Theory of Biological Control

The theory behind biological control is the attempt to duplicate the factors that keep plant populations in check in their native environments. The host (the invasive weed) and the natural enemy (the biological control) respond differently to each other depending upon environmental conditions and their individual responses to natural forces. The specific genotype of the host and the enemy may play an important role in determining which biocontrol will be effective. The ultimate goal of the Biological Control program in Asotin County is to achieve a natural balance until the status of the weed can be relegated to “just another plant in the landscape”.

Types of Biological Control Agents

Biological control agents may be insects, nematodes, mites, or rusts. There are often more than one agent released on the host plant. The agents may directly or indirectly attack a plant. A direct attack may include defoliation, or consumption of the developing seeds. An indirect attack may cause slight damage that subsequently allows pathogens to inflict even greater damage.

Advantages and Disadvantages of Biological Controls

Biological controls have advantages and disadvantages. The advantages include their ability to target specific species and their ability to reproduce and expand their range without further assistance. Their disadvantages include the many years that it may take to build up a population of sufficient size to impact the host plant and the fact that they do not eradicate a species, but rather reduce its population size.

Current Biological Controls in Asotin County

  • Yellow starthistle: 4 biological controls
  • Diffuse/spotted knapweed: 3 biological controls
  • Poison hemlock, Dalmatian toadflax, Rush skeletonweed, and  St. Johnswort: 1 biological control

Biological controls for the above weeds are well established in Asotin County. The Asotin County Weed Board distributes, free-of-charge, biological controls for diffuse knapweed, dalmatian toadflax, and poison hemlock. Since 2003 the Weed Board has been introducing new weevils for the control of puncturevine and a new moth for rush skeletonweed.  When these populations become established, the Board will disperse them throughout the county. Please call the Weed Board to check on available biological controls (509) 243-2098.