Squash Vine Borer

The squash vine borer (Melittia cucurbitae) is a common pest of cucurbits (Cucurbitaceae), which is the plant family containing squash, pumpkins, cucumbers, and melons. The larvae of the squash vine borer damage plants by boring into the hollow stems, where they live and feed on the plant. This more often than not results in death (without much warning) of a runner or the whole plant. Home gardeners, compared to commercial growers, are extremely affected by the squash vine borer because they usually have a small number of plants. Most lose 50-100% of their crop when squash vine borers are present.

Life of the Squash Vine Borer

Squash vine borers can be found throughout the eastern United States, southeastern Canada, and down through Mexico to near Guatemala (Jackson 2005). In the southern United States, there are generally two generations of this pest. In more northerly locations, only one generation occurs; however, a partial second generation may occur in northern states in warm years and in the mid-latitude states.

Adult

Squash vine borers are members of the clear-winged moth family. In their adult form, it is common to mistake them for a wasp. Adults have orange abdomens dotted with black spots, ‘hairy’ red hind legs, opaque front wings, and clear hind wings with dark veins. Adults emerge between late April and mid-August (Bessin 2019). Unlike most moths, adult squash vine borers are active during the day and rest on leaves at night (Canhilal 2006).

Egg

While active, adults lay disk-shaped, dark-reddish-brown eggs singly on upper and lower leaf surfaces, buds, and stems near the base of the plant (Adam 2006). According to a study from the Journal of Agricultural and Urban Entomology, adult females lay on average 40 eggs; however, a publication from the National Sustainable Agriculture Information Service reports an individual adult can lay up to 250 eggs. These eggs then take anywhere from 8 to 14 days to hatch (Canhilal 2006).

Larva

Once the eggs hatch, the young larvae immediately burrow and feed in the stems of the vines for 2-6 weeks. Larvae are whitish, heavily segmented, brown-headed worms that can grow to about 1 inch in length. A single larva can destroy an entire plant, so imagine the damage 40+ eggs can cause to an entire crop (Adam 2006).

Pupa

Fully grown larvae enter into the soil 1-2″ deep to pupate overwinter in cocoons (Canhilal 2006). They can overwinter as full-grown larvae or pupae depending on the climate that they are in (Jackson 2005). If they have not already done so, the larvae pupate in the spring (Bessin 2019). Pupae are mahogany brown in color and measure about 0.75″ long. 

Damage

First symptoms of squash vine borer infestation appear in mid-June when the adults emerge and fly, although they can appear as early as April and as late as August. The main sign of infestation is frass (excrement), which is found pushed out through holes in the side of the stem near the entry hole of the borer. The frass is orange to yellow-ish brown and is sawdust-like in texture (Canhilal 2006).

Infested vines usually die beyond the point of attack. Soon after the appearance of frass, the infested runner or even the entire plant will suddenly wilt. One to several borers are usually present if the stem is split open. Very early signs of larval feeding indicate that other eggs will be hatching soon (Bessin 2019). If a vine dies before the borer has completed its larval cycle, the larva can migrate to a neighboring plant and resume feeding there (Adam 2006).

How to Prevent Squash Vine Borers

In the case of squash vine borers, preventive management practices are usually more effective than control after an infestation. Once a borer enters the stem, it is very hard to control their presence and save the plant. Here are a few steps you can take to hopefully prevent an outbreak of squash vine borers in the future.

Tilling

Tilling in early fall will kill pupae in the soil and suppress adult populations the following spring (Dellinger 2015). Another round of tilling in early spring further helps to keep populations suppressed (Adam 2006).

Sanitation

Destroying crop residues, debris, adjacent areas of litter, leaf piles, and (especially) cucurbitaceous weeds that provide shelter for overwintering adults can help prevent an infestation (Adam 2006). Once harvest is complete, remove vines from the garden, and burn or trash them to prevent late larvae from completing their lifecycle (Bessin 2019). The same goes for plants that have died from infestation. Promptly crush or otherwise destroy any plants killed by squash vine borers to kill larvae still within the vines before they enter the soil to pupate (Dellinger 2015). 

Row Covers

According to a study from ATTRA (Appropriate Technology Transfer for Rural Areas), researchers saw the most success of controlling squash vine borers when row covers were used between the time of planting and flowering. Row covers must be tightly secured to be effective in excluding insects. To facilitate pollination, row covers can be removed just before female blossoms appear; otherwise, you will have to pollinate flowers by hand (Adam 2006).

Crop Rotation

Annual rotation can contribute to pest suppression by delaying population build-up early in the season. However, producers should be aware that the squash vine borer adults are strong fliers and have been known to find squash fields as far as one-half mile from their emergence site in another field (Adam 2006). 

Grow less susceptible varieties

Squash vine borers are found less frequently in varieties with narrow stems, such as cucumber (Cucumis sativus), cantaloupe (Cucumis melo), and watermelon (Jackson 2005). Of the squashes, butternuts are the least susceptible host (Canhilal 2006). The most susceptible are summer squash, some winter squash (particularly ‘Hubbard’), and pumpkins that have large, hollow stems (Adam 2006). Many heirloom summer squashes and winter squash varieties classified as Curcubita maxima will root along running vines when in close contact with the soil, which may be an advantage over bush-type curcubits (Dellinger 2015).

