Photo: Cacti & Creosote Bush, Chihuahuan Desert in Big Bend National Park. © 2017 Delena Norris-Tull
Impacts of biocontrol insects on non-target native plant species
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Suckling and Sforza, 2014, concluded that non-target impacts of insects in natural ecosystems have been rare. The vast majority of impacts on non-target plants have been minimal or minor, with no known long-term effects. The only major native plant injuries have stemmed from two biocontrol insects species (Cactoblastis cactorum impacts on native cacti in North America and Mexico, and Rhinocyllus conicus impacts on native thistles in the US), both of which had been introduced in the early days of biocontrol use, injuries they believe would not occur in today’s stricter requirements. They concluded that the benefits of using biocontrol agents outweigh any known negative impacts. They developed a five-step scale to use in evaluating adverse environmental effects from biological control introductions.
Louda et al., 1997, examined the major impact of the weevil Rhinocyllus conicus on native thistles. This weevil was introduced into Canada in 1968 and into the US in 1969, to control exotic thistles. They found that, “Between 1992 and 1996, the frequency of weevil damage to native thistles consistently increased, reaching 16% to 77% of flowerheads per plant…The density of native tephritid flies was significantly lower at high weevil density.” Thus, in addition to negative impacts on native thistles, the weevil also has had significant indirect impact on native flies.
Suckling and Sforza, 2014, report moderate impacts from only two additional biocontrol insects, Larinus planus and Trichosirocalus horridus, which have had impacts on two additional native thistles in North America. These two biocontrol agents were released between 1958-1988.
“The frequency and magnitude of nontarget [native] plant seed destruction, the time delay from introduction to host range expansion…, [and] the geographic extent of spread to native species were not predicted.” They noted that “diet specialization is one of the crucial criteria in the selection of biological control agent… [and] further evaluation of ecological interactions [should] be required.” They concluded that, “the outcome also reinforces suggestions that ecological consequences may be difficult to predict in advance” (Louda, et al., 1997).
Johnson and Stiling, 1998, examined the impact of the cactus moth, Cactoblastis cactorum, on native Opuntia species, prickly pear cactus, in Florida. This moth originated in the southern countries of South America, and thus had never existed in the US and Mexico, which are home to many native Opuntia species. The cactus moth was introduced successfully as a biological control on prickly pear cacti in Australia in 1925, and somewhat less successfully in South Africa. No known negative impacts have been documented in these areas, which are far away from the native habitat for Opuntia species in the Americas. But in 1957, the moth was introduced to Nevis and Saint Kitts, islands in the Caribbean, to reduce native Opuntia populations. Unfortunately, even today, quite a few states in the USA still allow native plant species to be treated as noxious weeds, thus increasing the likelihood for additional disastrous ecological results in the future.
From the Caribbean Islands, where it is now widespread, the cactus moth made its way into Florida by 1989. From 1991-1993, Johnson and Stilling examined the impact it had on native Opuntia species in six sites throughout Florida. They found damage on as many as 90% of larger cacti, and found that it could kill smaller plants. The moth has spread throughout the southeastern US and into Mexico. It has severely reduced populations of various species of native Opuntia species in the US, including one endangered species. The cactus moth arrived in Texas in 2017 or 2018 and now threatens native species there. Opuntia species, which include prickly pear, pencil cactus, and cholla, provide food for wildlife, including javelinas, and nests for birds, such as roadrunners and cactus wrens. Prickly pear cactus has great market value, as the source of foods such as nopalitos and sweets made from the cactus fruits.
Thus far, the number of species of introduced biocontrol agents that have had major or moderate ecological impacts remains small, representing only 0.8% (4 species) of biocontrol agents. Suckling and Sforza, 2014, report that of the 512 biocontrol agents released worldwide, 91.6% have had no known direct or indirect non-target impacts. Another 7.6% have had minor or minimal impacts, with no known reduction in non-target plant populations. 77% of impacted non-target plant species were in the same family as the target species, with 54% in the same genus. Thus very few biological agents (32) have impacted native species that were not in the same genus or family as the target weed species, and all of those impacts were minimal or minor.
But we must heed the warnings expressed by these authors, and others listed below, that future biocontrol introductions must include short- and long-term evaluations of ecological impacts, both direct and indirect.
