Photo: Broom snakeweed, NE New Mexico. © 2017 Delena Norris-Tull
Impacts of Herbicides on Butterflies and Their Host plants
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
Schultz & Ferguson, 2020, analyzed the impacts of herbicide-based restoration on habitat for the endangered Fender’s blue butterfly (Icaricia icarioides fenderi) and the threatened Kincaid’s lupine (Lupinus oreganus), the host plant for the butterfly larvae. These two species persist in highly degraded prairie remnants in western Oregon. They commented that, while there are a number of studies on the impact of herbicides on native plants, very few studies investigate the impacts of herbicides on animals (including insects).
Oregon has placed a priority on restoration of prairie remnants, as most of the habitats have been lost, and remaining habitats have been greatly degraded by invasive plants. Controlled fires and mowing have been useful in reducing invasive woody plants, but these have had limited success at reducing invasive grasses. In the Willamette Valley prairies, the main grass-specific herbicide in use is fluazifop-p-butyl, which has been shown to be effective at reducing invasive grasses, while having limited impacts on native forbs, native bunchgrass fescue, and butterflies.
Schultz & Ferguson, 2020, conducted field experiments in two remnant prairies west of Eugene, Oregon. Some plots received a chemical-free strategy of annual fall mowing plus seeding with native species in 2013, 2014, and 2015. The other plots were treated with the same strategy of annual fall mowing and seeding, and supplemented with an annual herbicide application in late March, 2014, 2015, and 2016.
Each June, butterfly eggs, lupine leaves, racemes, and seedpods were counted in five randomly selected lupine plots that had at least 30% lupine cover. The following April, larvae were counted, and larval feeding damage was assessed. Due to low counts of larvae in one site, in 2014 the plot sizes were increased. The counts were completed each year 2013-2016.
Schultz & Ferguson, 2020, results: “Herbicide treatment was associated with increases in [butterfly] reproduction, as measured by egg abundance and eggs per leaf, but only in some sites and some years… Density of eggs per leaf and overall egg abundance was much lower at Willow Creek than Fern Ridge and there were no years [at Willow Creek] in which higher egg abundance was associated with herbicide treatments….The effect of the herbicide treatment at [Fern Ridge] was not sustained and fecundity was significantly higher in the [chemical-free] plots in the following year. At Willow Creek, overall fecundity was much lower in 2015 than 2016. Here fecundity was higher in the [chemical-free] plots in 2015 and there were no notable effects of herbicide treatments in 2016.”
“Annual [butterfly] population growth rate was higher in herbicide-treated plots at Fern Ridge in 2014, but this effect was not sustained. There were no notable differences in estimated annual population growth rate in 2015 or 2016 at Fern Ridge or Willow Creek.”
“Overall, the abundance of Kincaid’s lupine leaves did not change in response to treatments… We did, however, see a response in Kincaid’s lupine seedpods. At Fern Ridge in 2016, there were more than twice as many pods in herbicide-treated plots than [chemical-free] plots…,and at Willow Creek, there were twice as many seedpods in herbicide-treated plots as [chemical-free] plots in both 2014 and 2016… There were more seedpods in herbicide-treated plots than [chemical-free] plots at Fern Ridge in 2016 and at Willow Creek in 2014 and 2016.” They also documented that “secondary invasion by non-native forb species increased over time in these plots.”
Schultz & Ferguson, 2020, “We conclude that grass-specific herbicide application can be included as a restoration tool in Fender’s blue butterfly habitat, but only if it results in reaching restoration goals unrelated to butterfly population enhancement….For the butterflies, and other nontarget invertebrates, there are no prior studies assessing effects on survival and fecundity in a field setting.”
