Weed Feeders

New plant species have been extensively introduced to new locations around the world. Often these new species spread rapidly because nothing eats them in their new locations. In the U.S., it is estimated that 50% to 75% of problem weeds were either accidental or intentional introductions from other areas. For example, purple loosestrife is a valued wildflower in its native Eurasian habitats, existing in small stands and as isolated plants controlled by a complex of natural enemies. It was introduced to North America in the late nineteenth century where it rapidly multiplied, choking waterways and crowding out native North American species. Classical biocontrol of purple loosestrife with the beetles Galerucella calmariensis and G. pusilla and the weevils Hylobius transversovittatus and Nanophyes marmoratus has subsequently been largely successful. Traditional methods of weed control, such as cultivation and the use of herbicides, are not practical or desirable for such wide-ranging stands of weeds, and biological control has become an attractive strategy for control of pest weeds.

Worldwide, classical biological control has successfully controlled introduced weeds on numerous occasions. In North America, there have been successes in controlling nodding thistle (Canada; Kansas, U.S.), ragwort (British Columbia, Canada; California and Oregon, U.S), klamath weed (Ontario, Canada; California, Oregon and southeast Washington, U.S.) alligator weed (Florida, Louisiana, and Texas, U.S.), and water lettuce (Florida). When not completely controlled, pest weeds may be reduced in varying degrees by natural enemy introductions. Sometimes a complex of natural enemies is used in an attempt to control a pest weed.

How weed-feeders manage weed problems

Insects can control weeds by feeding on seeds, flowers, leaves, stems, roots, or combinations of these, or by transmitting plant pathogens, which will infect plants. Other natural enemies of weeds include plant pathogens, nematodes, and vertebrates (fish).

Benefits of using Weed Feeders

Using biological agents for weed control is beneficial because:

• There is less overall expense compared to herbicidal sprays.
• Once a population of biocontrol agents is established, minimal effort is required to conserve it.
• The use of natural enemies does not require a high level of technology.
• Natural enemies are less disruptive ecologically and so natural biodiversity is maintained.

Weed-feeder selection

In considering species for introduction to control pest weeds, researchers first determine that the species feeds exclusively on the pest species. Other considerations include the effectiveness of control, favorable host-plant synchronization with the natural enemy's life cycle, the fecundity of the natural enemy, and its heartiness. Researchers have also determined that a successful introduction is more likely if the new locale is similar in ecology and climate to the area of origin, and if trials are made where population densities of the pest plant are similar to those in the new location. The reasons for success or failure are not yet well understood.

Desirable characteristics of weed-feeding natural enemies

• They are specific to one plant species
• They have a negative impact on plant individuals and the population dynamics of the target weed
• They are prolific
• They thrive and become widespread in all habitats and climates that the pest weed occupies
• They are good colonizers

Relative Effectiveness

North American introductions of weed-feeding natural enemies have ranged from very successful, with a 99% reduction of the pest species, to complete failures, with the introduced species unable to become established in the new location.

Predators, parasitoids, or pathogens of the introduced natural enemy species in the new location may limit its ability to control the pest plant.