Photo: Russian olive, southwest Montana. © 2020 Delena Norris-Tull
Other Challenges to adopting Biocontrol Agents
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Removing noxious weeds alone is not sufficient to solve the problems caused by the weeds. While in some cases, native plants may re-emerge on their own, when biocontrol agents are used to remove weeds, there must be a plan in place to re-seed or re-plant with native species or other desirable species, and the resources to fund that plan. If the follow-up work does not occur, just as with herbicides and other methods of removal, the likelihood is high that an alternate noxious weed will replace the targeted weed.
While biocontrol holds promise for management of a number of invasive plant species, they are clearly not always successful. For example, attempts to use biocontrol insects with Centaurea maculosa (spotted knapweed) have repeatedly failed in the past. By 2004, thirteen different insect species had been used to control knapweed. Ridenour and Callaway, 2004, suggested that “the lack of specialist insect herbivores is a minor component of its invasive success. In fact, specialist biocontrol root herbivory may stimulate the growth and competitive ability of C. maculosa.” In more recent years, Melissa Maggio reports better success with spotted knapweed control, by using a complex of insects, each of which attacks different parts of the plant. Refer to the section, “Biocontrol Interviews: Montana,” for more details.
[The following research summary is repeated in the section of this website on Invasive Success Hypotheses].
Ridenour, Vivanco, Feng, Horiuchi, and Callaway, 2008, conducted greenhouse experimental comparisons of the success of European populations of spotted knapweed with North American populations. They found that the North American plants not only grew larger than their European counterparts, but they were more resistance to insect herbivory than the European plants.
They used two specialist biocontrol insects, the European root boring weevil, Cyphocleonus achates (first released in the US in 1987), and a moth, Agapeta zoegana, that has root boring larvae (first released in the US in 1984). Both species do substantial damage to knapweed roots, and adult Cyphocleonus eat the leaves. Ridenour, et al., 2008, also used some native generalist insects in their experiments, insects that are not normally used for biocontrol.
The results from these experiments are somewhat astonishing. The biocontrol insects produced significantly more damage to the European knapweed plants than to the North American knapweed population. While the weevil caused significant damage to both populations of plants, nearly twice as many North American knapweed plants survived than did the European plants. The moth larvae also preferentially attacked the European plants. Each of the native generalist insects (native to North America) also caused more severe damage to the European plants.
Ridenour, et al., 2008, found that leaves on the North American plants “contained approximately two times higher concentrations of the defense compound precursor, phytol, in their leaves than the European populations.” North American knapweed leaves were also much tougher than the European plants. They concluded that North American spotted knapweed plants “were bigger, elicited stronger competitive effects, and demonstrated stronger competitive responses than European populations.” They were able to rule out phenotypic plasticity as a cause of the differences. They concluded that some of their results support the “evolution of increased competitive ability” hypothesis. But they also concluded that, “North American Centaurea genotypes were also consistently better defended against (or avoided by) specialist and generalist consumers, demonstrating both a stronger inhibitory effect on the consumers (resistance) and a better ability to grow in response to herbivory (tolerance), which questions the trade-off based assumptions of EICA as a consistent mechanistic basis for the continental differences between populations.” Ridenour, et al., 2008, suggest that “selection for effective competitive or defense traits may not be easily coupled to resource or energetic trade-offs for a simple reason: different defense or allelopathic chemicals may cost the same energetically or nutritionally, but differ a great deal in effectiveness… Physiological costs of a biochemical may be trivial in an ecological context if the biochemical is exceptionally effective or performs more than one job.”
This research by Ridenour, et al., 2008, has implications for biocontrol, as it provides some insights into the possible causes of failure, or at least partial failure, of some biocontrol agents.
