Great efforts have been made to control saltcedar in the United States since the mid-1940's (Sisneros 1990). Most chemical and mechanical controls have failed because of the ability of saltcedar to rapidly resprout from below-ground buds and to rapidly reinvade by the small, abundant, wind-blown seeds.
Foliage application of Arsenal® 1 Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. (Imazapyr) provides very effective control but also kills most other plant species except legumes. Cutting by hand, followed by stump treatment by Garlon®1 (Triclopyr) also is very effective but is highly labor intensive. Controlled burning kills the aerial parts but the plants rapidly resprout and then must be treated with herbicides. Burning also kills many associated beneficial plants, including all of the very valuable cottonwoods. All of these methods are expensive, require repeated applications to the entire infested area, and often result in severe damage to other vegetation, the preservation and increase of which (for wildlife habitat) often is the objective of control (see papers by John Smith and Suzanne Baird, this workshop).
Biological control, by the introduction of the host-specific insects or pathogens that damage saltcedar within its native distribution in Eurasia, promises effective, highly specific, permanent and relatively inexpensive control. It would not harm any species other than saltcedar, would not pollute the environment, and would allow natural revegetation by the native riparian plants needed by wildlife.
The suitability of saltcedar as a target for biological control was determined through an analysis of the amount of damage caused vs. its beneficial values (both economic and ecological) and its potential for successful control (Brown 1989, DeLoach 1990).
Damage caused - The damages caused by saltcedar are enormous, particularly to the plant and animal communities in the riparian ecosystem. Saltcedar has almost completely replaced the native vegetation along many western river and lake shores, with the worst infestations being in Arizona, New Mexico, western Texas, Nevada, and Utah; serious infestations also are present in Colorado, California, Oklahoma, Kansas, Wyoming, Montana, and Washington (Robinson 1965).
Some of the most severe infestations are along the Gila and Salt Rivers in Arizona, the Rio Grande and Pecos Rivers of New Mexico and Texas, the Virgin and Green Rivers of Utah, and the Humboldt and Walker areas of Nevada. In other large river basins, such as the lower Colorado of California-Arizona and the Brazos of Texas, saltcedar makes up 50% or more of the vegetation. Many smaller rivers, tributaries and lake shores are seriously infested. Robinson (1965) estimated that one million acres of riparian ecosystems would be occupied by saltcedar by 1970. The area is substantially larger today and the plant is still increasing and spreading into new areas. The saltcedar invasion has seriously damaged many nature reserves in the Southwest (Loope et al 1988).
In infested areas, saltcedar often occurs as dense thickets 15 to 20 feet high, and with 100% canopy cover, with a complete absence of other plants except in not-so-dense areas near the edges. The native cottonwoods (Populus spp.) and willows (Salix spp.), which are the prime wildlife habitat, and seepwillow baccharis (Baccharis salicifolia) have nearly disappeared in many areas; these are riparian obligate species and are not tolerant of saline soils or groundwater. Screwbean mesquite (Prosopis pubescens), another riparian obligate, is somewhat salt-tolerant but has also been seriously reduced. Honey mesquite (Prosopis glandulosa) and velvet mesquite (Prosopis velutina) have fared somewhat better by being only facultative riparian species and also by being somewhat salt tolerant (Horton and Campbell 1974; Robinson 1965). The lower stratum of grasses and forbs is essentially non-existent under the saltcedar canopy because of the combination of salt excretion from the foliage, dense shade and accumulation of saline litter. Saltcedar is a vigorous invader of wetlands where it shades out many native species, reduces their reproductive potential and contributes to the loss of biodiversity (Lovich and de Gouvenain, in press.)
Many native birds and mammals are unable to utilize saltcedar, very few native insects can develop on it, and the alterations produced by saltcedar reduce habitat value and threaten the existence of many species of fish and other animals. No native birds or other animals are able to feed on the tiny fruits and seeds and the lack of insects further limits their food supply and that of fishes.
On the lower Colorado River, the total number of all birds in saltcedar during the critical winter months was only 39% that in the average of cottonwood/willow, screwbean and honey mesquite (Anderson and Ohmart 1977); numbers from spring to fall were only 48 to 74% that in other vegetation. Species richness in saltcedar was only 48% that in the other vegetation during the winter and 59 to 93% from spring to fall. Effects on bird density and species richness in other river systems (Gila, Rio Grande, Pecos) is also severe, though less so than on the lower Colorado, especially if saltcedar is less dense and other vegetation in more abundant (Hunter 1984; Hink and Ohmart 1984; Hildebrandt and Ohmart 1982; Engle-Wilson and Ohmart 1978).
On the lower Colorado River, a few bird species showed a preference for saltcedar, or at least did not avoid it; these were ground feeders, granivores, or species that fed largely in nearby agricultural habitats (Cohan et al 1978). Half of the bird preference for saltcedar was by the summer tanager, nearly all of which was in athel which Hunter (1984) included with saltcedar. On the Pecos River, 34% of all birds in saltcedar were white-crowned sparrows that use it only for cover but feed in nearby areas of annual plants. If these and other such birds that occur in saltcedar but don't use it are excluded, bird preference would be substantially lower than calculated. Cohan et al (1978) also found that insectivores, and especially frugivores, were most intolerant of saltcedar, probably because of the lack of fruits and insects for food; cavity dwellers (including woodpeckers) are almost totally absent.
The effects of saltcedar on game birds has been less drastic and/or of little consequence. The white-winged dove is able to nest equally well in saltcedar as in the velvet mesquite it replaced; the doves fly out to nearby grain fields or areas of native herbaceous vegetation to feed and do not need to feed within the nesting habitat (Cottam and Trefethen 1968). The mourning dove is a generalist, able to nest in many habitats (Keeler et al 1977). Gamble's quail prefers areas of thorn scrub (Anderson and Ohmart 1984). None of these birds is a riparian obligate species. Small mammals were much less abundant in saltcedar than in native vegetation in Big Bend National park, TX (Boeer and Schmidly 1977).
