Field Sampling of Alfalfa for the Estimation of Guthion Residues1

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Estimates of Guthion® (O,O-dimethyl S-(4-oxo-3H-1,2,3- benzotriazine-3-methyl) phosphorodithioate) residues on alfalfa have been determined by collecting three types of samples and performing chemical analyses and biological assays on extracts of these samples. A 7.6-acre field of alfalfa was sprayed with a water emulsion of Guthion at the rate of 1 pound of Guthion per acre. A two-stage sampling scheme was used to obtain single, composite, and cross-plot composite samples at intervals of 1 day and 3, 7, 14,21, and 28 days following spraying. For each sampling day, 9 plots, 36 feet by 36 feet, were chosen at random; then 14 subplots, 2 feet by 2 feet, were similarly chosen from within each plot. All the alfalfa within a subplot constituted a sample. The alfalfa in 2 of the subplots in each of the 9 plots was analyzed separately as single samples, the alfalfa from 10 of the remaining 1:/ subplots was used to form 2 composite samples, each consisting of the combined alfalfa from 5 subplots. The alfalfa in the remaining two subplots in each plot was used to form two cross-plot composite samples. This design provided 228 samples for analysis.

One hundred-gram, well-chopped portions of each alfalfa sample were extracted by a procedure that yielded combined surface and internal residues of Guthion. A decolorized benzene extract was ultimately obtained and aliquots of this solution were used for colorimetric analyses and bioassays. The residue data obtained by both analytical procedures and for the three types of samples were very similar. As an example of these data, the weighted mean chemical residue values of single and composite samples were 51.5, 28.6, 8.7, 1.4, 0.2, and 0.2 p.p.m., respectively, for samples gathered at the six intervals after spraying that are listed above. Because the residues in the last three sets of samples were very low, it was not possible to estimate these residues accurately by biological assays even though a fortifying procedure was used. The residue data indicated that with this sampling scheme and a uniform stand of alfalfa, little, if any, advantage results in using composite samples over single samples to estimate mean residues. For practical purposes, if one were to adopt the two-stage sampling method used in this study for residue estimates involving more than one insecticide and rate of application, a convenient combination would consist of collecting five subplot samples (either single or composite) selected from three plots.

Additional information was obtained concerning the variation in sample weights and moisture, sample processing, and analytical procedures.

Document Type: Editorial

Publication date: October 1, 1959

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  • Journal of Economic Entomology is published bimonthly in February, April, June, August, October, and December. The journal publishes articles on the economic significance of insects and is divided into the following sections: apiculture & social insects; arthropods in relation to plant disease; forum; insecticide resistance and resistance management; ecotoxicology; biological and microbial control; ecology and behavior; sampling and biostatistics; household and structural insects; medical entomology; molecular entomology; veterinary entomology; forest entomology; horticultural entomology; field and forage crops, and small grains; stored-product; commodity treatment and quarantine entomology; and plant resistance. In addition to research papers, Journal of Economic Entomology publishes Letters to the Editor, interpretive articles in a Forum section, Short Communications, Rapid Communications, and Book Reviews.
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