Saltcedar, Tamarix ramosissima Ledeb., is one of several Tamarix species introduced into the United States for use as a wind break and to prevent soil erosion. In the southwestern United States, it became widely naturalized and during the 1930-50s its populations exploded and occupied large areas of bottomlands and drainages. Tamarix ramosissima is now considered the worst weed of southwestern riparian areas. In 1986, the United States Department of Agriculture, Agricultural Research Service, at Temple, Texas, initiated a biological control program against this invasive exotic weed. The goal of this program is to search for and obtain insects that are capable of feeding on and damaging T. ramosissima , that would not harm Frankenia or other native vegetation, and that do not feed on or at least not significantly damage Tamarix aphylla , a less-invasive species used for windbreaks and shade in the southwestern United States.
Insects exert heavy pressure on Tamarix in central Asia. Faunistic studies showed that 50-250 insect species feed on Tamarix in Asia and Europe. The highest diversity was observed in the former Soviet Union (<250 species), Israel (220 species) and Pakistan (190 species). As a result of these surveys, 15 insect species were identified as candidates for introduction into the United States. Two insects, the mealybug Trabutina mannipara from Israel and the leaf beetle Diorhabda elongata from China, have preliminary approval for release (see below), four species are currently being tested in quarantine in Temple, Texas, and one more is approved for introduction into quarantine. The other eight species are currently being tested overseas.
Trabutina mannipara is a small mealybug whose nymphs and adults feed on the twigs and branches of Tamarix . The mealybug inserts its mouthparts into the plant and remove juices and nutrients. Large densities can build up on a host plant and branches with very high densities may eventually be killed. The adult female is wingless and excretes a tough waxy egg sac that gradually encloses her and in which she lays her eggs. After hatching, the nymphs remain in the egg sac for a few days, then disperse onto the surrounding branches and plants. This mealybug has 2-3 generations per years. In quarantine the waxy young nymphs covered the young twigs of T. ramosissima and eventually killed several of the test plants.
The biotype of T. mannipara tested in quarantine is known only from deciduous Tamarix from the Dead Sea and Sinai areas in Israel. Specimens were never collected from Tamarix aphylla in the field. In quarantine, nymphs were forced to live on plants from 14 genera of Violales and willow and cottonwood but survived only on Tamarix . Some feeding on T. aphylla was observed but populations were not sustained while populations increased 4-20 times on T. ramosissima . Given these observations, this biological control agent is expected to have little or no impact on T. aphylla in the field.
Diorhabda elongata is a leaf feeding beetle obtained in China where it attacks only Tamarix spp. In China, D. elongata is common and causes appreciable damage. The adult is 5-6 mm long and is bright yellow with two black strips on its back. Adults live an average of 18 days. Females lay their eggs in singly or in groups of up to 14 in a cluster on tender leaves. A single female will lay an average of 30 (range 11-49) eggs in her lifetime. In China, adults become active in mid-April and begin over wintering by the end of September. There are usually three generations per year. Diorhabda elongata overwinters as an adult under leaf litter and in cracks in the soil.
Upon hatching from the egg, the young larva begins to feed on the tender leaves of its host. This beetle is an external foliage feeder and damages saltcedar by eating the leaves and tender twigs. Complete defoliation can occur in outbreaks of this beetle. The youngest larvae are black but the older, larger larvae have a broad yellow stripes on each side. The larvae grow through three instars. When finished feeding, D. elongata larvae will drop to the ground and pupate in litter or loose soil. The immature stage (egg-to-adult) lasts approximately 29 days at 25°C.
In quarantine tests, larvae fed almost entirely on Tamarix spp. When forced to feed on Frankenia , some larval feeding did occur but no healthy adults developed. Larvae fed on T. aphylla as well as T. ramosissima , however, adults tended to avoid ovipositing on T. aphylla . If released, D. elongata probably will damage T. aphylla only slightly, if at all.
In March 1994, petitions for field releases of these two insects were submitted to USDA, APHIS for approval for release. The technical advisory group of APHIS, a scientific panel that evaluates introduction requests, recommended that both insects be released and preparation of an Environmental Assessment (as required by NEPA) was initiated. On March 26 1995, the southwestern subspecies of the willow flycatcher, Empidonax trailii extimus Phillips, was placed on the endangered species list. This bird now uses Tamarix to nest in some of its range since willows, its natural nest trees, have been displaced by it. Concerns have been raised that no other tree will now grow in place of Tamarix if it were removed and thus the flycatcher would be negatively impacted. Approval of the Environmental Assessment is dependent on resolution of concerns regarding the flycatcher. It should be noted that this is not an issue regarding the use of biological control, rather it is an issue of whether Tamarix should be controlled at all. Thus resolution of these issues impacts all efforts directed at controlling Tamarix .
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