Photo: Salt cedar & Giant Cane on the Rio Grande, Big Bend National Park. © 2017 Delena Norris-Tull
Indirect impacts of biocontrol insects on native species
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Boettner et al., 2000, examined the indirect non-target effects of the introduction of Compsilura concinnata (a parasitoid fly introduced into North America repeatedly from 1906 to 1986, for control of gypsy moths) on native moths, including a silk moth.
They deployed, in the field, larvae of two native silk moth species to examine the attack rate by Compsilura concinnata. They found high levels of parasitism in all samples studied, up to 100% in some samples. They warned that indirect non-target effects could potentially be responsible for extinctions, at least locally, of native insect species.
They also found that few studies had been done on the ecological impact of this and other introduced invertebrates, including the Asian lady beetle (Harmonta axyridis), which was introduced for biological control in the eastern US and Canada over several decades into the 1990s, and is now abundant and widespread in the US and Canada. The Asian lady beetle is now considered one of the most invasive insects. This species has resulted in the decline of native ladybird species and some other insects.
Boettner et al., 2000, warned that, as of 2000, federal requirements and funding for testing the ecological impacts of biological control agents is inadequate.
Hennemann and Memmott, 2001, warned that looking solely at direct effects on non-target plant species is not adequate to determine ecological impacts of biocontrol agents. They found that, “Indirect effects on native [insect] species are the most difficult to assess. An insect herbivore introduced to control a weed could be attacked by generalist native parasitoids that also have native hosts. If the… biological control agent is abundant, then there is the potential for apparent competition between the agent and native [insect] herbivores, mediated via shared native parasitoids. Thus, even the introduction of an entirely species-specific herbivore, presumed to have no nontarget effects, still could have a community-wide impact. Only by understanding how invasive species interact within the context of the entire community can we hope to assess the risks to native species, whether they be direct effects on single species or indirect effects on several species across trophic levels.”
“There have been at least 122 releases of parasitic wasps and flies against agricultural pest Lepidoptera in Hawaii within the past 100 years, providing high potential for nontarget effects, particularly on leaf-feeding [native] caterpillars” (Hennemann & Memmott, 2001).
Hennemann and Memmott, 2001, conducted an extensive field study of parasitoids found in moths in the remote Alakai Swamp in the mountains on Kauai. They purposely chose a location distant from, and climatically and ecologically distinct from, the agricultural area in which the biological control agents had been introduced. The goal was to quantify the “mortality of native moths caused by alien parasitic wasps.” They collected 2112 caterpillars from two plots in the swamp. They found parasitoids in both leaf-eating caterpillars and carnivorous caterpillars. “Fifty-eight moth species (4 alien and 54 endemic) were reared from 60 plant species (47 endemic, 6 indigenous, and 7 alien). Out of 216 parasitoids reared from 2112 caterpillars collected, most (83%) were biological control agents introduced against lowland agricultural pests, followed by accidentally introduced wasps (14%). Native parasitoids were relatively rare (4%). The most common parasitoid species were the generalist braconid control agents Meteorus laphygmae, reared from at least 12 species of moths in six different families, and Cotesia marginiventris, reared from at least nine species in three families…The level of attack by alien parasitoids is estimated to be 19% in 1999 and 22% in 2000.” While some caterpillar species may be able to sustain this level of attack, it is not known how many native species had disappeared prior to the study.
Unfortunately, there has been little study of native parasitoids in Hawaii. The small number of native parasitic wasps found in this study may be due to displacement by alien parasitic wasps, or may be the result of historically low species of parasitoids on remote islands. Nonetheless, the study suggests the potential for introduced biological control agents, such as parasitic wasps, to invade nontarget ecosystems and cause significant damage to native caterpillars, and therefore, indirectly impact the ecology of native plant species.
Suckling and Sforza, 2014, consider that when the removal of an invasive plant species is followed by re-invasion by a different noxious weed species, this is also an example of an indirect effect. They point out that this result is the most common type of indirect effect documented. While some treated locations may naturally return to native plant growth, the prevalence of cases in which other noxious weed species invade the treated location, points to the importance of having a plan and the resources to re-seed areas with native plant species, before treating an area with either biocontrol agents or herbicides.
