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Pest of the Month - Squash Vine Borer

Squash Vine Borer Adult, Nebraska Extension Acreage Insights June 2017.
Adult squash vine borer moth. Adults are ½ an inch long and have a brightly colored body. They are active as adults from June-July. Photo by Jim Kalisch, UNL Entomology

How to Deal with Squash Vine Borer

The squash vine borer is the bane of a many a gardener’s summer. If you grow squash, zucchini, pumpkin, or gourd plants it is likely that you have encountered this caterpillar pest.

Quick Facts

  • As an adult this insect is a day flying moth. It is grey in color with red hind legs and green/silver/clear wings (Figure 1).
  • The caterpillar of this moth dwells in the lower portion of the stem and chews away the inside of the plant. The first symptom of a squash vine borer infestation is wilting and orange frass (fecal material) exuding from the base of the plant where the insect is feeding (Figure 2).
  • Insecticidal control requires sprays or powders be applied to the stem before the borer enters the stem. This means products like neem, spinosad, Sevin©, permethrin, and bifenthrin must be applied just before the eggs hatch.
  • If this application window is missed, you can perform squash surgery to remove the caterpillar and maybe save the plant.

Biology & Symptoms

Squash vine borer is a clearwing borer moth, just like the ash-lilac borer or peach borer. Clearwing moths are wasp mimics and are therefore day-time fliers with tapered tips to their abdomens. This pest overwinters as a cocoon in the ground or in leaf litter. In late June or early July adults emerge and then begin their flight and mating habits. After mating, females lay eggs individually at the base of their preferred host plants (squash, zucchini, pumpkin in particular). One week after being laid, eggs hatch and the caterpillar will bore into the plant’s stem to feed and gain protection. The caterpillar will feed for a month or maybe a month and half and then exit to pupate in the ground.  There is only one generation of squash vine borers a year.

As the caterpillar feeds inside of the plant it hollows out the base of the stem. This deprives the rest of the plant of food and water and initially leads to wilting. If you look at the stem near the soil you will find piles of yellow-orange frass exuding from a hole in the stem. If no control measures are taken the plant will continue to wither and the base may even rot.

Integrated Pest Management Approach to Squash Vine Borer

The best thing that gardeners can do is be vigilant in their squash gardens for this pest. It is active during the day when you may be weeding or watering and is highly visible. If you want to add an extra level of alertness you can trap for the females using yellow sticky traps or plastic, yellow bowls that contain soapy water. The egg laying females are attracted to the yellow squash blooms and gardeners can manipulate this to know when to apply an insecticide.

When you start to trap females it will be time to make an insecticide application. Sprays and powders will need to be applied to the stem rather than the leaves. Organic pesticides such as neem or spinosad are good for controlling vine borers and will need to be reapplied every three to five days over four to five weeks to protect the plant. Synthetic products like Sevin©, permethrin, and bifenthrin, also work and should be applied every seven to fourteen days over 4 to 6 weeks.

If a client is calling and describing the aforementioned symptoms it is likely too late to treat for the borer. One possible solution for these clients is to bury part of the vine ahead of the larvae in some soil. The hope is that the squash will re-root, and the infested section of vine can be removed. Gardeners can also cut a vertical slit in the stem and remove caterpillars by hand. Soil should be mounded over the stem to encourage re-rooting.

For long term maintenance and control, row covers and fall sanitation may work the best. Row covers are fabrics you can purchase at most garden stores. Pin the edges to the ground and drape the cover over frames or the plants themselves. The row covers should be removed once female flowers appear in order to allow for pollination. In the fall you should thoroughly remove plant debris that the borer may overwinter in. Gardeners should till the area as well. Planting squash in the same location year after year also encourages squash vine borer infestations, so consider rotating where you plant your squash.

Squash Vine Borer Damage
Caterpillars live inside of the stems of squash and other cucurbits, eating them from the inside out. Photo by Jim Kalisch, UNL Entomology
Image of Jonathan Larson
Jonathan Larson
Extension Educator - Entomology

Jonathan Larson is the Nebraska Extension entomologist for Douglas and Sarpy counties. His main focus is lawn and landscape pests but he also helps with bed bugs, roaches, and any other home invader that has six or more legs. Jonathan has his Bachelor of Science in Entomology from Purdue University, and his Ph.D. in Entomology from the University of Kentucky.

Contact Jonathan at:
Douglas/Sarpy County Extension
8015 W Center Rd.
Omaha, NE 68124-3175
(402) 444-7804