Saltcedar increases soil salinity through its ability to use saline groundwater and to excrete the excess salt through leaf glands. The excretions include Na, K Mg, Ca, Cl, NO3, HCO2, and SO4, the concentration of each depending on its concentration in the root environment (Waisel 1961; Berry 1970). This salt then drips to the soil surface on damp mornings or falls with the leaves in the fall, forming a layer of salt on the soil surface. The salt accumulates in arid areas, where the salinity level soon exceeds that in which most other species can grow. Dams built on many of the western rivers greatly worsen the problem by preventing the spring floods that formerly leached out accumulated salts (see paper by Stan Smith, this workshop).
On uncontrolled streams, saltcedar increases sedimentation by slowing flood flows, causing narrowing and eventual blockage of stream channels and the distribution of the flow into many small meandering streams (Busby and Schuster 1973; Burkham 1972, 1976; Culler et al 1970; Robinson 1958). This greatly alters streambank morphology, and the temperature and other characteristics of the water, to the detriment of many species of endemic fish and other aquatic organisms.
Saltcedar is highly susceptible to fire, but after the aerial portions burn, the plants rapidly resprout; then sprouts can grow 6 to 8 feet tall the first year after burning, rapidly gaining dominance over other vegetation that regrows more slowly. The associated cottonwoods have no fire tolerance and are eliminated after a fire (Horton 1977; Anderson et al 1977).
Saltcedar damages recreational use of parks and other natural areas by limiting access, eliminating shade trees, creating unpleasant, sticky brine on damp mornings or dusty conditions at other times, by reducing bird and other animal populations, by creating aesthetically unpleasant conditions, and by reducing non-consumptive uses of wildlife (such as bird watching and wildlife photography) (Horton and Campbell 1974).
Saltcedar uses large amounts of groundwater that causes water tables to fall and springs and small streams to dry up. This causes the reduction or elimination of plants dependent on a high water table and kills wildlife or forces it to other areas. Water usage has been studied on the middle Gila River, AZ (Gatewood et al 1950), the middle Rio Grande, NM (Bureau Reclamation 1973) the lower Gila River, AZ (van Hylckama 1980) and the lower Colorado, CA (Gay and Hartman 1982). Best estimates of water usage are ca. 5.5 feet per year on the lower Colorado and somewhat less at the higher elevations. Clearing saltcedar from a large area of the Gila River reduced evapotranspiration by 20.5 inches per year (Weeks et al 1987) but this was before revegetation occurred. Revegetation with cottonwoods and willows would use approximately the same amount as the saltcedar. Clearing saltcedar was not demonstrated to increase stream flow downstream but could result in more desirable vegetation on site. Clearing saltcedar on the Pecos River near Artesia, NM, resulted in an increase of several feet in the water table to the point of having free water again in small lakes that had long been dry (see paper by Keith Duncan, this workshop).
Beneficial Values - Both saltcedar and athel are occasionally used as ornamental or shade trees, each constituting ca. 0.24 to 0.28% of all shade trees in Arizona, New Mexico and western Texas. Athel was used only in the more southern areas because it is not cold tolerant (C.J. DeLoach, unpubl. data).
Saltcedar is of moderate value to beekeepers; in one survey, beekeepers in Arizona ranked it seventh for honey production and fourth for colony maintenance. The honey is not of table grade, but can be used in the baking industry. The greatest need for colony maintenance is in late summer and early fall when few other flowers bloom. However, control should allow the increase of the native seepwillow baccharis, which blooms during this time and is also used by bees to make a good quality honey (G.D. Waller, unpubl. data; Waller and Schmalzel 1976).
We estimate that the expected 75-80% control of saltcedar by introduced natural enemies would have little impact on beekeepers, given the amount of saltcedar that probably will remain, the replacement by seepwillow, and the fact that athel will not be harmed (Brown 1989). Ornamental saltcedar plants probably would be damaged and should be protected by insecticides or replaced by other species. Populations of white-winged doves may be reduced only moderately, given that populations now are food-limited, not nesting-habitat limited. The gradual nature of biological control would allow the native mesquites to return naturally and some saltcedars would remain.
Potential for successful control Several factors influence the success potential for biocontrol of a weed: 1) the number of known natural enemies (or the potential to find them) in other areas of the world that could be introduced, 2) whether the target weed is introduced or native and whether the site of origin of the weed genus is foreign or native, and 3) the number of species closely related to the weed in the area invaded. Saltcedar ranks very favorably in these factors.
Both saltcedar and the genus Tamarix are of foreign origin, with no species of the family Tamaricaceae native to the Western Hemisphere (Baum 1978). This degree of taxonomic isolation means that the risk of an introduced control agent attacking native plants is exceptionally small. The converse situation, that Tamarix is an ancient and very isolated genus in the Old World, has allowed a very large number of host specific insects to evolve on species of the genus there, many of which could be introduced if needed (Kovalev 1995) (Table 5, Table 6). The potential for successful control of saltcedar is as great as for any biocontrol project attempted anywhere in the world, and is closely analogous to the successful projects against pricklypear cacti or lantana in Australia. This contrasts sharply with some other biocontrol projects, for example of leafy spurge, Carduus thistles, or field bindweed, where the close taxonomic relationship with several U.S. native plants limits the number of control agent species that can be introduced.
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For information on the outcome of this workshop or integrated weed management in the Pacific Region (Region 1), U.S. Fish and Wildlife Service, Portland, OR, contact: Scott_Stenquist@fws.gov