Photo: Kochia, southwest Montana. © 2020 Delena Norris-Tull
The Impacts of Pesticides on Reptiles and Amphibians
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
In a study of the impact of spot-spraying glyphosate, used to control a Bitou Bush invasion, on an Eastern Australian native reptile, Martin & Murray, 2013, “found no significant short-term (7 - 10 months) differences in reptile abundance, species richness or assemblage composition among invaded, uninvaded and sprayed sites before and after glyphosate application.”
Martin & Murray, 2013, conclude that, “While we would not recommend basing management decisions on the outcomes of our study alone, we suggest that our findings can be used to assist in the development of strategic analyses of glyphosate impacts on native flora and fauna.”
Picone, 2015, compared the effects of aquatic herbicides and housing density on the abundance of five species of frogs in lakes in Massachusetts. At each lake, Picone assessed the housing density and frog abundance. Picone found that frog abundance declined with increased housing density. Picone compared seven untreated lakes with five lakes that had been treated annually or biannually for approximately 20 years, with Reward (diquat bromide), Captain (a chelated copper algaecide) and/or Rodeo (glyphosate). By itself, herbicide was not a significant main effect. But there was a “marginal interaction between herbicides and housing density; lakes with moderate-high housing densities seemed to have lower frog abundance with herbicide treatments.” Only one frog species, green frog, Rana clamitans, was less common as housing density increased.
King & Wagner, 2010, report that glyphosate-based herbicide products can be highly toxic to amphibians. They tested the toxicity of glyphosate to six pond-breeding amphibian species in the Northwestern U.S. Using six different concentrations of herbicide up to 5.0 mg AI/L, they exposed amphibian larvae to RoundUp Regular in a laboratory/aquaria setting. “Treatment concentrations were chosen to represent concentrations of glyphosate that have been measured in natural bodies of water and include a worst case scenario of a direct overspray at maximum application rate.” After placing amphibian larvae into aquaria with different herbicide concentrations, they recorded larval mortality each day over 15 days.
King & Wagner, 2010, results: The six species varied greatly in their response to exposure to the herbicide. “Anurans were more directly sensitive than salamanders at 24 hours with a higher incidence of mortality.” Within the anurans there were significant differences in toxicity, “which varied from 0.43 mg AI/L for [Pseudacris] regilla [Pacific tree frog] to 2.66 mg AI/L for [Anaxyrus] boreas [Western toad]… No P. regilla tadpoles survived above concentrations of >1.0 mg AI/L during the initial 24-hour exposure.” With longer exposure times, the LC50 (lethal concentration with 50% mortality) was lower for P. regilla at 15 days, meaning that the Pacific tree frog died at a lower dose if exposed to the herbicide for 15 days.
For all six species, “The LC50s for the longer exposure times of 7 and 15 days were lower compared to the 24-hour LC50s…. Overall, there was a trend for decreased mean time-to-death with increasing Roundup® Regular concentration… All the anurans died within 24 hours at the highest concentration.”
“There was also strong evidence for differences among species in their mean time-to-death.” P. regilla, Rana cascadae (Cascades frog), and Rana luteiventris (Columbia spotted frog) all had a short mean time-to-death compared to the salamanders (Ambystoma macrodactylum, long-toed salamander, and Ambystoma gracile, Northwestern Salamander) and to A. boreas.
“Psuedacris regilla was the most sensitive species with a mean time-to-death of 24 hours for the 2.0 mg AI/L Roundup treatment…, and A. boreas was the least sensitive… with a mean time-to-death of 8.5 days for the 2 mg AI/L Roundup treatment. In contrast, the mean time-to-death was 3.3 days for R. cascadae and 2.1 days for R. luteiventris for the 2 mg AI/L.”
King & Wagner, 2010, conclusions: Even though glyphosate is not supposed to be used in aquatic systems, it has been found in toxic concentrations in bodies of water. “Glyphosate-based herbicides are highly toxic to Pacific Northwestern amphibians and have the potential for synergistic interactions with other environmental stressors; therefore, we urge that less toxic formulations should be made widely available to the consumer.” They also advocate for restricted applications of glyphosate products, so that they are not used during the season when amphibian larvae are present.
Wagner, et al., 2013, conducted a meta-analysis of the research on the impact of glyphosate-based herbicides on amphibians. They located only 51 relevant toxicological studies. They concluded that, “Because little is known about environmental concentrations of glyphosate in amphibian habitats and virtually nothing is known about environmental concentrations of the substances added to the herbicide formulations that mainly contribute to adverse effects, glyphosate levels can only be seen as approximations for contamination with glyphosate-based herbicides. The impact on amphibians depends on the herbicide formulation, with different sensitivity of taxa and life stages. Effects on development of larvae apparently are the most sensitive endpoints to study… If and how glyphosate-based herbicides and other pesticides contribute to amphibian decline is not answerable yet due to missing data on how natural populations are affected.”
