Abstract
The effects of methyl parathion and methomyl applications upon yields and photosynthetic rates were examined on lettuce in the growth period between thinning and rosette formation. Reductions in lettuce head weight and density were linearly correlated with the increased numbers of methyl parathion applications made during the season. Applications of methomyl were correlated with increases in lettuce head weight. A positive correlation was found between the percentage of plants that bolted before formation of a marketable head and the number of applications of both insecticides; methyl parathion elicited the greater effect. Both insecticides reduced photosynthetic rates and leaf conductance rates, and reductions were linearly correlated with rates of insecticide applied. The effects of the insecticides on physiological processes were of short duration. Based on calculated reductions in yield, a pesticide threshold of not more than three applications of methyl parathion per season on lettuce in southern California was suggested.
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