Bennion, Ferguson, New, & Schultz, 2020, expanded on the study by Schultz & Ferguson, 2020, using the same data on treatments. They examined additional plant community dynamics, for one year prior to the treatments and for three years during treatment. They reported the following results: herbicide treatment did have the intended impact on invasive grasses, by reducing height of the grasses relative to the lupines. Surprisingly, lupine cover increased in the control plots but not in the herbicide plots. “Nectar production declined in 2015 but increased back to 2014 levels in 2016… We found control plots in Fern Ridge to have more nectar resources than herbicide plots... At Willow Creek in 2014–2015, there was no notable difference in nectar production between control and herbicide plots. However, in 2016 we noted a substantial increase in the herbicide plot relative to the control plot…. Annual sampling along transects detected 94 species of plants in the two conservation areas... We observed a general trend of exotic forbs (and one native fern) increasing in frequency relative to native forbs... We identified 10 exotic and one native forb species as well as one native fern that showed substantial increases in frequency in at least one plot following [herbicide] treatment.”
Bennion, et al., 2020, conclusions: “The application of grass‐specific herbicide met the management objective of altering the structure of Fender's blue habitat by reducing the height of invasive grasses and increasing the apparency of Kincaid's lupine, but resulted in unexpected costs…. Many restoration studies that focus on invasive species fail to monitor the response of native community…. We found 11 species, including false dandelion, each of which substantially increased in frequency as a result of herbicide treatment. Three of these species are native: coast tarweed, bracken fern, and wild strawberry… There are seven other exotic forbs that have increased substantially… We cannot predict the combined effect of all nine prominent invaders. Their interactions have the potential to result in a net gain for invaders relative to native species…. To promote establishment of nectar species we seeded native forbs throughout the experiment. Some species (e.g. Allium amplectans and Iris tenax var. tenax) gained in frequency while others declined or never established (e.g. Sidalcea malviflora ssp. virgata or Camassia leichtlinii) following seeding, but this change did not result in a net increase in nectar production in herbicide plots…. The treatment of invasive species with taxa‐specific herbicides can facilitate the establishment of species from other functional groups and thus begin the invasion treadmill.”
References:
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Impacts of Herbicides on Butterflies and Their Host plants
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
Schultz & Ferguson, 2020, analyzed the impacts of herbicide-based restoration on habitat for the endangered Fender’s blue butterfly (Icaricia icarioides fenderi) and the threatened Kincaid’s lupine (Lupinus oreganus), the host plant for the butterfly larvae. These two species persist in highly degraded prairie remnants in western Oregon. They commented that, while there are a number of studies on the impact of herbicides on native plants, very few studies investigate the impacts of herbicides on animals (including insects).
Oregon has placed a priority on restoration of prairie remnants, as most of the habitats have been lost, and remaining habitats have been greatly degraded by invasive plants. Controlled fires and mowing have been useful in reducing invasive woody plants, but these have had limited success at reducing invasive grasses. In the Willamette Valley prairies, the main grass-specific herbicide in use is fluazifop-p-butyl, which has been shown to be effective at reducing invasive grasses, while having limited impacts on native forbs, native bunchgrass fescue, and butterflies.
Schultz & Ferguson, 2020, conducted field experiments in two remnant prairies west of Eugene, Oregon. Some plots received a chemical-free strategy of annual fall mowing plus seeding with native species in 2013, 2014, and 2015. The other plots were treated with the same strategy of annual fall mowing and seeding, and supplemented with an annual herbicide application in late March, 2014, 2015, and 2016.
Each June, butterfly eggs, lupine leaves, racemes, and seedpods were counted in five randomly selected lupine plots that had at least 30% lupine cover. The following April, larvae were counted, and larval feeding damage was assessed. Due to low counts of larvae in one site, in 2014 the plot sizes were increased. The counts were completed each year 2013-2016.
Schultz & Ferguson, 2020, results: “Herbicide treatment was associated with increases in [butterfly] reproduction, as measured by egg abundance and eggs per leaf, but only in some sites and some years… Density of eggs per leaf and overall egg abundance was much lower at Willow Creek than Fern Ridge and there were no years [at Willow Creek] in which higher egg abundance was associated with herbicide treatments….The effect of the herbicide treatment at [Fern Ridge] was not sustained and fecundity was significantly higher in the [chemical-free] plots in the following year. At Willow Creek, overall fecundity was much lower in 2015 than 2016. Here fecundity was higher in the [chemical-free] plots in 2015 and there were no notable effects of herbicide treatments in 2016.”
