Cucumber Beetle
These may be of the Acalymna or Diabrotica species (the latter is also known as Corn Rootworm).
The larvae eat the roots of cucumbers, squash, corn and peas (and occasionally other crops) and can severely stunt or kill the young plants. Adults feed on flowers, leaves and young growth and when in sufficient numbers can destroy the plants. They may also spread Bacterial Wilt and Cucumber Mosaic diseases.
Prevention
You can use row covers to protect the plants from adults and larvae.
You should also attract predators by planting nectar and pollen producing flowers.
Also rotate plants annually and remove crop residue to prevent them overwintering.
Image: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org