Aphids: spray or pray?

There are two kinds of farmers who have never seen an aphid on a soybean plant: extraordinarily lucky ones, and ones who need to get their eyes checked. Most Ontario soybean farmers are all too familiar with this tiny crop pest. Aphids are a very small, greenish-brown insect that blows in on winds from the south and can cause huge issues in farmer’s soybean fields.

Current weather patterns that Ontario has been experiencing are perfect for aphid development: warm and dry. Aphids have a piercing/sucking mouth part that they use to suck the juice out of the soybean plant, causing yellowing, curled leaves, wilting leaves, and in severe cases, plant death. A dead plant at this stage of the growing season does not contribute anything to the final yield.

Farmers scout their fields this time of the year in order to monitor aphid populations. This means they walk through the field, inspecting random plants, as well as looking out across the field for any areas that might not look the same as the rest of the field. Farmers are especially concerned with lower elevation areas and areas protected by trees. When looking at individual plants, the farmer counts the number of aphids on each leaf and the stem–usually, they will make the decision to spray pesticides if the threshold of 250 aphids per plant is reached throughout a reasonable portion of the field. Aphids do have some natural predators, which are taken into account prior to a spray decision. If many lady bugs or lace wings are present in the field, the decision to spray may be held off; however, with ideal weather, aphid populations can double in approximately 3 days. If a soybean field has not reached the economic threshold for aphids the farmer will re scout the field every 3-7 days. Farmers do not want to spray any field unless they absolutely must, because it is extremely expensive.