Crops: Crop Scene Investigation 19 Solved - What caused Roger's aphid infestation?

Thursday, June 4, 2009

by SHAWN BRENNEMAN

Soybean aphids are attracted to fields under stress. A soil test confirmed my hypothesis – that the soybean plants on the knoll of Roger's field were stressed by a potassium deficiency.

Potassium-deficient soil can significantly reduce soybean yield and quality. On the flip side, potassium-rich soil can increase plant resistance to pests and diseases.   

Research in recent years has confirmed a link between potassium-deficient plants and soybean aphids, insects that thrive by sucking valuable nutrients from soybean plants.

Studies show that aphid numbers are higher and increase faster on plants with a low potassium status. When soybean plants lack this key nutrient, their ability to assemble proteins from amino acids is reduced. The amino acids which accumulate make the plant more attractive to pests. In other words, potassium-deficient plants are considered an irresistible meal for aphids.

Roger's history of good yields meant key nutrients had been pulled from his soil. Second only to nitrogen, potassium is the nutrient absorbed by soybean plants in the greatest amount. A large portion of this potassium is stored in the seed and is therefore removed from the field at harvest.

Because Roger hadn't sufficiently replaced the nutrients in his soil, the amount of potassium available for the plants was limited. In addition, the dry conditions – which were most significant on the knoll – compounded the problem and further reduced the uptake of potassium.

We couldn't do anything to correct the nutrient deficiency at this stage of the crop, but we could tackle the aphid problem. We flagged off the areas that had reached the action threshold of 250 aphids per plant and sprayed to help preserve Roger's yield.

So what should Roger do next year? First of all, he needs to keep in mind that insects as well as plants are affected by fertilization. Replacing the potassium removed by soybeans is essential, so he needs to get his fertility up by bulk-spreading potash in the fall. If not, yields in Roger's field could continue to suffer.

Ontario tends to experience soybean aphids to some extent almost every year. Scouting is an essential tool when it comes to managing these pests.

Roger could also consider planting a soybean variety with an aphid-resistant trait. These became available for the first time in Canada in 2009, so growers have a genetic choice in aphid management. Aphids prefer not to colonize on these soybean plants and populations eventually die off, preserving yield potential. Roger should also consider using a seed treatment that offers early season-aphid control. Congratulations to Heather Mackichan, Denfield, for her correct answer.

Crop Scene Investigation will return in the October. BF
 
Shawn Brenneman, CCA, is a Sales and Agronomy Manager for NK® Brand, Syngenta Seeds.