Impacts of Roundup Weathermax and Ignite 280 Sl on Amphibians in the Southern High Plains
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The Southern High Plains (SHP) region of the United States is dominated by agricultural enterprise. In many areas, playa wetlands are the only remaining patches of native habitat. As such, they are vital for the persistence of flora and fauna in this region, including amphibians. Because most playas are embedded in cropland, SHP amphibians may encounter a variety of agricultural chemicals due to contaminated runoff or direct terrestrial exposure. However, no previous work has examined whether commonly applied herbicides pose a threat to larvae or juveniles of SHP species. Initially, I investigated the toxicity of widely used herbicides Roundup WeatherMAX® and Ignite® 280 SL to larval New Mexico and Plains spadefoot toads (Spea multiplicata and S. bombifrons, respectively) from three cropland and two grassland playas. To assess the effects of short-term herbicide exposure, I obtained Roundup WeatherMAX® and Ignite® 280 SL acute toxicity estimates (i.e., LC50 values) for both species. Because larvae may experience prolonged exposure under field conditions, I also investigated how survival of larval New Mexico and Plains spadefoot toads was affected by 30-day exposure to these herbicides at environmentally-relevant levels. I hypothesized that, due to historical differences in herbicide exposure, larvae from cropland playas would be less sensitive to Roundup WeatherMAX® and Ignite® 280 SL compared to those from grassland playas. The toxicity of formulated glyphosate herbicides (like Roundup WeatherMAX®) toward aquatic organisms is thought to result primarily from surfactants. To increase mechanistic understanding of surfactant toxicity toward larval amphibians, I also examined the histological impacts of a non-ionic surfactant (ADSEE 907®) on skin and gills of Spea spp. larvae. Because surfactants disrupt skin and gill structure of aquatic organisms, I hypothesized that skin and gill lesions would be more extensive among Spea spp. larvae exposed to ADSEE 907®. Following metamorphosis, juvenile SHP amphibians may disperse or inhabit moist areas near drying playas. This may result in direct exposure to agrochemicals. Therefore, I also investigated how the short-term survival of juvenile New Mexico spadefoot and Great Plains toads (Bufo cognatus) housed on soil or moist paper towels was affected by exposure to environmentally-relevant levels of a glufosinate-based herbicide (Ignite® 280 SL) and several glyphosate-based herbicides (Roundup WeatherMAX®, Roundup Weed and Grass Killer Super Concentrate®, and Roundup Weed and Grass Killer Ready-To-Use Plus®). The hypothesis that Ignite® 280 SL or Roundup WeatherMAX® sensitivity would differ between spadefoot larvae from cropland and grassland playas was not supported. Acute toxicity tests indicated that New Mexico spadefoots were less sensitive to Ignite® 280 SL than Plains spadefoots. Ignite® 280 SL 48-hr LC50 values for both species (3.55-5.55 mg glufosinate/L) were well above environmentally relevant concentrations. These results agree with those from chronic toxicity tests; 30-day exposure to environmentally relevant levels of Ignite® 280 SL did not reduce survival among New Mexico or Plains spadefoot larvae. Acute toxicity tests with Roundup WeatherMAX® indicated no between-species variation in herbicide sensitivity. Roundup WeatherMAX® 48- and 216-hr LC50 values for New Mexico and Plains spadefoot larvae (1.65-2.30 mg glyphosate acid equivalents/L) were similar to environmental concentrations expected from accidental direct overspray. While chronic exposure to environmentally relevant levels of Roundup WeatherMAX® reduced survival of both species, the response of New Mexico spadefoots differed by landuse. Survival of New Mexico spadefoots from grassland playas was greater than those from cropland playas following the 30-day exposure. Contrary to expectations, skin and gill lesions were not consistently more extensive among larvae exposed to ADSEE 907®, even though mortality was greater among these larvae compared to controls. The extent of gill lesions was similar among control larvae and those exposed to surfactant. While one type of skin lesion (apical hyperplasia) was more extensive among larvae exposed to surfactant, several other lesions (apical and skein necrosis) were more extensive among control larvae. Because larvae were collected from a SHP playa wetland, the expected histological response may have been obscured by prior contaminant induced lesions. It is also possible that other environmental stressors present in cropland playas contributed to observed skin and gill lesions. Additionally, the histological profile of larvae may have been influenced by normal tissue restructuring associated with metamorphosis. These results may indicate that larval skin and gills are not the primary target of non-ionic surfactants. For example, it is possible that general narcosis is the primary mode of toxicity. Survival of New Mexico spadefoots and Great Plains toads juveniles was not affected by exposure to Roundup WeatherMAX® or Ignite® 280 SL on either paper towels or soil. However, New Mexico spadefoot and Great Plains toad survival was reduced by exposure to Roundup Weed and Grass Killer Ready-To-Use Plus® on moist paper towels or soil. The toxicity of this formulation may result from included "pelargonic and related fatty acids." However, since this product is intended for lawn and garden use, it is unlikely that large numbers of SHP amphibians will encounter this formulation under field conditions. Great Plains toads exposed to Roundup Weed and Grass Killer Super Concentrate® on paper towels also exhibited reduced survival. However, this result has limited ecological relevance to SHP amphibians because widespread exposure is unlikely and mortality occurred only on a highly artificial substrate (i.e., paper towels). Great Plains toad survival was not affected by exposure to this formulation under more realistic conditions (i.e., experimental tubs lined with soil). Acute and chronic toxicity data suggest that Ignite® 280 SL does not pose a mortality risk to larval New Mexico and Plains spadefoots. However, Roundup WeatherMAX® may pose a mortality risk to larvae of these species. Further studies under increasingly realistic conditions are needed to determine if this is the case. Also, it is important to investigate whether Ignite® 280 SL and Roundup WeatherMAX® exert sublethal impacts on larvae of these species. When used properly, the agricultural herbicides tested (Roundup WeatherMAX® and Ignite® 280 SL) likely do not pose a threat to juvenile New Mexico spadefoots and Great Plains toads. Future studies should examine whether sub-lethal endpoints (e.g., growth, reproduction) are negatively affected by common agricultural herbicides, and if environmental factors modulate herbicide toxicity.
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