As the 2005 crop season moves into its final stages, there is concern that insect infestations may still affect Iowa
soybean yields. Three Iowa State University experts have some recommendations for treatment options of these pests and a reminder about preharvest intervals.

Aphids

"In 2004, we did not see an impact on yield from soybean aphid populations that were well below 250 aphids per plant, therefore we are not recommending that growers treat these low populations," said Matt O'Neal, assistant professor, Entomology. "This does not mean that low populations now will not continue to increase."

O'Neal says there is still plenty of time for populations to increase and growers should continue to scout their fields. However, it is possible that fields may not reach 250 aphids per plant, or higher, threshold this year.

He says it is important to pay particular attention to at-risk soybean fields (K-deficient, drought, late-planted), watch for spider mites in drought-stressed fields and look at the label for preharvest intervals.

"In our 2005, trials we are noticing that the efficacy of the pyrethroids (Baythroid, Warrior, Decis and Proaxis) compared to an organophosphate applied alone (Lorsban) is not improved when applied in combination with a pyrethroid (Baythroid)," said O'Neal. "This tactic is thought to combine the longer residual activity of a pyrethroid with the faster immediate kill of an organophosphate."

"Our data suggests that these products work well alone and a combination is not necessary. This will be an issue later in the season as the longer pre-harvest intervals of certain pyrethroids (Warrior and Baythroid, both 45 days) may prevent the use of these products. Combining these products may also be a concern in the longer term as exposure to both active ingredients could select for a soybean aphid population that is resistant to both active ingredients."

There is more information about O'Neal's 2005 trials at www.soybeanaphid.info.

Bean Leaf Beetles

Bean leaf beetle populations are higher in 2005 than during the  previous two years according to Marlin Rice, ISU Extension entomologist.

"Producers who are concerned about injury to soybean pods will want to start scouting their fields before the end of August. Beetle feeding on pods during late August and early September can reduce both seed quantity and quality," said Rice.

Pesticides and Preharvest Intervals

Palle Pedersen, ISU Extension soybean agronomist, urges producers to consider the pre-harvest interval when considering pesticide applications at this time in the crop season.

"We expecting harvesting to start within 30 days in some parts of Iowa," said Pedersen.

Based on early planting (last week of April and first week of May) for an average maturity group, Pedersen expects soybean harvest to start in the drought-stricken eastern and southeastern portions of Iowa in the Sept. 10-15 time period.

He expects harvest to start about Sept. 15 in southern Iowa, Sept. 15 to 20 in the central part of Iowa, and Sept. 20 to Oct. 1 in the northern portion of the state.