This study was conducted to evaluate the effectiveness of a systemic insecticide, phorate, and varietal resistance in reducing earworm, Heliothis zea (Boddie), damage to maize. Phorate, also designated as Thimet, contained 0,0-diethyl S-(ethyl-thiomethyl) phosphorodithioate,
A split-plot design was employed with hybrids on whole plots and insecticide treatments on subplots. Significant differences among the four single cross hybrids were found for all the six characteristics studied, viz., ear and grain weights, larval penetration, damaged seed percentage, husk extension, and husk tightness. The effect of phorate was significant in reducing the larval penetration depth and in increasing the undamaged grain weight. There was a tendency for the differences among hybrids to be accentuated following the insecticide treatments. Husk tightness was an important morphological character in determining corn earworm resistance, but excess husk extension did not contribute additional resistance.
M. H. Yu* and J. D. Smith
*USDA-ARS, Agric. Res. Sta., Salinas, California
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