April 25, 2008 

Routinely treating for corn rootworms, regardless of pest population, can lead to unnecessary spending and pesticide application.

Peoples' attitudes toward integrated pest management (IPM) for crops seem to have changed, given the current increase in grain commodity values. Some adjustment in IPM decision-making is needed with higher commodity prices, but we've seen some growers making unwise changes.

Commodity value is a factor in calculating the economic injury level — the breakeven point where the value of potential yield loss from untreated pests is equal to control costs to protect the crop from future injury. As commodity values rise, economic injury levels decrease, if all other variables remain constant.

The role of the farmer or consultant is to monitor the crop, and if a pest population is present and increasing in number, use a pest control tactic before the pest population reaches the economic injury level. To time control measures correctly, economic thresholds have been developed. The economic threshold is the level where control should be applied to prevent a pest population from reaching the econoic injury level. These thresholds often are set at about 80% of the economic injury threshold.

Extension recommendations often use tables to show how economic thresholds change with control costs and commodity value. In these cases you can simply identify the appropriate economic threshold for your situation. If control costs or commodity values are higher or lower than those in the table, adjust the thresholds accordingly by following the rising or declining trend of the table.

Soybean Aphids

Soybean aphids on a single leaf, 2002. Entomologists are recommending a treatment threshold of 250 aphids per plant, despite higher commodity prices this year. The threshold is set to account for pest population increases and provides a treatment window of four to five days.

One important exception to this is the economic threshold for the soybean aphid. When the current economic threshold of 250 aphids per plant was developed, it allowed growers a week to apply an insecticide before aphids were likely to increase above the economic injury level (which is much higher than 250 aphids per plant). Midwest soybean entomologists are recommending that even with higher soybean market values, growers should continue to use the economic threshold of 250 aphids per plant. The higher soybean market values mean that growers have a four- to five-day window to treat rather than a seven-day window.

'Insurance' or 'Clean-up' Treatments

Other changes in grower practices may not be as wise. Frequently, growers may want to tank-mix insecticides with other pesticides without accounting for the level of pests in a field. The logic apparently is that with higher commodity values, an insecticide application has to save fewer bushels to break even or make a profit.

There are several flaws with this approach. First, using insecticides for low levels of pests is not likely to increase yield and may actually reduce grower profits. Second, there are longer term effects of insecticide use that should be considered. We have seen examples in Nebraska where growers tank mix an insecticide with the last glyphosate application in soybeans to "clean up the field." The problem with this is that current insecticides typically used for soybean aphid control, such as pyrethroids or chlorpyrifos, also kill many beneficial predatory and parasitic insects. This has led to increased survival of soybean aphids or spider mites later in the season, resulting in the need for additional pesticide use later in the season. Similar results can be seen in other crops.

We also have notice an increase in "insurance" or "prophylactic" insecticide treatments. These are when treatment applied before a pest is in the field, with the expectation that the pest will likely occur and cause economic loss. This strategy is best used when a pest is difficult to scout for, can cause severe economic loss, cannot be easily managed once it is injuring a crop, and has a relatively high probability of occurring. People who don't scout their fields and grow continuous corn often have treated for corn rootworms on a prophylactic basis, although not all continuous corn fields have economic problems with corn rootworms. Most pests for a given crop occur sporadically. Those that do occur more regularly can be managed effectively by scouting and using economic thresholds to determine treatment needs.

Continuous, widespread, and/or unnecessary use of insecticides increases the risk of insecticide resistance developing, and unnecessarily releases toxins or pesticides into the environment.

Integrating Control Options

Another aspect of integrated pest management is the use of a variety of control strategies to manage pests rather than relying solely on chemical treatments. As more people shift to corn after corn, there is the potential for increasing populations of corn rootworms to develop in local areas. Crop rotation is still the best approach to reduce densities of western corn rootworms in Nebraska. Where we have high populations of corn rootworms, all control practices have an increased possibility of reduced levels of control, whether growers are relying on traditional planting time liquid or granular insecticides, insecticidal seed treatments or Bt corn hybrids.

Although some adjustments in IPM recommendations are wise to make given the higher crop values, don't make unwise changes that will prove unprofitable in the short or long term. IPM is still a useful concept and strategy to profitably manage pests, while protecting human health and the environment.

Bob Wright
Extension Entomologist, Lincoln
Tom Hunt
Extension Entomologist, Northeast REC