Thimet as a Low-Temperature Fumigant against the Pea Aphid

Abstract

On February 25, 1957, two plots of alfalfa at the Walla Walla, Wash., laboratory, each about 0.2 acre, were treated with granulated 2% Thimet3 (0,0-diethyl S-(ethylthio) methyl posphorothioate) at rates of 1 and 2 pounds of active ingredient per acre for control of the pea aphid, Macrosiphum pisi (Harris). The granules were broadcast on the soil surface and not worked in. The odor of Thimet was perceptible for at least 10 days after broadcasting. On March 15 four 50-tip samples were taken from each treated plot and eight similar samples from adjacent untreated alfalfa. When examined in the laboratory, the untreated samples averaged 0.40 pea aphid per tip, samples from the 1-pound dosage 0.03, and those from the 2-pound dosage no aphids. At that time it was supposed that the action of the Thimet was systemic, and no further tests were made except to determine a month later that the alfalfa was not toxic to pea aphids. It was suggested, however, that fumigation might be involved, rather than systemic action, because the mortality occurred so soon after treatment, and the dosages used were too low for systemic action. Therefore, on February 21, 1958, twelve 20-×20-foot plots were treated with ½ or 1 pound per acre of 2% Thimet granules. These plots were laid out in randomized blocks, replicated four times. They were made small so that the Thimet vapor might possibly drift from treated to untreated plots. Counts made just before treatment showed a rather evenly distributed population of pea aphids, averaging 0.77 per tip. One week after treatment one 20-tip sample from each plot was examined for aphids. Out of the 12 samples, 1 contained 20 aphids, 2 had 1 aphid each, and the other 9 had no aphids. All samples containing aphids came from untreated plots. The sample containing 20 aphids was from an untreated plot that was up-wind from all the others, and thus escaped fumigation. The aphid mortality on the other untreated plots indicated considerable drift of the Thimet vapor. When these samples were taken, 5 more tips were taken from each plot, stripped down to a single leaf and the terminal bud, caged and infested with 5 aphids each, of various stages. After 48 hours at 77° F. the cages were examined and the aphid mortality and reproduction recorded. Much growth and reproduction had taken place in all samples, and there was no significant difference between the treated and untreated plots. These results would indicate that no detectable absorption or translocacation of Thimet had occurred at the time the field mortality was found. The field mortality must have been entirely from fumigation.