Wagner, et al., 2013, recommend that research needs to be expanded, particularly in these three areas:
“1) filling basic knowledge gaps through studies with comparable design and modeling, addressing especially chronic and delayed effects of glyphosate-based herbicides and added substances;
2) long-term monitoring of glyphosate-based herbicides in the environment and of free-living amphibians, aiming at the detection of effects on individuals and especially abnormal local population changes; and
3) an ongoing analysis of information and risk assessment (especially of new substances to come and costressors) in the context of worldwide amphibian decline and extinction.”
Lenkowski, et al., 2008, examined the impact of different doses of atrazine on the development of tadpoles of the amphibian Xenopus laevis (African clawed frog.) This species is often used as a vertebrate model in genetic research. “We found a significant dose-dependent increase in the percentage of atrazine-exposed tadpoles with malformations of multiple tissues including the main body axis, circulatory system, kidney, and digestive system. Incidence of apoptotic cells also increased in the both midbrain and kidney of atrazine-exposed tadpoles…. Our results demonstrate that acute atrazine exposure (10–35 mg/L for ≤ 48 hr) during early organ morphogenesis disrupts proper organ development in an amphibian model system. The concurrent atrazine-induced apoptosis in the pronephric kidney and midbrain begins to elucidate a mechanism by which atrazine may disrupt developmental processes in nontarget organisms.”
The decline of the Texas horned lizard, classified in Texas as a threatened species, is attributed to loss of habitat, the use of herbicides and other pesticides, and the increase of fire ants. These lizards primarily eat harvester ants. Increases in the non-native red fire ant populations has had a negative impact on the lizard populations (Linam, 2008). Insecticides used to kill ants have negatively impacted the native harvester ant population, which has had a negative impact on the lizards. In addition, the insecticides are toxic to the lizards.
The Houston toad, listed as an endangered species, has declined in numbers due to a variety of conditions, including drought, habitat loss, and pesticide use. One of the few remaining large populations has been in Bastrop County, Texas. Large fires in that area in recent years have also greatly impacted toad habitat.
References:
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The Impacts of Pesticides on Reptiles and Amphibians
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
In a study of the impact of spot-spraying glyphosate, used to control a Bitou Bush invasion, on an Eastern Australian native reptile, Martin & Murray, 2013, “found no significant short-term (7 - 10 months) differences in reptile abundance, species richness or assemblage composition among invaded, uninvaded and sprayed sites before and after glyphosate application.”
Martin & Murray, 2013, conclude that, “While we would not recommend basing management decisions on the outcomes of our study alone, we suggest that our findings can be used to assist in the development of strategic analyses of glyphosate impacts on native flora and fauna.”
Picone, 2015, compared the effects of aquatic herbicides and housing density on the abundance of five species of frogs in lakes in Massachusetts. At each lake, Picone assessed the housing density and frog abundance. Picone found that frog abundance declined with increased housing density. Picone compared seven untreated lakes with five lakes that had been treated annually or biannually for approximately 20 years, with Reward (diquat bromide), Captain (a chelated copper algaecide) and/or Rodeo (glyphosate). By itself, herbicide was not a significant main effect. But there was a “marginal interaction between herbicides and housing density; lakes with moderate-high housing densities seemed to have lower frog abundance with herbicide treatments.” Only one frog species, green frog, Rana clamitans, was less common as housing density increased.
King & Wagner, 2010, report that glyphosate-based herbicide products can be highly toxic to amphibians. They tested the toxicity of glyphosate to six pond-breeding amphibian species in the Northwestern U.S. Using six different concentrations of herbicide up to 5.0 mg AI/L, they exposed amphibian larvae to RoundUp Regular in a laboratory/aquaria setting. “Treatment concentrations were chosen to represent concentrations of glyphosate that have been measured in natural bodies of water and include a worst case scenario of a direct overspray at maximum application rate.” After placing amphibian larvae into aquaria with different herbicide concentrations, they recorded larval mortality each day over 15 days.
King & Wagner, 2010, results: The six species varied greatly in their response to exposure to the herbicide. “Anurans were more directly sensitive than salamanders at 24 hours with a higher incidence of mortality.” Within the anurans there were significant differences in toxicity, “which varied from 0.43 mg AI/L for [Pseudacris] regilla [Pacific tree frog] to 2.66 mg AI/L for [Anaxyrus] boreas [Western toad]… No P. regilla tadpoles survived above concentrations of >1.0 mg AI/L during the initial 24-hour exposure.” With longer exposure times, the LC50 (lethal concentration with 50% mortality) was lower for P. regilla at 15 days, meaning that the Pacific tree frog died at a lower dose if exposed to the herbicide for 15 days.
For all six species, “The LC50s for the longer exposure times of 7 and 15 days were lower compared to the 24-hour LC50s…. Overall, there was a trend for decreased mean time-to-death with increasing Roundup® Regular concentration… All the anurans died within 24 hours at the highest concentration.”
“There was also strong evidence for differences among species in their mean time-to-death.” P. regilla, Rana cascadae (Cascades frog), and Rana luteiventris (Columbia spotted frog) all had a short mean time-to-death compared to the salamanders (Ambystoma macrodactylum, long-toed salamander, and Ambystoma gracile, Northwestern Salamander) and to A. boreas.