Suckling and Sforza also reported on the potential for moderate to major indirect effects on non-plant species from only four biocontrol agents:
· Urophora affinis and U. quadri-fasciata (knapweed seedhead flies): target species, spotted knapweed (Centaurea maculosa); indirect effect on elevating deer mouse populations.
· Diorhabda elongata (Mediterranean tamarisk beetle): target species, salt cedar or tamarisk (Tamarix spp.); indirect effect on loss of bird nesting habitat, mainly for the endangered southwestern willow flycatcher. (However, the effects are complex and include some factors beneficial to birds, namely the beetles providing a source of food for birds).
· Rhinocyllus conicus (weevil): target species, musk thistle (Carduus nutans); indirect effect declining populations of native picture-wing flies.
· Chrysolina quadrigemina (Klamath weed beetle): target species, St. John’s wort (Hypericum perforatum); indirect effect, increasing populations of several other noxious weed species.
The northern tamarisk beetle, Diorhabda carinulata, is now restricted for use as a biocontrol agent (USFS, 2014), as the damage caused by its use can reduce nesting habitat for the southwestern willow flycatcher. The beetle has expanded its range beyond the original targeted States. Other birds also likely nest in salt cedar, and salt cedar may provide pollen for honeybees. Diverse species of Diorhabda are suited to different ranges in the Western States, but all species have generated concerns about reducing bird habitat in established stands of salt cedar.
Additional cases of indirect effects are described in the book, Biological Control, by Heimpel and Mills, 2017.
References:
Continue with, or return to, reports on Biocontrol Agents:
Indirect impacts of biocontrol insects on native species
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Boettner et al., 2000, examined the indirect non-target effects of the introduction of Compsilura concinnata (a parasitoid fly introduced into North America repeatedly from 1906 to 1986, for control of gypsy moths) on native moths, including a silk moth.
They deployed, in the field, larvae of two native silk moth species to examine the attack rate by Compsilura concinnata. They found high levels of parasitism in all samples studied, up to 100% in some samples. They warned that indirect non-target effects could potentially be responsible for extinctions, at least locally, of native insect species.
They also found that few studies had been done on the ecological impact of this and other introduced invertebrates, including the Asian lady beetle (Harmonta axyridis), which was introduced for biological control in the eastern US and Canada over several decades into the 1990s, and is now abundant and widespread in the US and Canada. The Asian lady beetle is now considered one of the most invasive insects. This species has resulted in the decline of native ladybird species and some other insects.
Boettner et al., 2000, warned that, as of 2000, federal requirements and funding for testing the ecological impacts of biological control agents is inadequate.
Hennemann and Memmott, 2001, warned that looking solely at direct effects on non-target plant species is not adequate to determine ecological impacts of biocontrol agents. They found that, “Indirect effects on native [insect] species are the most difficult to assess. An insect herbivore introduced to control a weed could be attacked by generalist native parasitoids that also have native hosts. If the… biological control agent is abundant, then there is the potential for apparent competition between the agent and native [insect] herbivores, mediated via shared native parasitoids. Thus, even the introduction of an entirely species-specific herbivore, presumed to have no nontarget effects, still could have a community-wide impact. Only by understanding how invasive species interact within the context of the entire community can we hope to assess the risks to native species, whether they be direct effects on single species or indirect effects on several species across trophic levels.”
“There have been at least 122 releases of parasitic wasps and flies against agricultural pest Lepidoptera in Hawaii within the past 100 years, providing high potential for nontarget effects, particularly on leaf-feeding [native] caterpillars” (Hennemann & Memmott, 2001).
