Crop Management Practices Favoring Soil Predators in Field Corn

Several studies in field corn demonstrate that cultivation reduces the abundance and diversity of soil inhabiting beneficial arthropods (particularly ground beetles and spiders, but also other groups). Part of this effect is direct mortality due to cultivation, but part is from the reduced surface residue and organic matter levels in cultivated fields compared with reduced or no-till production systems. Many predatory insects and mites build up in fields with higher levels of plant residues because they feed on organisms involved in breaking down organic matter. These predators are then available to feed on pest insects. Another practice influencing soil predators is the use of soil insecticides, most of which have a broad spectrum of activity against insects and mites. Either cultivation or insecticide effects on soil inhabiting beneficials can be demonstrated.

Compare the effect of tillage on natural enemies by examining two fields close together: one farmed using conventional tillage practices and the other farmed using reduced or no-till production. Ideally the field using reduced tillage should have been under this system for a few years, as it may take more than one year to fully develop the effects on soil insect predators. Soil inhabiting predators are commonly sampled using pitfall traps. These can be as simple as a mason jar or plastic cup buried in the soil so that the lip is flush with the ground surface. Insects and spiders walking over the soil surface fall into the jar and cannot crawl out. You may add a small amount of antifreeze to pitfall traps to kill and preserve insects, especially if you cannot check the traps frequently. Use 5 or more pitfall traps per treatment depending on the size of the area. Pitfall traps should be placed in the middle of the area being studied to minimize border effects. Depending on your indentification skills, spiders and ground beetles could be counted, or you could include other types of insects too. Compare the number of predators and the species found in the fields with different cultivation histories.

Similarly, if an untreated strip was left in a field normally treated with soil insecticide at planting for corn rootworm, this could be compared to the insecticide-treated area. Some extension entomologists have been conducting on-farm research evaluating the use of reduced rates (1/2 -3/4 labelled rate) of soil insecticides for corn rootworm control. Again, if you can sample in untreated areas or areas treated with the full labelled rate, you may observe differences in the soil predators associated with the level of insecticide use. A minimum plot size would be about 10 rows wide to minimize movement between treatments. Smaller plots are not desirable because predator movement between plots may mask any effect due to the treatments being compared.

- Bob Wright, University of Nebraska


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