Risks of biological control

Biologists have purposely introduced many alien species for the purpose of controlling others that have been accidentally introduced. In some cases these introductions have proved highly successful. Approximately 40% of projects introducing herbivorous insects purposely introduced into the United States to control weeds show some evidence of success, and 20% appear to have exerted some significant control.

• Hypericum perforatum - First reported from California in 1900, by 1944 it occupied more than 800,000ha.

• Four herbivorous insects were released for biological control efforts in 1945.

• Within a decade, it was reduced from an important pest of range lands to an occasional road-side weed.

Biological control is attractive because:

• May allow pests to be controlled without use (or with limited use) of chemical pesticides or herbicides.

• Potential for control over a broad geographical region continuously and without additional investment of time or money once control agents are established.

Depends for its effectiveness on:

• Ability of control agent to limit numbers of target species.

• Specificity of control agent for target species.

It's the second of these I want to say a few words about in this context. Consider the case of Rhinocyllus conicus, a flowerhead weevil, introduced into North America in 1968 in an effort to control populations of European thistles (Carduus).

• Original releases made even after initial feeding tests showed that the weevil's host range included native North American thistles (Cirsium, Silybum, and Onopordum). Stronger oviposition preference for Carduus and more successful larval development on Carduus were expected to limit use of North American thistles (Table 2).

  
  

  Table 2: Results of a 1996 survey in Colorado, South Dakota, and Nebraska for Rhinocyllus conicus in thistle (from [7]).

  	percent heads infested
  Native	($<1$)16-78%
  Introduced	25-100%

  
  

• Significant increase from 1992 in native infestation.

• Seed production in infested heads 14% that of uninfested heads.

• Threatens persistence of Cirsium canescens, a sparsely distributed thistle restricted to the Sandhills prairie of Nebraska. C. canescens is presumed progenitor of C. pitcheri, which is a federally listed threatened species in the Great Lakes dunes. Foreshadowing of future impact if the weevil makes it to that ecosystem?

A similar study on parasitoids of native Hawaiian moths found that 83% were biological control agents and 14% were accidentally introduced wasps (216 parasitoids collected from 2112 caterpillars) [4].

We also face other difficult choices. In 1999 a biological control project was started in the desert southwest with a Chinese beetle. The control effort's objective is to reduce the abundance of saltcedar (tamarisk), which was originally introduced as a windbreak and to control soil erosion. It now covers almost 500,000 hectares in 15 states. It has crowded out many native species, including cottonwoods and willows that are important resources for desert bighorn. It provides 90% of the nesting habitat for the southwestern willow flycatcher, which is listed as endangered under the Endangered Species Act.⁷