Insect Pests Affecting Squash Plants

Monitoring your growing garden regularly is important to identify and control insect pests. Sanitation is also essential in preventing and controlling insect populations. Remove and destroy any crops affected by insect pests. Do not put these in your compost pile. Keep your garden weeded to limit places for insects to hide. Many insects winter over in leaf and plant litter, so removing all plant debris at the end of the growing season is vital. Also, be sure to practice crop rotation so that if any insects do winter over, they do not have their favorite food waiting for them in the spring. Insect damage may occur as they feed on the plants, but they may also introduce disease to the plants.

Figure 2. Striped cucumber beetle by Whitney Cranshaw, Colorado State University on Bugwood.org. CC BY

The spotted cucumber beetle (Diabrotica undecimpunctata howardi) and striped cucumber beetle (Acalymma vittatum) are two examples of just such a pest.  Both adult beetles are approximately ¼ inch long and yellowish-green. The spotted cucumber beetle has twelve black spots on its wing covers and a black head with black antennae. The striped cucumber beetle has three longitudinal, black stripes on its back. While both may cause damage through feeding, a more devastating issue is their ability to transmit the bacteria that cause bacterial wilt. Caused by the bacterium Erwinia tracheiphila, infected plants will suddenly wilt and die. While this bacteria will affect summer and winter squash, it causes the most damage in cucumbers and muskmelon, as they may wither and die quickly. This bacteria overwinters in the guts of infected striped and spotted cucumber beetles. As these infected beetles emerge in the spring and begin feeding, they introduce the bacteria into the plant, infecting its vascular system, causing it to wilt. Beetles that are not infected with the bacteria may become infected if they eat an infected plant, thus spreading the disease to more plants. Row covers may be used as a preventative measure from planting until bloom but must be removed when the plants begin to bloom to allow for pollination.

Figure 3. Squash vine borer larva by Jim Jasinski, Ohio State University Extension on Bugwood.org. CC BY-NC

The squash vine borer (Melittia satyriniformis) larvae can also cause wilt in summer squash, winter squash (with the exception of butternut and green striped cushaw), gourds, and pumpkins. The squash vine borer moth emerges in early summer. These moths are ½ inch long and have an orange abdomen with black spots. They are considered clearwing moths because their back pair of wings are clear. They are also day-flying moths and are often mistaken for wasps. The females lay their eggs at the base of susceptible plants. Once hatched, the white or cream-colored larvae with brown heads feed through the center of the stems, blocking the flow of water to the rest of the plant and causing the plant to wilt. Damage from these boring insects may appear as a greenish or orange sawdust on the stems of the plants. Monitoring in June and July is important. It is possible to take advantage of their attraction to yellow and trap the adult moth. In mid-June, place a yellow-colored container filled with water and a drop of dish soap in your garden. Attracted to the yellow, they will fly into the container and become trapped in the water. You may also consider a second planting of your vegetable if your growing season is long enough, in early July, after the moths have laid their eggs. 

Figure 4. Squash beetle by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo on Bugwood.org. CC BY-NC

Squash beetles (Epilachna borealis) are a type of ladybug beetle. They are yellow or orange and have seven large, black spots on each wing covering and four smaller black spots just behind the head on the thorax. The larvae, which are yellow with six rows of barbed, black spines, feed on the underside of leaves. Adults may feed on the leaf surface and the skin of the vegetable itself, leaving a telltale spiral-shaped feeding injury. These beetles generally exist in small enough numbers that they do not require control, and hand-picking may be enough to limit populations.

Figure 5. (side-by-side) Squash bug nymphs and Squash bug adult by Whitney Cranshaw, Colorado State University on Bugwood.org. CC BY

Squash bugs (Anasa tristis) may be more challenging to control and can cause significant damage to cucurbits, with hubbard squash being the most severely damaged. Adult squash bugs are 5/8 inch long, winged, and brownish-gray in color. Their thorax, the area directly behind the head, is triangular in shape. When crushed, they have an unpleasant odor. They overwinter under leaves, wood, or other garden debris. Emerging in the spring, they mate and lay clusters of shiny, elliptical, bronze to red-colored eggs on the underside of leaves and stems. Nymphs hatch in one to two weeks. Newly hatched nymphs are pale green with black legs and are spidery in appearance, but as they go through five growth stages, over four to six weeks, they will become more gray in color. Both nymphs and adults have mouthparts that allow them to pierce foliage and suck out plant juices, causing the plants to wilt and, in some cases, die.  Evidence of feeding first appears on the leaves as numerous, small, white dots known as stipples. Eventually, the leaves may become yellow or brown, appear tattered, and, if the damage is severe, die. Row covers may be used, but again they must be removed once the plant starts blooming to allow for pollination. Check leaves regularly for egg masses. Tear off that portion of the leaf and dispose of it. Nymphs can be hand crushed, drowned in soapy water, or removed using the sticky side of duct tape to trap them. Another method of trapping nymphs and adults is by placing a board or small piece of heavy cardboard under each affected plant. The bugs may hide under it for protection. Lift the boards in the morning and destroy the bugs you find hiding there.

While insects may cause damage to your cucurbits, careful monitoring of your garden may allow you to identify a potential problem before it gets out of hand and allow you to manage them without the use of pesticides. Remember that a good garden cleanup at the end of the year can set you up for a good start the following year.