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, updated November 2024.
Biocontrol alone is not enough
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 stoebe ( formerly C. maculosa) (spotted knapweed) have repeatedly failed in the past. By 2004, thirteen different insect species had been used to control knapweed. Callaway and Ridenour, 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.
Native generalist herbivores sometimes more effective than introduced specialists
[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 (where it is native) with North American invasive populations. They found that the North American plants not only grew larger than their European counterparts, but they were more resistant 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.
Parker, Burkepile, & Hay (March 6, 2006) conducted a meta-analysis of 63 field studies that involved experiments in herbivore exclusion. Overall, these studies examined the impact of herbivore removal on more than 100 exotic plant species. 35 of these studies monitored herbivore effects on entire plant communities. They reviewed an additional 28 studies that monitored herbivore effects on specific species of exotic plants. Many diverse environments and many diverse herbivores (from cattle to bison, elk, deer, antelope, waterfowl, rabbits, rodents, fishes, insects, etc.) were included among these studies.
They challenged the effectiveness of biocontrol herbivores. For example, insects who are the natural predators of invasive species have been brought into the USA because it has been believed that they are the best predators to control invasive species. Rather, Parker, et al. concluded that generalist herbivores native to the USA are significantly more effective at controlling invasive species, than are the herbivores from the home environment. This makes sense if you consider that, in their native environment, plants that become invasive in the USA are NOT eliminated by the herbivores that naturally prey upon them. Rather, an ecological balance is reached between predator and prey in the home environment.
The meta-analysis revealed that "native herbivores strongly suppressed, whereas exotic herbivores strongly enhanced, the relative abundance of exotic plants... Overall, the relative abundance of exotic plants was 52% higher in communities grazed by exotic herbivores.. than in communities grazed by native herbivores." In addition, "exotic herbivores suppressed the abundance of native plants." They also found that, "Native vertebrate herbivores had a three- to five-fold larger impact on exotic plant survival than did native invertebrate herbivores."
Impact of biocontrol agents on native species
Rand & Louda (June 2004) conducted a field study in a mixed grass prairie in Nebraska. They examined grassland patches, both those with high surrounding agricultural cover and those with low agricultural cover, and agricultural patches. And within the grasslands and the agricultural fields, they found sections with and without significant densities of the exotic thistle Carduus nutans. The grasslands also included two native thistle species, Cirisum undulatum and C. flodmanii. A biocontrol weevil, Rhinocyllus conicus, had been introduced to these fields in Nebraska in 1969. In 2001 and 2002, Rand & Louda measured weevil egg load and larval entrance holes on flowers of each thistle species. In 2001, they found that mean weevil egg density on native thistles was nearly twice as great in patches with high density of Carduus nutans, than in patches with a low density of the invasive thistle. In 2002, egg densities on native thistles were more than four times higher in patches with high densities of Carduss nutans, than in patches with a low density of the invasive thistle.
This study provides a long-term examination of the impacts of a biological control agent, and presents concern for the negative impact on native species, when biocontrol herbivores are used to attack invasive species.
Russell, et al. (2007), followed up on the work of Rand & Louda. They state: "Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species...We quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie... Over three years (2001–2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0–220 m) from (the exotic) musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density (≤5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle's co-occurrence with the coevolved preferred exotic host plant and not from the weevil's response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants."
Exotic herbivores can impact native plant populations
Over three years, Lau & Strauss (2005) conducted censuses of native Lotus wrangelianus plants and invasive Medicago polymorpha plants in the grasslands of the McLaughlin Natural Reserve in Napa and Lake Counties, California. Medicago polymorpha, from the Mediterranean, had likely been in California since the late 1800s. Both species are attacked by a foliovore, Hypera brunneipennis, an exotic alfalfa weevil, and a hymenopteran seed predator, Bruchophagous sp..
Lau & Strauss (2005) sampled plants along 25m transects. The plants were examined for foliar damage, bud-galling damage and seed predation. Plant densities were estimated for both species.
