POPULATION SUPPRESSION OF ADULT CORN ROOTWORMS (DIABROTICA SPP.) AND THE EFFECT OF ROOTWORM ADULTS ON POLLINATION IN INDIANA CORNFIELDS

During 1975 - 1979, 2 areas of corn rootworm pest management were examined: population suppression of adult corn rootworms as a means of larval damage prevention and the effect of corn rootworm (CRW) adults on pollination in Indiana cornfields. From 1975 - 1979, 17 fields were treated aerially with 1.13 kg/ha of carbaryl during early stages of CRW oviposition. The efficacy of adult suppression was compared to the efficacy of soil insecticides applied to corn the following year as a means of larval damage prevention. Where economic infestations of CRW larvae were present, the aerial spray provided adequate control (root damage \u3c 2.5 on a scale of 1-6) in 50% of the fields, the soil insecticide 88%. Soil insecticides alone also provided adequate control in 75% of the fields where aerial sprays were not efficacious. Results indicate that using an aerial spray the previous season will not significantly improve the performance of soil insecticides at planting time. Results of these studies also suggest that adult suppression will not provide satisfactory control in fields with adult CRW populations (GREATERTHEQ) 3 beetles/plant. Results of 1977-78 pollination damage studies indicate that 3 beetles/ear for the inbred and 5 beetles/ear for hybrid corn can significantly reduce ear weight and the number of kernels/ear. Corn plants compensate for reduced kernel set by increasing kernel weight, but the response is weak when compared to the trend for reduction in ear weight. Compensation may mask some pollination damage at very low CRW populations. Fields of inbred corn where plants are uniformly infested (100%) with 3 beetles/ear would suffer a loss of 7 ql/ha using individual ear grain loss estimates from this study. For hybrid corn, a uniform infestation of 5 beetles/ear would cause a field loss of 6.5 ql/ha. However, several factors must be considered in converting field loss estimates to economic thresholds. CRW adults are not uniformly distributed in cornfields, and the stage of pollination can vary considerably over individual fields. Both factors can directly change the proportion of acreage needing protection. Economic thresholds reflecting the percentage pollination and the percentage of the field infested with CRW adults were developed for both inbred and hybrid corn. Inbred corn is of considerable value as parent lines in the production of hybrid seed corn. With a price of $24/ql, marginal economic damage (1.1 ql or $28/ha) would occur when an individual field is 20% pollinated with 20% of the field infested with 3 beetles/ear. For hybrid corn, using a value of $8/ql, marginal economic damage (3.1 ql or $25/ha) would occur when an individual field is 20% pollinated with 60% of the field infested with 5 beetles/ear. These studies also show that if a field is (GREATERTHEQ) 70% pollinated, even 8 beetles/ear would not cause an economic loss. The results of this research suggest that when evaluating potential CRW pollination damage, the stage of pollination and the field percentage infestation must be considered