Figure 1. Base of a zucchini plant with obvious feeding injury. Courtesy: Adam Varenhorst
For many of us, our zucchini, squash, or pumpkin plants might not be looking so great this time of the year. Personally, my zucchini plants have many leaves that are beginning to wilt, leaving them appearing as if they have simply given up. Similar symptoms may be observed in other cucurbit vine crops as well. Further inspection of my wilting zucchini revealed a sawdust like material around the base of the plant at the surface of the soil. When I pushed on the plant, it broke, revealing clear evidence of insect feeding through the stem (Figure 1). The insect that is causing these issues is commonly known as the squash vine borer.
Squash vine borer adults are part of the clear-winged moth family (Sesiidae). These moths are often confused with wasps or bees due to their coloration and their clear wings. The squash vine borer moths are approximately 1/2 an inch long. They have an orange abdomen with a black spot on the top of each abdominal segment. The front pair of wings (closer to the head) are metallic green while the hind wings are clear. The antennae have a white band and the legs have similar white banding (Figure 2).
The larvae of the squash vine borer are cream colored with a dark brown head and dark brown pronotum (segment directly behind the head). The larvae will also have dark spots on each segment of their body (Figure 3). They are commonly found near the base of the plant in the stems. Their feeding will result in a saw-dust like material being pushed out onto the stem or leaves.
Figure 3. Squash vine borer larva. Courtesy: Adam Varenhorst
Squash vine borer adult emergence occurs in late June through early July. They are good fliers, so they are capable of traveling long distances. Moths are commonly thought of as being only nocturnal, but many are active during the day – including the squash vine borer. After emergence, the female moth will lay eggs at the base of cucurbit plants. The eggs soon hatch, and the larvae bore into the stems to feed. The larvae will feed within the stem, which blocks water and nutrient flow to the rest of the plant which results in wilting. The squash vine borer larvae will feed for 4-6 weeks before exiting the stems and pupating in the soil.
If you observe an infested plant, remove it from your garden immediately and destroy it. This will reduce the likelihood of the larvae successfully pupating and increasing the population for subsequent years. In early summer, watch for moth activity on squash plants. If they are present, a homemade bait trap made from a yellow container filled with soapy water can be used to reduce adult populations. Alternatively, an early and late planting of squash can be used to ensure that some plants will not be affected by the squash vine borers. Although insecticides can be used, applications have to occur when the adults are active because the larvae are safe inside the plant.