Four treatments were used on plant plots over three years, to examine their impacts on the native Lotus. Some plots had both Medicago and herbivores. Some plots were sprayed to eliminate herbivores. In some plots, Medicago seedlings were removed, while herbivores remained. And in some plots Medicago seedlings and herbivores were removed. Treatments were imposed on 3X3 meter plots established in four invaded patches in 2002, and three invaded patches in 2003.
Lau & Strauss (2005) found that "Medicago presence significantly affected herbivory on the native Lotus... Subsequent univariate analyses on folivory, bud-galling, and seed predation indicated that Lotus individuals in patches invaded by Medicago received approximately twice the foliar damage as Lotus in uninvaded patches... This result was consistent across three years of observations... Additionally, densities of the predominant folivore H. brunneipennis observed feeding on Lotus increased linearly with increasing Medicago density..., indicating that Medicago may be increasing herbivory rates on the native Lotus by increasing the density of this shared enemy. In contrast to the strong effect of Medicago, Lotus density had no effect on folivore density... In contrast to folivory by H. brunneipennis, bud-galling and seed predation were not significantly different in invaded vs. uninvaded patches... Lotus individuals produced significantly more (302%) seed in uninvaded vs. invaded patches, but mean seed mass did not differ with invasion status."
They found that, "Medicago removal and insecticide treatments significantly affected attack by natural enemies... Univariate analyses on folivory levels revealed consistent trends towards decreased folivory on Lotus in the Medicago-removal plots over both years of the study; however, this difference was only significant in 2003... Rates of bud-galling and seed predation varied substantially across the two years of the study. In 2002, both bud-galling and seed predation rates were high, and Medicago removal increased attack on Lotus from both of these herbivores... In contrast, in 2003, bud-galling and seed predation were low, and neither herbivore responded significantly to Medicago removal or insecticide treatments."
Lau & Strauss (2005) found that, "Medicago removal and insecticide treatments both significantly increased Lotus fitness...; however, the mechanism underlying the Medicago removal effect varied between years. We calculated the effects of Medicago removal within each insecticide treatment. This analysis revealed that Medicago removal only increased Lotus fitness in the absence of insects (insecticide plots) in 2002... In the absence of herbivores, we attribute negative effects of Medicago on Lotus fitness to the competitive effect of Medicago. In contrast, in plots where insects were present, Medicago removal both decreased the competitive effect of Medicago and decreased folivory on Lotus; however, Lotus also exhibited delayed flowering time that caused increased rates of bud-galling and seed predation. These dual direct and indirect effects (release from competition and from folivory, but increased flower and fruit predation) resulted in the net effect that Medicago removal did not affect Lotus fitness in the presence of insects in 2002. Thus, in 2002, net fitness effects due to competition with Medicago were stronger than the net fitness consequences of herbivore-mediated indirect effects, despite the fact that herbivory rates were very high.... In contrast, in 2003, the net fitness consequences of herbivore-mediated indirect effects exceeded the direct competitive effect. Medicago only significantly decreased Lotus fitness in the presence of herbivores... We interpret this result as Medicago decreasing Lotus fitness indirectly by boosting folivory rates."
Lau & Strauss (2005) state that, "Exotic plant species can affect native plant species both directly via competition and indirectly by modifying native species’ interactions with other organisms in the community. Both direct and indirect effects can have strong fitness impacts on the native species and can result in cascading effects throughout the invaded community."
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, updated November 2024.
Biocontrol alone is not enough
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 stoebe ( formerly C. maculosa) (spotted knapweed) have repeatedly failed in the past. By 2004, thirteen different insect species had been used to control knapweed. Callaway and Ridenour, 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.
Native generalist herbivores sometimes more effective than introduced specialists
[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 (where it is native) with North American invasive populations. They found that the North American plants not only grew larger than their European counterparts, but they were more resistant 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.