“Psuedacris regilla was the most sensitive species with a mean time-to-death of 24 hours for the 2.0 mg AI/L Roundup treatment…, and A. boreas was the least sensitive… with a mean time-to-death of 8.5 days for the 2 mg AI/L Roundup treatment. In contrast, the mean time-to-death was 3.3 days for R. cascadae and 2.1 days for R. luteiventris for the 2 mg AI/L.”
King & Wagner, 2010, conclusions: Even though glyphosate is not supposed to be used in aquatic systems, it has been found in toxic concentrations in bodies of water. “Glyphosate-based herbicides are highly toxic to Pacific Northwestern amphibians and have the potential for synergistic interactions with other environmental stressors; therefore, we urge that less toxic formulations should be made widely available to the consumer.” They also advocate for restricted applications of glyphosate products, so that they are not used during the season when amphibian larvae are present.
Wagner, et al., 2013, conducted a meta-analysis of the research on the impact of glyphosate-based herbicides on amphibians. They located only 51 relevant toxicological studies. They concluded that, “Because little is known about environmental concentrations of glyphosate in amphibian habitats and virtually nothing is known about environmental concentrations of the substances added to the herbicide formulations that mainly contribute to adverse effects, glyphosate levels can only be seen as approximations for contamination with glyphosate-based herbicides. The impact on amphibians depends on the herbicide formulation, with different sensitivity of taxa and life stages. Effects on development of larvae apparently are the most sensitive endpoints to study… If and how glyphosate-based herbicides and other pesticides contribute to amphibian decline is not answerable yet due to missing data on how natural populations are affected.”
Wagner, et al., 2013, recommend that research needs to be expanded, particularly in these three areas:
“1) filling basic knowledge gaps through studies with comparable design and modeling, addressing especially chronic and delayed effects of glyphosate-based herbicides and added substances;
2) long-term monitoring of glyphosate-based herbicides in the environment and of free-living amphibians, aiming at the detection of effects on individuals and especially abnormal local population changes; and
3) an ongoing analysis of information and risk assessment (especially of new substances to come and costressors) in the context of worldwide amphibian decline and extinction.”
Lenkowski, et al., 2008, examined the impact of different doses of atrazine on the development of tadpoles of the amphibian Xenopus laevis (African clawed frog.) This species is often used as a vertebrate model in genetic research. “We found a significant dose-dependent increase in the percentage of atrazine-exposed tadpoles with malformations of multiple tissues including the main body axis, circulatory system, kidney, and digestive system. Incidence of apoptotic cells also increased in the both midbrain and kidney of atrazine-exposed tadpoles…. Our results demonstrate that acute atrazine exposure (10–35 mg/L for ≤ 48 hr) during early organ morphogenesis disrupts proper organ development in an amphibian model system. The concurrent atrazine-induced apoptosis in the pronephric kidney and midbrain begins to elucidate a mechanism by which atrazine may disrupt developmental processes in nontarget organisms.”
The decline of the Texas horned lizard, classified in Texas as a threatened species, is attributed to loss of habitat, the use of herbicides and other pesticides, and the increase of fire ants. These lizards primarily eat harvester ants. Increases in the non-native red fire ant populations has had a negative impact on the lizard populations (Linam, 2008). Insecticides used to kill ants have negatively impacted the native harvester ant population, which has had a negative impact on the lizards. In addition, the insecticides are toxic to the lizards.
The Houston toad, listed as an endangered species, has declined in numbers due to a variety of conditions, including drought, habitat loss, and pesticide use. One of the few remaining large populations has been in Bastrop County, Texas. Large fires in that area in recent years have also greatly impacted toad habitat.
References:
- King, J.J., & Wagner, R.S. (2010). Toxic effects of the herbicide RoundUp Regular on Pacific Northwestern amphibians. Northwestern Naturalist, 91:318-324.
- Lenkowski, J.R., Reed, M., Deininger, L., & McLaughlin, K.A. (Feb., 2008). Perturbation of organogenesis by the herbicide atrazine in the amphibian Xenopus laevis. Environmental Health Perspectives, 116(2). https://ehp.niehs.nih.gov/doi/10.1289/ehp.10742
- Martin, L.J., & Murray, B.R. (Jan., 2013). A preliminary assessment of the response of a native reptile assemblage to spot-spraying invasive Bitou Bush with glyphosate herbicide. Ecological Management & Restoration, 14(1):59-62.
- Picone, C. (March, 2015). Effects of aquatic herbicides and housing density on abundance of pond-breeding frogs. Northeastern Naturalist, 22(1):26-39.
- Wagner, N., Reichenbecher, W., Teichmann, H., Tappeser, B., & Lotters, S. (Aug., 2013). Questions concerning the potential impact of glyphosate-based herbicides on amphibians. Environmental Toxicology and Chemistry, 32(8): 1688-1700.
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