Squash bugs are a major cause of squash plants turning yellow. The bugs feed on the leaves and inject a toxin that causes the leaves to turn yellow and wilt. Without treatment, squash plants and surrounding plants may die due to squash bug infestation.
Adult squash bugs and nymphs live under the plant''s leaves or other nearby coverings. Adults are about 5/8 of an inch long, and they can be picked off by hand if there are only a few. A large infestation requires an insecticide application on the underside of the leaves. Bugs that survive the growing season live throughout the winter, hiding in stacks of leaves or yard debris.