K Deficiency and the Soybean Aphid

Given the high input costs for P and K in soybean many growers either cut back or as I have been told “took a year off”. I wanted to remind growers as well as crop consultants of the relationship between K deficiency and soybean aphid fecundity. Below please read a blog contribution from Dave Hogg: Professor of Entomology, University of Wisconsin, Madison.

Soybean aphid infestations remain at sub-economic levels throughout Wisconsin. Will they stay there, or will we have a repeat of last year’s late season aphid buildup? We will have to wait to see what aphid populations do over the next several weeks. However, one thing we do know – that soybean fields with suboptimal potassium levels are at greater risk of soybean aphid population increase and yield loss.

Following the 2000 discovery of the soybean aphid in Wisconsin, entomologists and agronomists noticed that soybean aphid infestations seemed to be more severe in K deficient soybeans. [The below photo of a soybean field in Grant County (taken by John Wedberg in August, 2000) illustrates this. The yellow beans on the left were literally dripping with soybean aphids and were presumed to be K deficient, whereas the healthy beans on the right had few aphids and were thought to have adequate K. The demarcation line follows the field contour.]

Subsequent research has proved this observation to be correct, plus we now have a better understanding of why this occurs. What happens is that low K actually makes soybean more nutritious for soybean aphids, promoting higher aphid reproduction and leading to more rapid aphid population increase. To give an idea of how this might work, under field conditions in a K deficient field an aphid infestation can increase from 10 per plant to 230 per plant in 10 days; in a field with adequate K, that same population would increase from 10 to 150 aphids per plant. Further research suggests that K deficient beans have a greater percentage of asparagine in the plant phloem where the aphids are feeding. Asparagine is known to be an important amino acid for aphid nutrition.

Finally, we think the yellowing of K deficient soybean leaves may preferentially attract migrating soybean aphids, placing K deficient fields at a further disadvantage. The color yellow has been shown to be highly attractive to a number of aphids.

Bottom line, maintaining adequate K levels in soybean goes a long way toward managing soybean aphid.