Parker, Burkepile, & Hay (March 6, 2006) conducted a meta-analysis of 63 field studies that involved experiments in herbivore exclusion. Overall, these studies examined the impact of herbivore removal on more than 100 exotic plant species. 35 of these studies monitored herbivore effects on entire plant communities. They reviewed an additional 28 studies that monitored herbivore effects on specific species of exotic plants. Many diverse environments and many diverse herbivores (from cattle to bison, elk, deer, antelope, waterfowl, rabbits, rodents, fishes, insects, etc.) were included among these studies.
They challenged the effectiveness of biocontrol herbivores. For example, insects who are the natural predators of invasive species have been brought into the USA because it has been believed that they are the best predators to control invasive species. Rather, Parker, et al. concluded that generalist herbivores native to the USA are significantly more effective at controlling invasive species, than are the herbivores from the home environment. This makes sense if you consider that, in their native environment, plants that become invasive in the USA are NOT eliminated by the herbivores that naturally prey upon them. Rather, an ecological balance is reached between predator and prey in the home environment.
The meta-analysis revealed that "native herbivores strongly suppressed, whereas exotic herbivores strongly enhanced, the relative abundance of exotic plants... Overall, the relative abundance of exotic plants was 52% higher in communities grazed by exotic herbivores.. than in communities grazed by native herbivores." In addition, "exotic herbivores suppressed the abundance of native plants." They also found that, "Native vertebrate herbivores had a three- to five-fold larger impact on exotic plant survival than did native invertebrate herbivores."
Impact of biocontrol agents on native species
Rand & Louda (June 2004) conducted a field study in a mixed grass prairie in Nebraska. They examined grassland patches, both those with high surrounding agricultural cover and those with low agricultural cover, and agricultural patches. And within the grasslands and the agricultural fields, they found sections with and without significant densities of the exotic thistle Carduus nutans. The grasslands also included two native thistle species, Cirisum undulatum and C. flodmanii. A biocontrol weevil, Rhinocyllus conicus, had been introduced to these fields in Nebraska in 1969. In 2001 and 2002, Rand & Louda measured weevil egg load and larval entrance holes on flowers of each thistle species. In 2001, they found that mean weevil egg density on native thistles was nearly twice as great in patches with high density of Carduus nutans, than in patches with a low density of the invasive thistle. In 2002, egg densities on native thistles were more than four times higher in patches with high densities of Carduss nutans, than in patches with a low density of the invasive thistle.
This study provides a long-term examination of the impacts of a biological control agent, and presents concern for the negative impact on native species, when biocontrol herbivores are used to attack invasive species.
Russell, et al. (2007), followed up on the work of Rand & Louda. They state: "Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species...We quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie... Over three years (2001–2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0–220 m) from (the exotic) musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density (≤5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle's co-occurrence with the coevolved preferred exotic host plant and not from the weevil's response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants."
Exotic herbivores can impact native plant populations
Over three years, Lau & Strauss (2005) conducted censuses of native Lotus wrangelianus plants and invasive Medicago polymorpha plants in the grasslands of the McLaughlin Natural Reserve in Napa and Lake Counties, California. Medicago polymorpha, from the Mediterranean, had likely been in California since the late 1800s. Both species are attacked by a foliovore, Hypera brunneipennis, an exotic alfalfa weevil, and a hymenopteran seed predator, Bruchophagous sp..
Lau & Strauss (2005) sampled plants along 25m transects. The plants were examined for foliar damage, bud-galling damage and seed predation. Plant densities were estimated for both species.
Four treatments were used on plant plots over three years, to examine their impacts on the native Lotus. Some plots had both Medicago and herbivores. Some plots were sprayed to eliminate herbivores. In some plots, Medicago seedlings were removed, while herbivores remained. And in some plots Medicago seedlings and herbivores were removed. Treatments were imposed on 3X3 meter plots established in four invaded patches in 2002, and three invaded patches in 2003.
