INFLUENCE OF TERBUFOS AND NICOSULFURON ON IRON CHLOROSIS AND GROWTH OF CORN

Abstract

The larval stage of the spotted cucumber beetle (Diabrotica vigifera zeae), the Mexican corn rootworm, is a highly devastating pest of cornfields in Texas and the Southwest. A systemic organophosphate insecticide, standard release terbufos (ST), applied in row with the corn seed at planting, has been termed effective in reducing crop damage from the Mexican corn rootworm. However, recent field observations have indicated possible association of micronutrient deficiency symptoms with use of terbufos and over-the-top sulfonyl urea herbicides (nicosulfuron, primisulfuron) on crops such as corn. This experiment was conducted to determine if this systemic soil insecticide would affect plant metabolic activities as measured by nutrient deficiency chlorosis and plant growth. The study also involved testing a controlled release terbufos (CR) version of the same insecticide. Each soil insecticide was compared with and without foliar treatments with one of the sulfonyl urea herbicides (nicosulfuron) used in controlling grass-type weeds in corn. The greenhouse study was conducted on a Monteola sandy clay loam (Typic Pellusterts), which is a typical soil used in corn production in southern Texas. Standard release (ST) and CR forms of terbufos were compared at the label recommended rate (1.1 kg a.i./ha) and also at 2 kg a.i./ha. All materials were banded in the seed row at planting. Treatments were arranged in a randomized block design with three replications. Early visual chlorosis ratings (7 days) and chlorophyll readings at 28 days indicated increased interveinal chlorosis with soil insecticide applications. Plants overcame these early effects with time and showed higher chlorophyll than untreated plants at 42 days. Depressed early plant growth and reduced tissue levels of certain micronutrients indicated these systemic insecticides both with and without over-the-top herbicide could be injurious during early stages of plant growth. However, the affected plants were able to overcome most of the effects later in the growing season. Tissue iron (Fe) concentrations were slightly reduced by terbufos alone and reduced further when nicosulfuron was applied. Zinc was reduced substantially by terbufos but appeared to not change when the over-the-top herbicide was included.