Integrated Crop Management

Soybean aphid numbers increase...and decrease

As might be expected, soybean aphid populations have increased slightly in some locations around Iowa. The most notable increases, although they have been small, were reported from northeastern Iowa by Brian Lang, extension field agronomist in Decorah.

The average number of aphids per plant increased from 6.6 to 30 with the percent infested plants also increasing from 59 to 95 percent (Table 1). The population in northeastern Iowa most closely parallels the same trends from 2005 with respect to aphid numbers and percent infested plants. Fortunately, we are not yet observing a population increase that would mirror the major outbreak year of 2003, and that is good news.

Ants on a soybean plant often indicate the presence of aphids. The dark-colored ants can be used as 'crop scouts' to help in locating small colonies of aphids that are difficult to see with the naked eye. (Marlin E. Rice) [1]Ants on a soybean plant often indicate the presence of aphids. The dark-colored ants can be used as 'crop scouts' to help in locating small colonies of aphids that are difficult to see with the naked eye. (Marlin E. Rice)

Table 1. Soybean aphid population trends, Decorah, 2002-2007.

2002 2003 2004 2005 2006 2007
Date- - - - - - - - - - - - Infestation (%) - - - - - - - - - - - - - -
June 1 -- -- 0 6 0 2
June 7 -- -- 0 15 2 23
June 14 -- 40 0 33 6 56
June 21 7 90 0 31 12 59
June 28 15 100 0 85 13 95
July 5 70 100 4 99 24
July 12 93 100 8 100 71
2002 2003 2004 2005 2006 2007
Date - - - - - - - - - - - - - Aphids/Plant - - - - - - - - - - - - - -
June 1 -- -- 0 0.1 0 0.1
June 7 -- -- 0 1.4 0.4 3.4
June 14 -- 10 0 2.5 0.3 5.8
June 21 1 115 0 4.0 0.8 6.6
June 28 2 341 0 48 0.7 30
July 5 14 745 0.5 179 1.8
July 12 25 2,803 1 713 6.6

The aphid population in some areas also appears to have decreased in some areas. Jim Fawcett, extension field agronomist in Iowa City, reports that soybean aphids couldn't be found during the last week of June whereas they were present in eastern Iowa the third week of June. He attributes rainfall as one possible reason for this population decline.

Not only can aphid populations go up, or down, but they are also expected to be variable and uneven within the same field. We scouted a field near Nashua on June 28 and found eight consecutive plants in a row where each plant had 300 or more aphids, but on the plants in the immediate adjacent rows, there were 0-1 aphid per plant. This example should serve as a reminder that scouting across a field as opposed to a single spot in a field is imperative. The critical concept to remember coming into July is that soybean aphid populations are going to change--sometimes dramatically--during the next four weeks and that field scouting is absolutely critical for making informed decisions regarding pest management. Additional information on soybean aphids and the "speed scouting technique" can be found at www.soybeanaphid.info.

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

This article originally appeared on page 207 of the IC-498(17) -- July 2, 2007 issue.


Source URL:
http://www.ipm.iastate.edu/ipm/icm//ipm/icm/2007/7-2/soybeanaphid.html