–by Dr. Raymond Cloyd — Horticultural Entomology
Squash vine borer, Melitta curcurbitae, larvae feed on squash, pumpkin, cucumber, and muskmelon plants. Adults are moths that are 5/8 inches long, orange-red, with gray bands and three to four black markings along with orange-red hairs on the abdomen (Figure 1).
Figure 1. Squash Vine Borer Adult
Adults are active during the day with females laying eggs at the base of plants near the soil. Eggs are 1/30 inches in diameter, red-brown, and flattened (Figure 2).
Figure 2. Squash Vine Borer Eggs At Base of Plant
A single female can lay up to 200 eggs during her lifetime. Larvae that emerge (eclose) from the eggs are white, with a dark head capsule. Young larvae are 1/4 to 3/4 inches in length and taper toward the end of the abdomen. Mature or fully-grown larvae are 1 to 1-1/2 inches long (Figure 3).
Larvae that emerge (eclose) from eggs immediately tunnel into the base of plants and feed for approximately 30 days inside the plant
Figure 3. Mature Squash Vine Borer Larva inside Plant Stem (Raymond Cloyd, KSU)
Figure 4. Squash Vine Borer Larvae Collected From Two Pumpkin Plant Stems (Raymond Cloyd, KSU)
stem. The larvae increase in size as they mature. There is usually one larva per stem; however, multiple larvae may be present in a single stem. On July 24, 2021, we found over 20 larvae in two pumpkin plant stems (Figure 4). It was awesome J. Mature larvae emerge from plant stems, burrow into the soil and construct brown, silken cocoons for overwintering. Squash vine borer overwinters as a pupa in the cocoon located 1 to 2 inches deep in the soil. In early spring, adults emerge from the soil. There is one generation of squash vine borer in Kansas.
During this time of year, squash vine borer larvae are feeding within the internal vascular tissues, inhibiting the ability of plants to take-up water and nutrients. Consequently, you may notice sudden wilting of vines and/or plants collapsing (Figure 5). Once larvae are inside the plant, there is not much you can do to manage squash vine borer and prevent plant damage. The tunnels inside infested plants are filled with
Figure 5. Plant Wilting Due To Feeding By Squash Vine Borer Larvae (Raymond Cloyd, KSU)
Figure 6. Frass Associated With Squash Vine Borer Larva in Plant Stem (Raymond Cloyd, KSU)
moistened frass (fecal matter) (Figure 6). Yellow-green sawdust-like frass may also be found around feeding sites at the base of vines or plants, which is a direct indication that larvae have entered the plant.
Because larvae are feeding inside the plant there is not much that can be done to kill the larvae. However, there are plant protection strategies that can be implemented during the remainder of the growing season such as, sanitation and physical control.
Sanitation: remove and dispose of all wilted plants before larvae exit and enter the soil. Discard all plant debris, including vines and fruits after harvest.
Physical control: rototilling in fall will kill squash vine borer pupae directly or cause the pupae to reside on the soil surface where they are exposed to cold weather or predation by birds. In addition, the process of deep plowing may bury the pupae deeper in the soil profile, which may inhibit adult emergence from the soil. Another technique that may have limited use in large plantings, but may be an option for smaller plantings, is to locate infested stems and vines, and create slits at the base of the plant. Then, tweezers may be used to remove larvae from inside the plant stem. Larvae should be killed and the plant base covered with moist soil and mulch, which will stimulate the production of secondary vines and/or root growth, thus helping the plant to re-establish. The previous technique will only be effective if there are one or two squash vine borer larvae inside the plant stem.
For more information on how to manage the squash vine borer, refer to the following extension publication:
Squash Vine Borer (MF3309 July 2016)
http://www.bookstore.ksre.ksu.edu/pubs/MF3309.pdf