Abstract
2,4-dichlorophenoxyacetic acid and monosodium methanearsonate are often sold in commercial herbicide mixtures. Toxicity studies have been performed for each herbicide individually, but there is a dearth of information concerning the toxicity of these herbicides in a mixture. The following study examined the toxicity of a mixture of these two herbicides in the red swamp crawfish, Procambarus clarkii. 96-hour acute toxicity assays were performed to determine whether surfactant significantly altered the toxicity of these herbicides individually or in combination. Markings additive index was calculated to identify the interactions of the herbicide mixture. Surfactant was observed to significantly increase the toxicity of 2,4-dichlorophenoxyacetic acid and the toxicity of the herbicide mixture. The herbicide mixture alone displayed half the toxicity of the individual herbicides, but the mixture with surfactant was twice as toxic as the individual herbicides. The synergistic action of surfactant may be attributed to increased pesticide absorption across biological membranes. 2,4-dichlorophenoxyacetic acid and surfactant may also compromise gill function, increasing the sensitivity of the crawfish to herbicide toxicity. The antagonistic effects of the herbicide mixture in the absence of surfactant may be caused by competition of both herbicides for the same sites of activity.
Highlights
When precipitation washes herbicides from soil and foliage surfaces into waterways in close proximity to sprayed areas, nontarget organisms may be directly affected through exposure or indirectly affected through consumption of exposed organisms
Because of the rapid microbial decomposition 2,4-D undergoes when applied to soil rather than sprayed directly onto foliage, it may be paired with another herbicide such as monosodium methanearsonate (MSMA) to control germinating annuals [2]
The relatively high solubilities of 2,4-D and MSMA made possible the use of the static with renewal method of acute toxicity testing, in which test crawfish were moved to fresh dechlorinated water containing the same concentration of herbicide as the original tank after 24 hours to minimize waste buildup, oxygen depletion, and degradation of the herbicides
Summary
When precipitation washes herbicides from soil and foliage surfaces into waterways in close proximity to sprayed areas, nontarget organisms may be directly affected through exposure or indirectly affected through consumption of exposed organisms. MSMA is widely used to clear weeds from non-cropland areas such as public rights-of-way; it is used to control weed growth in sugar cane, a major money crop in Louisiana. Because of the rapid microbial decomposition 2,4-D undergoes when applied to soil rather than sprayed directly onto foliage, it may be paired with another herbicide such as MSMA to control germinating annuals [2]. This well illustrates the field practice of using multiple herbicides to increase cost effectiveness and effectiveness to target vegetation while supposedly reducing negative impacts on nontarget species and the environment in general [3]. Even when not directly applied in a mixture, herbicides may undergo subsequent mixing due to environmental factors
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