Aphids attack!

Here's some trivia--2007 is the 20th Anniversary of Shark Week (July 29-August 4) on the Discovery Channel. Since 1987, shark attacks and some interesting, if marginally educational, material is featured during this cable channel's push for new viewers. By now, it's a pretty familiar formula: these predators come out of nowhere to attack seals, surfers, and swimmers.

Since 2000, growers are familiar with a similar pattern as soybean aphids continue to invade Iowa. Although it may appear as surprising, soybean aphids have been mounting a steady attack on soybeans, too. What follows is a reminder--without commercial interruptions--to scout, even previously treated soybean fields, to estimate if you are experiencing a local version of "aphid week."

First, the bad news

We reported three weeks ago that soybean research plots in central Iowa had aphid populations at the economic threshold (ET). This was a month earlier than the last time an aphid outbreak occurred in Iowa. Northeast Iowa was on track for a "typical" aphid outbreak, with populations reaching the threshold in early July.

To add insult to injury, the number of aphids collected in suction traps has increased tenfold in northwest and central Iowa (Table 1). In the same way that some shark species can travel hundreds of miles looking for sushi, soybean aphids migrate over great distances to colonize soybean fields. The combination of early season aphid populations and an increase in winged aphids has increased the risk to soybeans, even those treated with seed- or foliar-applied insecticides.

Table 1. Winged soybean aphids collected in four Iowa traps during July 16-27, 2007.

Location July 16-20 July 23-27
Northwest--O'Brien Co. 7 22
Northeast--Floyd Co. 9 233
Central--Story Co. 17 278
South-central--Lucas Co. 0 9

These observations came from untreated plots. Last week, seed-treated plots in Story County went from below the ET to over 600 aphids per plant (Figure 1). Recently, Brian Lang, extension field agronomist in northeast Iowa, has observed a rebound of soybean aphid numbers above the ET in plots that had been sprayed on July 9.

Performance of a seed treatment (thiamethoxam, dotted line) compared to untreated soybean and soybean treated with a foliar insecticide (lambda-cyhalothrin, solid line with square).Figure 1. Performance of a seed treatment (thiameconomic thresholdhoxam, dotted line) compared to untreated soybean and soybean treated with a foliar insecticide (lambda-cyhalothrin, solid line with square). Aphids were scouted every 7 to 10 days on five plants in six, half-acre plots. Foliar insecticide was applied on 19 July, 2007.

The risk for a resurgence of soybean aphids is high. All growers, even those who used a seed-applied insecticide or foliar insecticides, should scout their fields.

Now, the good news

Despite the high numbers on the ground and an increase in winged aphids, not all of Iowa is experiencing this outbreak. Research plots in Lucas County are still below threshold as of July 27. Very few winged aphids were collected from the suction trap at this location (n = 9) and in northwest Iowa (n = 22). Although growers are still recommended to scout their fields, it is likely that they will find populations at or below the economic threshold. Recall that aphid populations at the economic threshold do not result in a yield loss greater than the cost of an insecticide application. If these populations increase, then the economic injury level (EIL) may be reached. The EIL is estimated at 654 aphids per plant for a range of control cost ($6.64 to $13.33) and markeconomic threshold values ($5.50 to $6.50/bushel per acre). Thus, we recommend treating when the populations reach the economic threshold and are increasing.

Just like chum in calm waters stirs sharks to attack (hey, did we mention it's the 20th Anniversary of Shark Week?!), aphids attract several different insect predators. As lady beeconomic thresholdles and other insect predators start to respond, there is the potential for these relatively low aphid populations that the rest of the Iowa is experiencing to be held in check without the need for a foliar insecticide.

Matt O'Neal is an assistant professor of entomology with research responsibilities in field crops. Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. Kevin Johnson is a graduate research assistant in the Department of Entomology.

This article originally appeared on pages 255-256 of the IC-498(22) -- August 6, 2007 issue.

Updated 08/13/2007 - 3:53pm