“Annual [butterfly] population growth rate was higher in herbicide-treated plots at Fern Ridge in 2014, but this effect was not sustained. There were no notable differences in estimated annual population growth rate in 2015 or 2016 at Fern Ridge or Willow Creek.”
“Overall, the abundance of Kincaid’s lupine leaves did not change in response to treatments… We did, however, see a response in Kincaid’s lupine seedpods. At Fern Ridge in 2016, there were more than twice as many pods in herbicide-treated plots than [chemical-free] plots…,and at Willow Creek, there were twice as many seedpods in herbicide-treated plots as [chemical-free] plots in both 2014 and 2016… There were more seedpods in herbicide-treated plots than [chemical-free] plots at Fern Ridge in 2016 and at Willow Creek in 2014 and 2016.” They also documented that “secondary invasion by non-native forb species increased over time in these plots.”
Schultz & Ferguson, 2020, “We conclude that grass-specific herbicide application can be included as a restoration tool in Fender’s blue butterfly habitat, but only if it results in reaching restoration goals unrelated to butterfly population enhancement….For the butterflies, and other nontarget invertebrates, there are no prior studies assessing effects on survival and fecundity in a field setting.”
Bennion, Ferguson, New, & Schultz, 2020, expanded on the study by Schultz & Ferguson, 2020, using the same data on treatments. They examined additional plant community dynamics, for one year prior to the treatments and for three years during treatment. They reported the following results: herbicide treatment did have the intended impact on invasive grasses, by reducing height of the grasses relative to the lupines. Surprisingly, lupine cover increased in the control plots but not in the herbicide plots. “Nectar production declined in 2015 but increased back to 2014 levels in 2016… We found control plots in Fern Ridge to have more nectar resources than herbicide plots... At Willow Creek in 2014–2015, there was no notable difference in nectar production between control and herbicide plots. However, in 2016 we noted a substantial increase in the herbicide plot relative to the control plot…. Annual sampling along transects detected 94 species of plants in the two conservation areas... We observed a general trend of exotic forbs (and one native fern) increasing in frequency relative to native forbs... We identified 10 exotic and one native forb species as well as one native fern that showed substantial increases in frequency in at least one plot following [herbicide] treatment.”
Bennion, et al., 2020, conclusions: “The application of grass‐specific herbicide met the management objective of altering the structure of Fender's blue habitat by reducing the height of invasive grasses and increasing the apparency of Kincaid's lupine, but resulted in unexpected costs…. Many restoration studies that focus on invasive species fail to monitor the response of native community…. We found 11 species, including false dandelion, each of which substantially increased in frequency as a result of herbicide treatment. Three of these species are native: coast tarweed, bracken fern, and wild strawberry… There are seven other exotic forbs that have increased substantially… We cannot predict the combined effect of all nine prominent invaders. Their interactions have the potential to result in a net gain for invaders relative to native species…. To promote establishment of nectar species we seeded native forbs throughout the experiment. Some species (e.g. Allium amplectans and Iris tenax var. tenax) gained in frequency while others declined or never established (e.g. Sidalcea malviflora ssp. virgata or Camassia leichtlinii) following seeding, but this change did not result in a net increase in nectar production in herbicide plots…. The treatment of invasive species with taxa‐specific herbicides can facilitate the establishment of species from other functional groups and thus begin the invasion treadmill.”
References:
- Bennion, L.D., Ferguson, J.A., New, L.F, & Schultz, C.B. (Jan. 9, 2020). Community-level effects of herbicide-based restoration treatments: Structural benefits but at what cost? Restoration Ecology, 28(3). https://doi.org/10.1111/rec.13118
- Schultz, C.B., & Ferguson, J.A. (May, 2020). Demographic costs and benefits of herbicide-based restoration to enhance habitat for an endangered butterfly and a threatened plant. Restoration Ecology, 28(3): 564-572.
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