DOSE-RESPONSE OF WEEDS TO METHYL-IODIDE AND METHYL-BROMIDE

Laboratory bioassay and field experiments were conducted to characteri ze the dose response of weeds to methyl iodide and methyl bromide as s oil fumigants. The patterns in potency of both fumigants and in sensit ivity of different weed species to the fumigants were distinguished wi th the use of logistic dose-response models. Similar to its response t o methyl bromide fumigation, Amaranthus retroflexus L. was the most se nsitive to methyl iodide fumigation. Cyperus rotundus L. was the least sensitive to methyl iodide fumigation, whereas Portulaca oleracea L. was the least sensitive to methyl bromide. Lolium multiflorum Lam., Ab utilon theophrasti Medik., Chenopodium album L., P. oleracea, Brassica kaber (D.C.) L.C. Wheeler and Cyperus esculentus L. were similar in s ensitivity to methyl iodide. Methyl iodide was as potent as methyl bro mide for A. retroflexus but more potent than methyl bromide for L. mul tiflorum, A. theophrasti, C. album, P. oleracea, B. kaber, C. esculent us and C. rotundus. The dose response for weeds in the field was simil ar to that obtained in laboratory bioassays. Under field conditions, 2 80 kg ha(-1) methyl iodide killed all species tested except Solanum ni grum L. Methyl iodide appears to be a suitable replacement for methyl bromide because it can be used in situations similar to methyl bromide fumigation, has superior efficacy against a broad spectrum of pests a nd has a low potential for degrading the earth's ozone layers.