Lau & Strauss (2005) found that "Medicago presence significantly affected herbivory on the native Lotus... Subsequent univariate analyses on folivory, bud-galling, and seed predation indicated that Lotus individuals in patches invaded by Medicago received approximately twice the foliar damage as Lotus in uninvaded patches... This result was consistent across three years of observations... Additionally, densities of the predominant folivore H. brunneipennis observed feeding on Lotus increased linearly with increasing Medicago density..., indicating that Medicago may be increasing herbivory rates on the native Lotus by increasing the density of this shared enemy. In contrast to the strong effect of Medicago, Lotus density had no effect on folivore density... In contrast to folivory by H. brunneipennis, bud-galling and seed predation were not significantly different in invaded vs. uninvaded patches... Lotus individuals produced significantly more (302%) seed in uninvaded vs. invaded patches, but mean seed mass did not differ with invasion status."
They found that, "Medicago removal and insecticide treatments significantly affected attack by natural enemies... Univariate analyses on folivory levels revealed consistent trends towards decreased folivory on Lotus in the Medicago-removal plots over both years of the study; however, this difference was only significant in 2003... Rates of bud-galling and seed predation varied substantially across the two years of the study. In 2002, both bud-galling and seed predation rates were high, and Medicago removal increased attack on Lotus from both of these herbivores... In contrast, in 2003, bud-galling and seed predation were low, and neither herbivore responded significantly to Medicago removal or insecticide treatments."
Lau & Strauss (2005) found that, "Medicago removal and insecticide treatments both significantly increased Lotus fitness...; however, the mechanism underlying the Medicago removal effect varied between years. We calculated the effects of Medicago removal within each insecticide treatment. This analysis revealed that Medicago removal only increased Lotus fitness in the absence of insects (insecticide plots) in 2002... In the absence of herbivores, we attribute negative effects of Medicago on Lotus fitness to the competitive effect of Medicago. In contrast, in plots where insects were present, Medicago removal both decreased the competitive effect of Medicago and decreased folivory on Lotus; however, Lotus also exhibited delayed flowering time that caused increased rates of bud-galling and seed predation. These dual direct and indirect effects (release from competition and from folivory, but increased flower and fruit predation) resulted in the net effect that Medicago removal did not affect Lotus fitness in the presence of insects in 2002. Thus, in 2002, net fitness effects due to competition with Medicago were stronger than the net fitness consequences of herbivore-mediated indirect effects, despite the fact that herbivory rates were very high.... In contrast, in 2003, the net fitness consequences of herbivore-mediated indirect effects exceeded the direct competitive effect. Medicago only significantly decreased Lotus fitness in the presence of herbivores... We interpret this result as Medicago decreasing Lotus fitness indirectly by boosting folivory rates."
Lau & Strauss (2005) state that, "Exotic plant species can affect native plant species both directly via competition and indirectly by modifying native species’ interactions with other organisms in the community. Both direct and indirect effects can have strong fitness impacts on the native species and can result in cascading effects throughout the invaded community."
References:
- Callaway, R.M., & Ridenour, W.M. (Oct., 2004). Novel weapons: Invasive success and the evolution of increased competitive ability. Frontiers in Ecology and the Environment, 2(8): 436-443.
- Lau, J.A., & Strauss, S.Y. (2005). Insect herbivores drive important indirect effects of exotic plants on native communities. Ecology, 86(11), 2990-2997.
- Parker, J.D., Burkepile, D.E., & Hay, M.E. (March 10, 2006). Opposing effects of native and exotic herbivores on plant invasions. Science, 311, 1459-1461.
- Rand, T.A., & Louda, S.M. (June, 2004). Exotic weed invasion increases the susceptibility of native plants to attack by a biocontrol herbivore. Papers in the Biological Sciences. Digital Commons: University of Nebraska-Lincoln.
- Russell, F.L., Louda, S.M., Rand, T.A., & Kachman, S.D. (2007). Variation in herbivore-mediated indirect effects of an invasive plant on a native plant. Ecology, 88(2), 413-413.
- 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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