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Pesticide usage on the Southern High Plains and acute toxicity of four chemicals to the fairy shrimp Thamnocephalus platyurus (Crustacea: Anostraca).

Article Excerpt
Abstract -- Extensive pesticide use on crops grown on the Southern High Plains (SHP) represents a considerable anthropogenic stressor to ephemeral aquatic ecosystems. These short-lived aquatic ecosystems, known in the southwest as playas, are epicenters of biodiversity on the High Plains. Cotton is the major agricultural crop grown on the SHP, accounting for over half of all cotton produced in Texas. Currently there are 67 different chemicals used to control cotton pests, and when other crops such as grain sorghum are included the number approaches 100. Thamnocephalus platyurus is a fairy shrimp indigenous to the Southern High Plains that is also available commercially. In addition it is used as an invertebrate model for water quality and toxicity testing. Acute toxicity of four agricultural pesticides widely used on the SHP (Methyl Parathion 4E, Tempo[R] SC Ultra [active ingredient cyfluthrin], Roundup[R] [glyphosate], and Karmex[R] DF [diuron]) was determined using laboratory-derived T. platyurus. Twenty-four hour old nauplii experienced mortality (48 hour LC50) at concentrations ranging from 10.99 [micro]g/L for Tempo[R] SC Ultra and 1.248 mg/L for Roundup[R]. These results suggest that the current pesticide application rates have the potential to endanger the native playa invertebrate T. platyurus.

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The Southern High Plains (SHP) is an 8.2 million hectare, 27 county region that starts at the Canadian River in west Texas and extends south past Lubbock, Texas, and west into eastern New Mexico (Fig. 1; Smith 2003). The major land use throughout this region is agriculture, with cotton being the most commonly grown crop species contributing 64% of Texas' crop (Crop Profile for Cotton in Texas 1999). Grain sorghum, soybean, corn, alfalfa, and sunflowers are relatively minor crops and although they may be important in local ecosystems, their limited areas of growth make them "islands" in a sea of cotton (Agricultural Statistics Board 2001). Therefore, the primary focus of this study is with the chemicals currently associated with the production of cotton in the SHP.

Cotton is grown throughout eleven different states in the southern United States, and geographic variation in agricultural practices results from differences in weed and insect pressures. This, in turn, results in major variation in the types and usage of pesticides (Gianessi & Puffer 1990).

[FIGURE 1 OMITTED]

Cotton typically is planted between May 5 and May 25 in Lubbock and the surrounding counties. Farther to the south, the planting window is slightly longer, lasting through early June. Blooms usually appear in mid July with heat unit accumulation ending around 5 October. Harvest begins around 15 October and lasts through November (Ritchie et al. 1994).

Cotton farming requires three to five times more kilograms of chemicals per hectare (ha) than corn or soybeans, and as much as 7 kg/ha of herbicides and 5 kg/ha of insecticides are applied annually to cotton fields throughout the United States. Cotton also requires more applications per year; 4.7 chemical applications as opposed to 1.2 applications for corn (Coupe et al. 1998).

Herbicides are one of the major chemical classes used in the SHP to control cotton weeds. Twenty-four different active ingredients in herbicide formulations are used on the SHP. Herbicides are applied at ten different times throughout the year starting with winter treatments and ending in preharvest applications. The most commonly used herbicide on the SHP was Roundup[R] and its other brand names used as a post-emergent herbicide, which can be applied at five different times throughout the growing season. It is used in high concentrations, with up to 4.5 kg active ingredient/hectare (a.i/ha) as the recommended application rate. Roundup[R] is very water soluble, therefore it can infiltrate into playa ecosystems through runoff or spray drift. The LC50 value for glyphosate (the active ingredient in Roundup[R]) (Relyea 2005) is relatively high (4,000 [micro]g/L for Daphnia magna), indicating that it poses minimal risk to aquatic invertebrates. However, the surfactant used in most Roundup formulations, polyethoxylated tallowamine [POEA],...

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