Yellow Starthistle Information
Mech control
-hand
-tillage
-mowing
Cultural control
-grazing
-burning
-re-vegetation
Biocontrol
-insects
-biocontrol table
-plant pathogens
Chem control
- risks
--spray
--water
--toxicology
--herbicide
resist
--effects
-herbicides
--preemergence
--postemergence
--late season
--pre- & post-
--imazapic
--clopyralid
--picloram
Management
Biological control
The goal of a biological control program is not to eradicate the target weed, but to exert sufficient environmental stress to reduce its dominance in the plant community and shift the competitive balance to more desired species (Wilson and MacCaffery 1999). With insect agents this can be achieved by boring into roots, shoots and stems, defoliation, seed predation, or extracting plant fluids. All these effects can reduce the competitive ability of the plant relative to the surrounding vegetation.
Ideally, biological control of weeds is considered to be environmentally safe, energy self-sufficient, cost-effective, and often self-sustaining. Over the past 100 years, over 200 control agents have been released against 114 weed species worldwide (Blossey et al. 1994). Of these, 165 have been imported and released in the continental United States and Canada (Goeden 1993). Although biological control agents can include nematodes, pathogens, and vertebrates, 114 of the 165 released organisms are arthropods (insects and mites) (Julien 1989). The vast majority of released agents for terrestrial weeds are targeted for non-indigenous weeds of rangeland (Julien 1992).
Despite the overwhelmingly positive aspects of biocontrol, some risks do exist. Most biocontrol agents introduced to the United States are native to other continents. Although the host specificity of these organisms is studied under quarantine conditions, little is known of their impact on the ecosystem as a whole, including other insect populations (DiTomaso 1997). In addition, our understanding of the nature of host specificity is poor. Consequently, there is no guarantee that the introduced biocontrol agent will not itself become a pest by changing its food preference from weeds to desirable plants after it is released. For example, based on chemical similarity yellow starthistle is most closely related to purple starthistle (Centaurea calcitrapa) and safflower (Stevens et al. 1990). Thus, there is the potential that biocontrol agents for starthistle may shift to this commercially important crop. Fortunately, this host shift has only rarely occurred with plant biocontrol organisms (Rees 1978). It has, however, arisen on many occasions with the introduction of animal control agents (Civeyrel and Simberloff 1996).
In some cases, accidental introductions of pathogens or insects can occur when biocontrol agents are released. For example, the pathogen Nosema was accidentally introduced as a contaminant of Trichosirocalus horridus, a weevil introduced for the control of musk thistle (Carduus nutans) (Andres and Rees 1995). More recently, a second species of peacock fly (Chaetorellia succinea) was identified as a contaminant of released populations of C. australis for the control of yellow starthistle (Balciunas and Villegas 1999).
Insects for yellow starthistle control
The USDA, Agricultural Research Service's Exotic and Invasive Weed Management Research Unit in Albany and the CDFA's (California Department of Food and Agriculture) Biological Control Program have been active in developing an effective biological control program for yellow starthistle in California and other western states. Since the first established biological control insect in California (seed-head weevil; Bangasternus orientalis) was reported in California in 1986 (Maddox et al. 1986), there have been five additional insects released and established in the state. Today, three introduced biocontrol insects have become widespread including the seed-head weevil, seed-head fly (Urophora sirunaseva), and the hairy weevil (Eustenopus villosus). A fourth insect, the false peacock fly (Chaetorellia succinea) was accidentally released in 1991, and is now widespread and more effective against yellow starthistle than the intentionally released peacock fly (Chaetorellia australis) (Balciunas and Villegas 1999). Fortunately, the false peacock fly appears to be host specific to yellow starthistle and does not attack native thistles or related commercial crops, such as safflower (Villegas et al. 1999, 2000b). One other insect has been released, the flower weevil (Larinus curtus), but has yet to become well established in California (see table). All six insects attack the flower heads of yellow starthistle and produce larvae that develop within the seedhead and feed on seeds.
Of the four insects that are well established in California (Villegas 1999, Villegas et al. 2000a) only two, the false peacock fly and the hairy weevil, have any significant impact on seed production (Pitcairn and DiTomaso 2000, Pitcairn et al. 1999a, 2000a). The combination of these two insects has been reported to reduce seed production by 43 to 76% (Pitcairn and DiTomaso 2000). Balciunas and Villegas (1999) reported a 78% reduction in seed production when seed heads contained false peacock fly larvae. Although this level of suppression is not sufficient to provide long-term starthistle management, the use of biological control agents can be an important component of an integrated management approach (see Integrated Approaches). A more successful biological control program will likely require the introduction of plant pathogens or other insect organisms capable of feeding on roots, stems, or foliage. Currently, a root-boring weevil (Ceratapion basicorne) is in quarantine and may have potential for release on yellow starthistle in the next few years.
