October 27, 2008

Bob Wright, Extension Entomologist
Tom Hunt, Extension Entomologist

Save $5-$10/ac by avoiding unnecessary insecticide applications.

Plan IPM treatments based on the Economic Injury Level — the point where the value of potential yield loss from untreated pests is equal to control costs to prevent future injury.

The Economic Injury Level

Peoples' attitudes toward integrated pest management practice for crops seem to change as commodity prices increase. Some adjustments in IPM decision making are needed with higher commodity prices, but unwise or untested changes can decrease profit.

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 the 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 (EIL). To time this correctly, the economic threshold (ET) is used. The economic threshold is the level where controls should be applied to prevent an increasing pest population from exceeding the EIL. Economic thresholds are often set at about 80% of the EIL.

Table 1. Economic threshold for treating bean leaf beetle in soybean at the VC growth stage.
	Management Costs
Crop Value	$6	$8	$10	$12
$5	3	4	4	6
$6	2	3	4	5
$7	2	3	3	5
$8	2	2	3	4
$9	2	2	3	3
$10	1	2	2	3
$11	1	2	2	2
$12	1	1	2	2

Often our extension recommendations are presented in the form of tables (for example, Table 1) or worksheets (for example, the Worksheet for First Generation European Corn Borer Larvae) which show how ETs change with control costs and commodity value. In those cases you can simply identify the appropriate ET for your situation. If control costs or commodity values are higher or lower than those in the table, the ETs can be adjusted accordingly by following the rising or declining trend of the ETs in the table.

Soybean leaf infested with aphids.

The Exception — the Soybean Aphid

One important exception to this is the economic threshold for the soybean aphid. When the current threshold of 250 aphids per plant was developed, it was set so growers had a week to apply insecticide before aphids were likely to increase above the EIL (which is much higher than 250 aphids per plant). Midwestern soybean entomologists are recommending that even with higher soybean market values, the economic threshold of 250 aphids per plant should still be used. The influence of higher soybean market values means that growers have a four- to five-day window to treat rather than a seven-day window.

Ill-Advised Practices

Other changes in grower practices we have heard about may not be as wise. Frequently, people are considering tank mixes of insecticides with other pesticides, without first considering the level of pests in a field. Their logic apparently is that with higher commodity values it takes fewer bushels saved by a pesticide application to break even or make a profit from the use of an insecticide.

There are several flaws with this approach. First, using insecticides with low pest levels is not likely to increase yield, and may actually reduce grower profits. For example, if an insecticide is applied when insect levels are not above an economic threshold, we would not expect there to be a yield increase. In this case the grower profits would be reduced by the cost of the insecticide (e.g., $5-10/acre depending on product and rate).

Further Resources Insect Treatment Threshold Tables Weed Treatment Recomendations

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 us. Again, this is an additional expense of at least $5-10/acre that reduces profits and could have been avoided if the "clean up" application had not been applied earlier in the season. Similar results would occur in other crops.

Also see: Crop Prices Determine When to Use Pesticides, a Market Journal segment (3:00)   IPM — as Important Now as Ever, an Ag Almanac segment (4:25)

We also have noticed the increase of "insurance" or "prophylactic" insecticide treatment. This treatment is applied before a pest is in the field, based on the expectation that the pest will likely occur and cause economic loss. This strategy is best employed when a pest

1. is difficult to scout for,
2. can cause severe economic loss,
3. cannot be easily managed once it is injuring a crop,
4. and has a relatively high probability of occurring.

People who don't scout their fields and grow continuous corn often treat for corn rootworms on a prophylactic basis, although not all fields of corn after corn have economic problems with corn rootworms. The majority of other pests for a given crop occur sporadically, and most pests that occur more regularly can be managed effectively by scouting and using economic thresholds to determine the need to treat.

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

Nonchemical Solutions

Another aspect of IPM is to use a variety of control strategies to manage pests, rather than relying on pesticides exclusively. 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, control by any method may be reduced, whether growers are relying on traditional planting time liquid or granular insecticides, insecticidal seed treatments or Bt corn hybrids.

Recommendation

While some adjustments in IPM recommendations are wise with higher crop values, avoid changes that can prove unprofitable in the short- or long-term. IPM is still a useful strategy to profitably manage pests, while protecting human health and the environment.