Although almost all corn and a majority of the soybean seed now come with insecticide seed treatments, growers should not assume they will get much benefit from their application. Seed treatments have shown great control of seedcorn maggot, but only if and when that insect pest occurs.
Research over 20 years has shown that maggot populations only reach economic levels when a green, living organic crop such as an alfalfa field, grass cover crop, or heavy weed growth, or possibly manure, is incorporated into the soil. Tillage of green matter is what draws female flies into the field for egg laying, resulting in possible higher numbers. In those situations, seed treatments have been shown to work very well. However, for most of the other secondary pests, grower experience has shown the lack of adequate control.
True grubs and grubs of the Asiatic garden beetle are able to greatly damage corn that has been treated with the various seed treatments. Although labeled for black cutworm control, we still see significant injury from larvae, often needing an insecticide foliar treatment for control; thus, the reason we state the need to always scout for black cutworm injury in no-still situations especially when winter annual weeds are present.
In soybean, we do have good evidence that because of the systemic activity of seed treatments, they will offer control of foliar feeding insects such as the bean leaf beetle or soybean aphid in early spring. However, these pests are seldom if ever at high enough levels to cause concern in the spring in Ohio; the seed treatments are no longer effective and do not offer meaningful control in mid to late summer. In terms of bean pod mottle virus vectoring by the bean leaf beetle in the spring, the jury is still out. At most, we only advise farmers growing seed or food grade soybean to consider using them for this purpose, and at best that would only provide a limited amount of success. Thus, when given a choice, we usually do not recommend application of an insecticide seed treatment except for those specific situations mentioned above.
Source : osu.edu