Time of Planting

In early spring, overwintering pupa will not have yet emerged, and in early fall, adult females have finished laying eggs. So plantings corresponding with these periods may bear a crop without intervention of the squash vine borer. 

Maintain healthy plants

Sustaining vigorous plant growth is a very important part of a borer control strategy. Supplemental fertilization may be necessary to promote the vigorously growing plants that can tolerate one or two borers and still produce a crop through additional rooting along the stem. 

Wrapping stems

As an additional preventative, home gardeners can wrap the base of vines with a barrier such as aluminum foil, netting, or pantyhose to keep larvae from boring into the vine. 

How to Control Squash Vine Borers

It is much easier to prevent squash vine borers rather than control them, but here are a few methods that have been tested when infestation is present.

Sprays

Even though it is difficult to kill squash vine borer larvae with any spray (organic or not) after they have entered the stem of a plant, insecticidal sprays are still a primary management tool for this pest.

Organic Sprays

Concern for the detrimental impact of chemical insecticides on human health, non-target organisms, and the environment has prompted development of alternate management techniques for squash vine borers (Jackson 2005). Azadirachtin, neem oil, pyrethrins, and spinosad are labeled for use against squash vine borers, but their effectiveness is questionable. I always try to use organic products first, but in my experience, they aren’t very effective against squash vine borers.

Pheromones

Pheromone-baited traps can be used to detect early infestations of a pest insect, to monitor established adult male populations, to assist in the timing of insecticide applications in relation to population increases, and for mass trapping control efforts. In a study performed by the USDA’s US Vegetable Laboratory, researchers observed that “males of this species are highly attracted to the commercial pheromone formulations and fly immediately to the vicinity of the lure” (Jackson 2005).

Physical Control

These control methods are generally too labor-intensive for large plantings. Home gardeners may have some success with deworming the vines. At the first signs of frass on vines, slice lengthwise near visible damage and remove the borers from the stem. Alternatively, use a stiff wire to probe the damaged part of the stem through the entrance hole to kill the larva without slitting open the stem. Cover any openings along the stem immediately with soil. Burying a few nodes along each vine will encourage rooting at these nodes, which will lessen the impact if squash vine borers girdle the base of the vine (Bessin 2019).

Bt Injection

For home gardens, slowly inject with a syringe the bacterial insecticide Bt (Bacillus thuringiensis) into each entrance hole by hand. 

Nematodes

Parasitic nematodes can be effective predators; although, Cornell Extension points out that soil treatments will not reliably control squash vine borers, as adults are strong flyers. 

In summary

I’m a part of a few local Facebook gardening groups, and as of recent, many people are posting about wanting to give up on growing squash altogether. They feel completely defeated by the squash vine borer after losing 50-100% of their crop. Squash vine borers can be quite the nuisance, but hey, they’re just trying to survive too… 

I understand the struggle my fellow Facebook-users feel, as I myself have battled with squash vine borers since I began growing vegetables several years ago. I have tried so many methods to get rid of them! I’ve wrapped the bases of the plants with aluminum foil, sprayed with neem constantly, and dug out larvae from stems. My efforts have so far not been successful, but I’m far from giving up. I challenge you to also keep growing squash!! Every year is different, and in the worst-case scenario you rule out another method of controlling the squash bug that doesn’t work. Even “failure” is still progress!

What do you think? Have you dealt with the squash vine borer before? What tactics did you use to prevent/control them? Let us know in the comments!

Sources

Adam, K.L. (2006). “Squash Bug and Squash Vine Borer: Organic Controls.” ATTRA. https://www.canr.msu.edu/foodsystems/uploads/files/borer-and-squash.pdf.

Bessin, R. (2019). “Squash Vine Borer and Squash Bug”. University of Kentucky College of Agriculture. Retrieved from https://entomology.ca.uky.edu/ef314.

Canhilal, R. (2006). Life History of the Squash Vine Borer, (Melittia cucurbitae) (Harris) (Lepidoptera: Sesiidae) in South Carolina. Retrieved from https://www.researchgate.net/profile/D_Jackson2/publication/265552984_Life_history_of_the_squash_vine_borer_Melittia_cucurbitae_Harris_Lepidoptera_Sesiidae_in_South_Carolina/links/5412145f0cf2bb7347dadf30.pdf.

Dellinger, T.A. and Day, E. (2015). “Squash Vine Borer”. Virginia Polytechnic Institute and State University. Retrieved from https://vtechworks.lib.vt.edu/bitstream/handle/10919/75429/3104-1566.pdf?sequence=1.

Jackson, D.M. (2005). Trap Monitoring Squash Vine Borers in Cucurbits. USDA. Retrieved from https://pdfs.semanticscholar.org/d554/b5d6bd3001c184619553bc515dab00daf480.pdf.