When beetle numbers first reach or exceed the above threshold, apply a residual insecticide. Residual activity is reduced by overhead irrigation or rainfall after application. If beetles reinfest the field, make a second application when population levels reach one beetle per two plants. The cost of two treatments will exceed that of a single soil treatment applied at planting or first cultivation the following spring. In continuous corn, if you do not have scouting data from the previous season for the full beetle activity period, consider a soil insecticide application at cultivation or planting as a precautionary measure.
Many fields never develop a rootworm problem.
Using both adult control the previous year plus soil insecticide the current year is not recommended. Insecticides applied too early may break down or lose effectiveness before rootworm eggs hatch, resulting in poor control. Generally, this is at cultivation time. Avoid planting time applications before mid-May. Do not reduce the rate and don't split it, i.e. half at planting and half at cultivation.