect enemy of I. cylindrica that has been subjected to host range testing is the gall midge O. javanica. O. javanica was studied on corn, sorghum, five species of rice, and two other grasses, and found to be specific to I. cylindrica (Mangoendihardjo, 1980). Further host range testing is necessary.
The fungus, C. caudatum proved to be host specific to I. cylindrica in limited host range tests in Malaysia (Caunter and Wong, 1988), which led to an examination of its potential as a bioherbicide in Malaysia (Caunter, 1996). Applications of spore suspensions of this fungus failed to kill whole plants, but it may be possible to enhance disease severity with the addition of yeast or other amendments. Because the two fungi currently being investigated in the United States as potential bioherbicides (i.e., B. sacchari and D. gigantea) are not specific to I. cylindrica, host range testing of the bioherbicidal mixtures are planned (R. Charudattan, pers. comm.).
above sea level (Mangoendihardjo, 1980), although infestations occur up to an elevation of 800 m (Soenarjo, 1986). Densities of O. javanica were significantly higher in areas where I. cylindrica was regularly cut or slashed (Soerjani, 1970; Soenarjo, 1986). However, parasitism of O. javanica larvae by a chalcid wasp also increased in slashed areas (Mangoendihardjo, 1980).
Females can produce from 200 to 560 eggs, which hatch two to three days after being laid and have a 98% viability rate (Soerjani, 1970; Mangoendihardjo, 1975). After hatching, less than 2% of the larvae successfully enter the plant, primarily due to predation by ants (Mangoendihardjo, 1980). Resultant infestation of I. cylindrica by the gall midge varies from 0 to 18% (Mangoendihardjo, 1975).
The larva enters I. cylindrica between the lower leaf sheaths to penetrate the shoot apical meristem, where it forms a cell in which it develops and pupates (Soerjani, 1970). In laboratory studies, only 1% of the total eggs produced survived to adulthood (approximately 50% of the larvae that entered the plant) (Mangoendihardjo, 1980).
The potential of O. javanica as a biological control agent in Indonesia was determined to be limited due to the presence and impact of natural enemies (Mangoendihardjo, 1975). Key natural enemies of O. javanica include a parasite (Hymenoptera: Platygasteridae, Platygaster sp.), which has been found to attack more than 20% of field-collected larvae; and predaceous ants, which attack gravid females, eggs, and larvae (Mangoendihardjo, 1975). Three other hymenopteran larval parasites of O. javanica are Obtusiclava sp. (Pteromalidae), Euplemes sp. (Eupelimidae), and Tetrastichus sp. (Eulophidae) (Mangoendihardjo, 1980). Pupae are parasitized by the wasp Platygaster oryzae (Cameron) (Soenarjo, 1986). In the southeastern United States, the red imported fire ant (Solenopsis invicta Buren) is one potential predator that may reduce the potential of O. javanica to suppress I. cylindrica.
Infestation by O. javanica is likely to reduce photosynthesis due to leaf blade reduction, leading to lower rhizome carbohydrate reserves. Infestation by O. javanica also may vector various pathogens. However, because O. javanica does not directly harm the plant’s rhizomes, it is unlikely to control the plant by itself (Brook, 1989).
Recommendations for Future Work
The gall midge O. javanica needs to be evaluated for potential introduction into the United States. It is likely to be highly host specific and may cause more damage to infested plants than suggested by the amount of tissue consumed. Removed from its native parasites and predators, O. javanica may prove to be an effective biocontrol agent in the United States, as occurred with the Australian bud-galling wasp, Trichilogaster acaciaelongifoliae Froggatt, released in South Africa against Acacia longifolia (Andr.) Willd. (Center et al., 1995). However, gall midges are notoriously parasitized by generalist parasitoids after introduction, severely limiting their effectiveness (B. Blossey, pers. comm.).
Secondly, DNA fingerprinting (Amplified Fragment Length Polymorphisms) should be used to identify the native range of U.S. cogon grass varieties. This information could then be used to direct survey efforts to areas most likely to have the widest range of natural enemies (Evans, 1987).
Thirdly, intensive surveys of natural enemies of the native I. brevifolia in the southwestern United States might yield indigenous biological control agents able to attack I. cylindrica if introduced into the southeast. Interestingly, I. brevifolia is a minor component of the flora where it occurs in the southwest and has been difficult to cultivate, unlike cogon grass. However, preliminary, limited surveys of I. brevifolia in 2000 failed to identify any potentially useful natural enemies.
In addition to the above, work with existing and new pathogens is needed both in the area of developing effective bioherbicides and to explore the potential of possible introductions of host specific foreign pathogens.
Lastly, the most commonly practiced method of biological control of I. cylindrica in southeast Asia is the use of competitive vegetation (Soerjani, 1970). To control cogon grass in the United States, an integrated program of biological control and revegetation with more desirable species will be needed (Shilling et al., 1998).
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