Corn Rootworm
Corn rootworms are the larvae of various species of Diabrotica beetle. The larvae eat the roots of Corn (and some other grasses) and when abundant they can reduce yields and cause plants to fall over. Adults like to feed on silks and pollen and can sometimes reduce pollination. Adults may also feed on cucumbers and Squash plants and can transmit Bacterial Wilt and Cucumber mosaic virus.
Prevention
Deal with this problem by rotation and by removing crop residues, so they don't overwinter there. You can also use row covers.
Control
Adults may be controlled with botanical insecticides.
Image: William M. Brown Jr., Bugwood.org