Female western corn rootworm beetles are yellow with black stripes; male beetles vary from striped to nearly black. They are ~1/6" to 1/4" long.

Adult beetles begin emerging in July and begin feeding on corn leaves, producing white, parchment-like areas. Beetles later feed on silks and pollen.

July to first frost (statewide).

Examine 50 plants per field, searching over the whole plant. Also, unbaited yellow sticky traps may be used; 12 traps per field. See NebGuide 774.

In continuous corn, ~0.75 beetle/plant or 6 beetles/trap/day may produce an economically damaging rootworm population in corn the following year. Numbers of beetles/plant that may interfere with pollination varies; controls are justified only when severe silk clipping occurs at 25 - 50% pollen shed.

http://www.unl.edu/entomology/extension/crops/fieldcorn.shtml