Dr. Jack DeLoach, USDA-ARS
Grasslands Research, 808 E. Blockland Rd.
Temple, TX 76502
Biological control has been a tried and proven weed control methodology for many years. The 130 years of experience worldwide (762 projects in 55 countries against 118 weed species), in North America (control agents released to control 34 weeds since 1945) and Hawaii (control agents released to control 20 weeds since 1902) have demonstrated a high degree of safety and a ca. 33% rate of complete or substantial control to date. The objective of biological control is to reduce the weed below the threshold of important damage; the method has never eradicated a target weed. Attack on non-target plants has been rare, especially during the past 30 years since strict safeguards have been in place, and such attack has always been of minor importance. The research protocol and methodologies of biological control of weeds are well understood and strong safeguards are in place to minimize risk to non-target plants. The method is highly specific to the target weed species, very environmentally compatible, relatively inexpensive, and provides permanent control. Candidate weeds are evaluated on damage caused, lack of beneficial (economic and environmental) values, and potential for successful control; salcedar ranks high in all three categories.
The taxonomic isolation of Tamarix in the Old World has promoted the evolution of many host specific insects: 26 genera (with over 200 species) are restricted to developing entirely or in major part on the genus Tamarix. The taxonomic isolation of saltcedar in the Western Hemisphere (no species of Tamarix or of the family Tamaricaceae are native) implies a very small risk that introduced insects might attack non-target plants. The somewhat beneficial athel (Tamarix aphylla) is distinct from the weedy species and the insects considered for introduction do not damage it.
Our cooperators in France, Israel, Turkmenistan, Kazakhstan, and China have conducted preliminary testing on 21 insect species, including 10 species being tested in quarantine at Temple, TX. Two of these have been recommended by the APHIS Technical Advisory Group for the Introduction of Biological Control Agents of Weeds (TAG) of the United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA-APHIS) for field release, pending approval of an Environmental Assessment; these are the mealybug Trabutina mannipara from Israel and the leaf beetle Diorhabda elongata that occurs from China to Turkey. Other promising species under study are two other Trabutina species; two weevils, Coniatus sp. and Corimalia sp.; two gall midges, Psectrosema spp.; two foliage-feeding moths, Ornativalva sp. and Agdistis sp.; two psyllids, Crastina sp. and Colposcenia sp., another leaf beetle, Cryptocephalus sp.; and a scale insect, Adiscodiaspis sp.
Biological control is expected to reduce the abundance of saltcedar by up to 75-85% but will probably require the introduction of several agents over several years. Control at a given site may require 5 to 10 years and control may not be satisfactory in all areas. As saltcedar is controlled, the native vegetation is expected to gradually return, though some areas may already be too saline for any but salt-tolerant species. Ultimately, saltcedar should be reduced to an uncommon (or common), but not a dominating, member of the plant community. The insects introduced to control saltcedar will also be used as food to some extent by wildlife, especially by insectivorous birds.
The research protocol includes extensive analyses of possible conflicts of interest between harmful and beneficial aspects of the target weed, exploration for host specific and effective control agents and ecological and biological studies within the native distribution of the weed overseas, and extensive host range testing both overseas and in quarantine in the United States. All aspects of the research are reviewed by the multi-agency and multi-disciplinary Technical Advisory Group on the Introduction of Biological Control Agents of Weeds, of the US Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). Field release of control agents required APHIS to make a "finding of not significant impact: (FONSI) on an Environmental Assessment and, if endangered species may be affected, concurrence of the US Fish and Wildlife Service on a Biological Assessment finding that release permit signed by state agricultural authorities for each state of release. Recent projects require monitoring not only of the degree of control but also extensive monitoring of the ecological effects of biological control.