Field Crops News

Do Organic and Conventional Fields Differ in Pest and Beneficial Insect Numbers?

Many farming practices are known to affect the seasonal growth patterns of plants and corresponding levels of pest and beneficial insects. In this study, we examined how conventional and organic farming systems differ in this regard. For the purpose of the study, "conventional" systems are defined as having high inorganic fertility inputs, a shorter crop rotation, a relatively early planting date, and medium to high herbicide use. A system with low or no inorganic fertility inputs, a longer crop rotation, a relatively late planting date, and low to no herbicide use is defined as "organic."

Field corn grown on three pairs of organic and conventional farms in Ohio was monitored for pest and beneficial insects during the 1995 growing season. All fields were first year corn, with the exception of one organic field of second year corn. Soil nitrogen was tested in the late spring, and plant growth was monitored over the growing season.

In conventional systems, early planting combined with higher nitrogen inputs resulted in taller plants early in the season. This greatly impacted insect colonization, as evidenced by significantly higher levels of damage by the European corn borer (Ostrinia nubilalis), which exceeded economic thresholds in two of the three conventional fields, but in none of the organic fields. On the other hand, late planting and slow early-season growth of plants in the organic systems appeared to result in low levels of corn pests. Although significantly higher levels of weeds on organic farms attracted several pests associated with grassy fields, such as armyworms, none of these pests approached economic thresholds. Grain yields were significantly different in only 1 of the 3 farm pairs, where deficient nitrogen levels on an organic farm resulted in reduced yield. These data suggest that variation in nitrogen level, planting date, crop rotation, and herbicide usage may promote very different pest complexes, without necessarily affecting yield.

Food source played an important role in determining population levels of beneficial insects. In conventional systems, significantly higher levels of aphids supported greater numbers of predators and parasites, such as ladybird beetles and parasitic wasps. In organic systems, habitat appeared to be the most important factor. Higher levels of weeds may have provided additional habitat or food sources for the predatory bugs (e.g. minute pirate bugs and assassin bugs) present at higher levels in organic fields.

It is clear that organic and conventional corn fields supported different insect communities. Planting date, fertilization rate, and weed level all appear to have played important roles.

- Charlotte Bedet, OARDC, Wooster, Ohio

The Impact of Insecticides on Ground Beetles

Insecticides are widely used in controlling corn soil insect pests in the U.S. corn belt, but have undesirable side effects, such as suppression of non-target organisms. Because soil insecticides may affect natural enemies differently than pests, studies were conducted to determine the impact of common soil insecticides on ground beetles in corn. In a two year laboratory bioassay and field microplot study at the Ohio State University, three insecticides commonly used in corn fields (permethrin, tefluthrin and chropyrifos) were tested against the most abundant ground beetles in Ohio corn fields. All of the products tested were highly toxic to the beetles. In each case, the LD50 of topical applications were lower than 4 ppm, and the mortality of ground beetles introduced into treated microplots was greater than 70%.

Using a specialized trapping technique, a field study was performed to determine the movement pattern of ground beetles in response to the application of insecticides in the field. It was found that ground beetles left treated plots and moved into untreated plots or the grassy areas between plots in the first three days after application of insecticides. However, the mortality of ground beetles moving out of the treated plots was high. The exact reason for the movement of ground beetles in response to the insecticide application is not clear yet. However, other entomologists have shown that ground beetles may be stimulated by pesticides, walking faster with less turning. While some may escape, others simply expose themselves to more chemical residues in the process. Given the high mortality of ground beetles in treated fields, the only way for the within-field population to recover is by dispersal from the boundary or non-treated fields.

The potential role of ground beetles as predators of insect pests in corn suggests that they should be conserved in an integrated pest management program. Based on these studies, it is likely that ground beetles can be aided by reducing the use of insecticides within fields and by providing untreated field borders. Untreated areas appear to be essential to maintaining predator populations in these farming systems. To encourage activity of predatory ground beetles in corn agroecosys-tems, insecticides should be applied with care and integrated with other pest management strategies.

- Z. Chen and H. R. Wilson, The Ohio State University (Based on a poster presented at the 1996 Ent. Soc. Amer. Annual Meeting)


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