Responses of high-oil and hybrid corn to rootworm beetles during pollination

High-oil corn (Zea mays L.) may be produced as a physical mixture consistin g of 91% male-sterile hybrid plants and 9% pollinator plants. Two field stu dies were conducted to determine if corn blends consisting of only 9% polli nator plants are more sensitive to high populations of rootworms (Diabrotic a spp.) feeding on silks during pollination than hybrid blends consisting o f 50 to 100% male-fertile plants. In one study, silks of hybrid corn and si lks of the male-sterile hybrid within the high-oil corn blend were clipped daily during pollination to simulate silk clipping caused by rootworms. In a separate study, 165 locations of high-oil corn and 134 locations of hybri d corn were monitored for rootworm population and severity of silk clipping during pollination. Grain yield was significantly reduced (P less than or equal to 0.05) if average lengths of exserted silks were <37 min for high-o il blends with 9% pollinator plants and <25 ram for hybrid corn. Oil, prote in, and starch concentrations of the high-oil grain were not significantly different from those of the untreated check if exserted silks were at least 25 min during pollination. Severity of beetle infestation correlated poorl y with daily average silk length (r = -0.06) and,,vith grain yield (r = -0. 01). Economic thresholds for grain yield were crossed, and appropriate pest control measures should be considered when rootworm beetles consistently c lip exserted silks to <37 mm for high-oil blends with 9% pollinator plants or <25 mm for hybrid corn.