squash vine borers
When strong, healthy zucchini plants wilt and then collapse into piles of yellow mush, blame the squash vine borer. This inch-long white grub with a brown head is the larvae of an orange and black clearwing moth (Melitta curcurbitae) that grows about ½ inch long.

damage
In late June or early July, adult squash vine borers lay eggs at the base of squash and pumpkin vines (and sometimes cucumbers and melons). The hatched larvae crawl inside the stems to feast there for several weeks. The larvae block the flow of water to the leaves, which causes the plant to wilt and eventually die.   

control
By the time you see wilting and yellow mush (called frass), the plants are most likely beyond saving. But you can prevent these larvae from becoming adults and laying eggs that will destroy next year’s crop. Either cut the vines lengthwise to find and destroy the white caterpillars inside or pull up affected crops as soon as they finish bearing and destroy them. Next year, set out traps for the adults: any yellow bowl or pan filled with water. If you see brownish eggs near the base, pick them off.

—text by Elizabeth Noll, photo courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden