Many producers are planting late this year due to continued wet weather and may be wondering how insecticidal seed treatments should factor into their planting decisions. While individual situations vary, here are some rules of thumb to consider.
The most commonly available class of insecticidal seed treatments are neonicotinoids such as thiamethoxam, clothianidin, and imidacloprid. The conventional wisdom is that late-planted crops stand to benefit less from these products than early-planted crops. Warmer soil and air temperatures get the plant get off to a faster start and faster growth, allowing it to outpace insect pests. Another important factor to keep in mind about insecticidal seed treatments is their window of activity. The insecticide applied to the seed coat is taken up by the germinating plant and translocated through the plant in the growing tissue. The amount of product that goes on to the seed is finite – when it runs out, it runs out. Studies have shown that on average, new plant tissue added 3 weeks after planting does not contain the insecticide product. This means that pests that affect plants after the 3-week planting window will not be managed by the insecticide. Thus we do not recommend these products for use against anything but the earliest season pests (usually soil pests). We generally do not recommend insecticide seed treatments as a prophylactic against early-season bean leaf beetles. Feeding on early V soybeans is rarely economic, only cosmetic. In the rare cases where feeding may be economic (considerable stem clipping or over 40% defoliation on most plants) a foliar insecticide can be applied.