MANAGING CORN ROOTWORMS - (COLEOPTERA-CHRYSOMELIDAE) ON DAIRY FARMS -THE NEED FOR A SOIL INSECTICIDE

Field trials were conducted at 20 locations in New York during 1993 an d 1994 to evaluate the effects of using manuring practices and soil in secticide on rootworm injury plant height, lodging, and yield of corn. All fields had adult densities of western corn rootworm, Diabrotica v irgifera virgifera LeConte, exceeding 1 beetle per plant during the ye ar preceding the test. The following 4 treatments were tested: (1) ino rganic fertilizer, (2) inorganic fertilizer plus soil insecticide, (3) manure, and (4) manure plus soil insecticide. Root injury was signifi cantly reduced by soil insecticides during both years but was unaffect ed by manure treatments. Both manure applications and soil insecticide s increased plant heights during June and July. The effect of treatmen ts on yields differed by year. In 1993, soil insecticides significantl y increased silage yields an average of 3.05 metric tons (MT)/ha but h ad no effect on grain yield. In contrast, manuring did not significant ly affect either silage or grain yield. In 1994, silage yields were 3. 45 MT/ha higher in manured treatments compared with nonmanured treatme nts. Soil insecticides increased silage yield 2.3 MT/ha in nonmanured treatments but did not affect silage yield in manured treatments. Grai n yields were not affected by either insecticide or manure treatments in 1993 but were significantly higher (4.7 quintals/ha) in insecticide treatments in 1994. The effect of manure on corn tolerance to western corn rootworm seemingly is greater during years when corn is not unde r moisture stress. Multiple regression models, incorporating soil char acteristics, nutrients in fertilizers, and planting information, were developed to predict crop yield and change in yield expected with the addition of a soil insecticide. Yield models also suggest the need for more controlled experiments to characterize directly the effects of d rainage, potassium and phosphorus availability, pendimethalin herbicid e injury, soil pH, and rainfall on soil insecticide benefits.