EFFECTS OF SOIL-MOISTURE, NO-TILLAGE AND PREDATORS ON SOUTHERN CORN-ROOTWORM (DIABROTICA-UNDECIMPUNCTATA-HOWARDI) SURVIVAL IN CORN AGROECOSYSTEMS

Field experiments in 1987 and 1988 were conducted to discern the effects of no-tillage (NT), conventional-tillage (CT), and soil moisture (irrigated and non-irrigated) on a below-ground corn pest, the southern corn rootworm (SCR), Diabrotica undecimpunctata howwadi Barber (Coleoptera: Chrysomelidae), and its natural enemies. Soil moisture, number of SCR eggs, and soil predators found in each system were recorded, as were observations of below-ground SCR/predator interactions. To measure the impact of SCR feeding in each treatment, root ratings, percent stand loss and grain yields were taken. There were 80% more SCR eggs in NT than CT, and 62% more eggs in irrigated systems than non-irrigated systems. Soil moisture levels were not significantly (P > 0.05) different between any of the treatments when SCR were ovipositing in April and May. However, in early June, CT without irrigation (CT/WOI) had very low soil moisture levels, while the other three treatments had adequate to good soil moisture. Predatory arthropods were found in nearly equal numbers in April and May, but numbers decreased in June and July, then increased in August. Predator numbers fluctuated, but remained relatively high in all NT treatments and slowly decreased in CT with irrigation (CT/WI) treatments. Although the numbers of arthropods were generally the same among treatments in the early season, predator activity was greatest in NT systems, while in CT systems (including irrigated) predator activity was much lower. Seven predator species were found responsible for removing large numbers of SCR stages. Mesostigmatid and Tyrophagus putrescentiae Shrank (Acaridae: Acarinae) mites, cantharid, carabid, and staphylinid larvae, centipedes, and formicids (ants) all fed extensively on SCR immatures. Root ratings and percent stand loss showed that SCR feeding was greatest in CT systems and least in NT systems, regardless of irrigation. NT systems had greater grain yields compared with CT systems. The importance of early season SCR damage and predators and mid-to-late season water-stressed corn is discussed. © 1990.