References:
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Impacts of biocontrol insects on non-target native plant species
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Suckling and Sforza, 2014, concluded that non-target impacts of insects in natural ecosystems have been rare. The vast majority of impacts on non-target plants have been minimal or minor, with no known long-term effects. The only major native plant injuries have stemmed from two biocontrol insects species (Cactoblastis cactorum impacts on native cacti in North America and Mexico, and Rhinocyllus conicus impacts on native thistles in the US), both of which had been introduced in the early days of biocontrol use, injuries they believe would not occur in today’s stricter requirements. They concluded that the benefits of using biocontrol agents outweigh any known negative impacts. They developed a five-step scale to use in evaluating adverse environmental effects from biological control introductions.
Louda et al., 1997, examined the major impact of the weevil Rhinocyllus conicus on native thistles. This weevil was introduced into Canada in 1968 and into the US in 1969, to control exotic thistles. They found that, “Between 1992 and 1996, the frequency of weevil damage to native thistles consistently increased, reaching 16% to 77% of flowerheads per plant…The density of native tephritid flies was significantly lower at high weevil density.” Thus, in addition to negative impacts on native thistles, the weevil also has had significant indirect impact on native flies.
Suckling and Sforza, 2014, report moderate impacts from only two additional biocontrol insects, Larinus planus and Trichosirocalus horridus, which have had impacts on two additional native thistles in North America. These two biocontrol agents were released between 1958-1988.
“The frequency and magnitude of nontarget [native] plant seed destruction, the time delay from introduction to host range expansion…, [and] the geographic extent of spread to native species were not predicted.” They noted that “diet specialization is one of the crucial criteria in the selection of biological control agent… [and] further evaluation of ecological interactions [should] be required.” They concluded that, “the outcome also reinforces suggestions that ecological consequences may be difficult to predict in advance” (Louda, et al., 1997).
Johnson and Stiling, 1998, examined the impact of the cactus moth, Cactoblastis cactorum, on native Opuntia species, prickly pear cactus, in Florida. This moth originated in the southern countries of South America, and thus had never existed in the US and Mexico, which are home to many native Opuntia species. The cactus moth was introduced successfully as a biological control on prickly pear cacti in Australia in 1925, and somewhat less successfully in South Africa. No known negative impacts have been documented in these areas, which are far away from the native habitat for Opuntia species in the Americas. But in 1957, the moth was introduced to Nevis and Saint Kitts, islands in the Caribbean, to reduce native Opuntia populations. Unfortunately, even today, quite a few states in the USA still allow native plant species to be treated as noxious weeds, thus increasing the likelihood for additional disastrous ecological results in the future.
From the Caribbean Islands, where it is now widespread, the cactus moth made its way into Florida by 1989. From 1991-1993, Johnson and Stilling examined the impact it had on native Opuntia species in six sites throughout Florida. They found damage on as many as 90% of larger cacti, and found that it could kill smaller plants. The moth has spread throughout the southeastern US and into Mexico. It has severely reduced populations of various species of native Opuntia species in the US, including one endangered species. The cactus moth arrived in Texas in 2017 or 2018 and now threatens native species there. Opuntia species, which include prickly pear, pencil cactus, and cholla, provide food for wildlife, including javelinas, and nests for birds, such as roadrunners and cactus wrens. Prickly pear cactus has great market value, as the source of foods such as nopalitos and sweets made from the cactus fruits.
Thus far, the number of species of introduced biocontrol agents that have had major or moderate ecological impacts remains small, representing only 0.8% (4 species) of biocontrol agents. Suckling and Sforza, 2014, report that of the 512 biocontrol agents released worldwide, 91.6% have had no known direct or indirect non-target impacts. Another 7.6% have had minor or minimal impacts, with no known reduction in non-target plant populations. 77% of impacted non-target plant species were in the same family as the target species, with 54% in the same genus. Thus very few biological agents (32) have impacted native species that were not in the same genus or family as the target weed species, and all of those impacts were minimal or minor.
But we must heed the warnings expressed by these authors, and others listed below, that future biocontrol introductions must include short- and long-term evaluations of ecological impacts, both direct and indirect.
References:
- Johnson, D.M., & Stiling, P.D. (1998). Distribution and dispersal of Cactoblastis cactorum, an exotic Opuntia-feeding moth, in Florida. The Florida Entomologist, 81 (1), 12-22.
- Louda, S.M., Kendall, D., Connor, J., & Simberloff, D. (22 August, 1997). Ecological effects of an insect introduced for biological control of weeds. Science 277 (5329), 1088-1090.
- 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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