Hennemann and Memmott, 2001, conducted an extensive field study of parasitoids found in moths in the remote Alakai Swamp in the mountains on Kauai. They purposely chose a location distant from, and climatically and ecologically distinct from, the agricultural area in which the biological control agents had been introduced. The goal was to quantify the “mortality of native moths caused by alien parasitic wasps.” They collected 2112 caterpillars from two plots in the swamp. They found parasitoids in both leaf-eating caterpillars and carnivorous caterpillars. “Fifty-eight moth species (4 alien and 54 endemic) were reared from 60 plant species (47 endemic, 6 indigenous, and 7 alien). Out of 216 parasitoids reared from 2112 caterpillars collected, most (83%) were biological control agents introduced against lowland agricultural pests, followed by accidentally introduced wasps (14%). Native parasitoids were relatively rare (4%). The most common parasitoid species were the generalist braconid control agents Meteorus laphygmae, reared from at least 12 species of moths in six different families, and Cotesia marginiventris, reared from at least nine species in three families…The level of attack by alien parasitoids is estimated to be 19% in 1999 and 22% in 2000.” While some caterpillar species may be able to sustain this level of attack, it is not known how many native species had disappeared prior to the study.
Unfortunately, there has been little study of native parasitoids in Hawaii. The small number of native parasitic wasps found in this study may be due to displacement by alien parasitic wasps, or may be the result of historically low species of parasitoids on remote islands. Nonetheless, the study suggests the potential for introduced biological control agents, such as parasitic wasps, to invade nontarget ecosystems and cause significant damage to native caterpillars, and therefore, indirectly impact the ecology of native plant species.
Suckling and Sforza, 2014, consider that when the removal of an invasive plant species is followed by re-invasion by a different noxious weed species, this is also an example of an indirect effect. They point out that this result is the most common type of indirect effect documented. While some treated locations may naturally return to native plant growth, the prevalence of cases in which other noxious weed species invade the treated location, points to the importance of having a plan and the resources to re-seed areas with native plant species, before treating an area with either biocontrol agents or herbicides.
Suckling and Sforza also reported on the potential for moderate to major indirect effects on non-plant species from only four biocontrol agents:
· Urophora affinis and U. quadri-fasciata (knapweed seedhead flies): target species, spotted knapweed (Centaurea maculosa); indirect effect on elevating deer mouse populations.
· Diorhabda elongata (Mediterranean tamarisk beetle): target species, salt cedar or tamarisk (Tamarix spp.); indirect effect on loss of bird nesting habitat, mainly for the endangered southwestern willow flycatcher. (However, the effects are complex and include some factors beneficial to birds, namely the beetles providing a source of food for birds).
· Rhinocyllus conicus (weevil): target species, musk thistle (Carduus nutans); indirect effect declining populations of native picture-wing flies.
· Chrysolina quadrigemina (Klamath weed beetle): target species, St. John’s wort (Hypericum perforatum); indirect effect, increasing populations of several other noxious weed species.
The northern tamarisk beetle, Diorhabda carinulata, is now restricted for use as a biocontrol agent (USFS, 2014), as the damage caused by its use can reduce nesting habitat for the southwestern willow flycatcher. The beetle has expanded its range beyond the original targeted States. Other birds also likely nest in salt cedar, and salt cedar may provide pollen for honeybees. Diverse species of Diorhabda are suited to different ranges in the Western States, but all species have generated concerns about reducing bird habitat in established stands of salt cedar.
Additional cases of indirect effects are described in the book, Biological Control, by Heimpel and Mills, 2017.
References:
- Boettner, G.H., Elkinton, J.S., & Boettner, C.J. (Dec., 2000). Effects of a biological control introduction on three nontarget native species of Saturniid moths. Conservation Biology, 14, (6), 1798-1806.
- Heimpel, G.E, & Mills, N.J. (2017). Biological Control: Ecology and Applications. Cambridge: Cambridge University Press.
- Henneman, M.L., & Memmott, J. (Aug. 17, 2001). Infiltration of a Hawaiian community by introduced biological control agents. Science, New Series, 293 (5533), 1314-1316.
- Suckling, D.M., & Sforza, R.F.H. (January, 2014). What magnitude are observed non-target impacts from weed biocontrol? PLoS ONE 9(1). Doi:10.1371/journal.pone.0084847
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