References:
More challenges of using Biocontrol Agents:
Other Challenges to adopting Biocontrol Agents
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Removing noxious weeds alone is not sufficient to solve the problems caused by the weeds. While in some cases, native plants may re-emerge on their own, when biocontrol agents are used to remove weeds, there must be a plan in place to re-seed or re-plant with native species or other desirable species, and the resources to fund that plan. If the follow-up work does not occur, just as with herbicides and other methods of removal, the likelihood is high that an alternate noxious weed will replace the targeted weed.
While biocontrol holds promise for management of a number of invasive plant species, they are clearly not always successful. For example, attempts to use biocontrol insects with Centaurea maculosa (spotted knapweed) have repeatedly failed in the past. By 2004, thirteen different insect species had been used to control knapweed. Ridenour and Callaway, 2004, suggested that “the lack of specialist insect herbivores is a minor component of its invasive success. In fact, specialist biocontrol root herbivory may stimulate the growth and competitive ability of C. maculosa.” In more recent years, Melissa Maggio reports better success with spotted knapweed control, by using a complex of insects, each of which attacks different parts of the plant. Refer to the section, “Biocontrol Interviews: Montana,” for more details.
[The following research summary is repeated in the section of this website on Invasive Success Hypotheses].
Ridenour, Vivanco, Feng, Horiuchi, and Callaway, 2008, conducted greenhouse experimental comparisons of the success of European populations of spotted knapweed with North American populations. They found that the North American plants not only grew larger than their European counterparts, but they were more resistance to insect herbivory than the European plants.
They used two specialist biocontrol insects, the European root boring weevil, Cyphocleonus achates (first released in the US in 1987), and a moth, Agapeta zoegana, that has root boring larvae (first released in the US in 1984). Both species do substantial damage to knapweed roots, and adult Cyphocleonus eat the leaves. Ridenour, et al., 2008, also used some native generalist insects in their experiments, insects that are not normally used for biocontrol.
The results from these experiments are somewhat astonishing. The biocontrol insects produced significantly more damage to the European knapweed plants than to the North American knapweed population. While the weevil caused significant damage to both populations of plants, nearly twice as many North American knapweed plants survived than did the European plants. The moth larvae also preferentially attacked the European plants. Each of the native generalist insects (native to North America) also caused more severe damage to the European plants.
Ridenour, et al., 2008, found that leaves on the North American plants “contained approximately two times higher concentrations of the defense compound precursor, phytol, in their leaves than the European populations.” North American knapweed leaves were also much tougher than the European plants. They concluded that North American spotted knapweed plants “were bigger, elicited stronger competitive effects, and demonstrated stronger competitive responses than European populations.” They were able to rule out phenotypic plasticity as a cause of the differences. They concluded that some of their results support the “evolution of increased competitive ability” hypothesis. But they also concluded that, “North American Centaurea genotypes were also consistently better defended against (or avoided by) specialist and generalist consumers, demonstrating both a stronger inhibitory effect on the consumers (resistance) and a better ability to grow in response to herbivory (tolerance), which questions the trade-off based assumptions of EICA as a consistent mechanistic basis for the continental differences between populations.” Ridenour, et al., 2008, suggest that “selection for effective competitive or defense traits may not be easily coupled to resource or energetic trade-offs for a simple reason: different defense or allelopathic chemicals may cost the same energetically or nutritionally, but differ a great deal in effectiveness… Physiological costs of a biochemical may be trivial in an ecological context if the biochemical is exceptionally effective or performs more than one job.”
This research by Ridenour, et al., 2008, has implications for biocontrol, as it provides some insights into the possible causes of failure, or at least partial failure, of some biocontrol agents.
References:
- Ridenour, W.M., & Callaway, R.M. (Oct., 2004). Novel weapons: Invasion success and the evolution of increased competitive ability. Frontiers in Ecology and the Environment, 2(8): 436- 443.
- Ridenour, W.M., Vivanco, J.M., Feng, Y., Horiuchi, J. & Callaway, R.M. (2008). No evidence for trade-offs: Centaurea plants from America are better competitors and defenders. Ecological Monographs, 78, 369–386.
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