Cucumber beetle damage is most apparent at the seedling stage and later, when plants are mature. Adult beetles live through the winter and feed on seedlings as soon as they emerge in spring. Often they devour the young stems and cotyledons (seed leaves) entirely. They’ve been known to enter cracks in the soil to get at the sprouting seeds before they can reach the surface! You don't see cucumber beetles until you set out plants or your seedlings come up. Then they show up the very next day. They can quickly decimate a stand of seedlings.
Check mature plants for beetles by shaking the vines; the startled beetles will scatter in flight around the plants. Also look for irregularly-shaped holes in leaves, chewed areas on flowers, girdled stems, and holes gnawed in fruits. Be aware that sudden wholesale wilting of leaves and stems may mean that either striped or spotted cucumber beetles have infected your plants with bacterial wilt disease. If the beetles carry the bacteria, then just one bite on the leaves of a mature plant can infect the plant, which will die in only a few days. There is nothing you can do after the disease hits. The beetles also transmit a mosaic virus which causes less damage but discolors cucumbers and melons.
Early Warning Trick: Cucumber beetles seem to attack weaker plants first. To draw them out, grow a cucumber plant indoors earlier than usual so that it is quite a bit larger than your normal plants at transplant time. Pull the large plant out of the soil and lay it out in the garden near the new transplants, where it will wilt. Early-arriving beetles will attack the weakened and dying plant first while leaving the rest of the plants alone. Pick the bugs off the bait plant and cover your good plants with garden fleece or other barriers.
Most common targets of the cucumber beetle:
Cantaloupe, Cucumber, Gourd, Pumpkin, Summer squash, Watermelon, and Winter squash
Other occasional targets of the cucumber beetle in the veggie garden:
Asparagus, Beets, Cabbage, Eggplant, Lima Beans, Peas, Potatoes, and Tomatoes.