VOL. 24 #17 August 18, 2004

Soybean Aphid Update

Quite a bit has happened on the soybean aphid front in northeast Nebraska over the last week. Many people observed a jump in populations during the week of August 9 through 15. This is exactly what happened during 2003. Some growers have opted to spray their fields and some have not. Once again we have been “treated” to an amazing example of the reproductive potential of the aphids. Talking to quite a few consultants, growers and dealers has led us to conclude that populations have not been very high until the last 7-10 days. Some treatments have taken place before last week, hopefully in fields that had reached the threshold of 250 per plant.

Now the decision becomes more difficult. Should you treat or not? A heavy rainfall may wash aphids off the plant, but the weather forecast as of this writing (Wednesday AM) is not very promising. Lady beetle numbers do not appear to be as high as last year’s, but we feel this is an indication of overall lower aphid numbers. Several fields scouted on Monday and Tuesday showed an increase in lady beetle and parasitoid numbers and we believe that many aphid populations will now be stabilizing or even declining. Looking for predators, especially lady beetles, is now going to become a key decision making factor. How many lady beetles will be needed to stabilize or reduce aphid populations? In 2003, fields where we believe the aphid populations were being reduced had several hundred aphids per plant and 1 or more lady beetles per foot of row visible on the tops of the soybean plant.

On paper, the moderate weather of the last few days could have favored the aphid population increase. Also, later maturing fields (younger than R5) appear to have borne the brunt of the increase. This is consistent with the observation of others that later in the growing season later maturing plants tend to have more aphids. The aphids prefer moderate temperatures for the most rapid reproduction. When will they leave the fields? That is the big question, which unfortunately we have no sure answer for. Last year, aphid populations peaked on August 20 at the Haskell Ag Lab and declined rapidly afterward, with populations going from over 1000 per plant to less than 100 in nine days. Again, we appear to be mirroring 2003 as far as what has happened so far.

What makes things tougher is that the longer you wait to treat, the less likely the return on investment as the beans continue to mature and the aphids get nearer the time where they will leave the fields to search for overwintering sites. At the current time with most beans entering the R5 stage we feel that at least 400 or more aphids per plant would be necessary to get a possible economic return from treating (this is assuming that populations are still increasing in that particular field). Fields that are delayed in maturity (less than R5) may benefit more from treating as the yield potential may be affected more.

Once again we stress the importance of leaving untreated check plots in fields receiving an insecticide treatment. There are not enough researchers to go around and while we have our own research plots, results from farmer fields are very important and we would like you to share them with us when the yield results are in. This is still a very new problem and we need much more information to refine our thresholds.