Squash Vine Borer

What is this bug?

While doing a spot of weeding I noticed a gorgeous orange and black insect with big wings, sitting innocently on a tomato plant. I ran for my camera and took his picture as he posed innocently. I know it's a he because he is brighter in colour, I found out the lady is not quite so brightly coloured.

Later I did some checking and figured out that he was a Squash Vine Borer. Melitta cucurbitae.

This year's tomato plants are where last year's squash were living so most likely the insect had recently hatched from a cocoon that survived the turning over and soil preparation. I bet he was surprised to find tomato rather than squash.

squash vine borer side view squash vine borer top view

He looks like a beetle but he's actually a clear wing moth. Unlike most moths he is active during the day rather than at night. He's about half an inch long not counting the long antenna. He's a good flyer and flies more like a wasp than a moth. Apparently he buzzes when he flies around but I did not see or hear him fly.

According to the University of Minnesota, they prefer summer and winter squash including pumpkins and zucchini and hubbard and if given a choice will stay away from cucumber and melon, or butternut squash, go figure.

Life cycle of the Squash Vine Borer

zucchini plant

Late in June or early July, after a specific amount of time spent at a given temperature, the moth emerges from a coccoon in the ground and heads off to find a suitable plant. Zucchini is a favourite as is hubbard squash. Cucumber, melon and butternut are a poor second but all members of the cucumber/squash family can be used.

It lays an egg near the ground. Eggs are brownish and flattened. The eggs are not laid in big bunches but rather singly at the base of the plant.

hole larva

Photo of grub: By Clemson University - USDA Cooperative Extension Slide Series [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

As soon as I figured out I had an adult vine borer, I checked my baby plants and found 2 with tiny holes in the stem. I used a needle to poke inside and hopefully kill the larva. I then sprinkled the whole area with diatomaceous earth. It might slow down any other larva from burrowing into the zucchini.

If left undisturbed, the egg hatches after a week or so and a larva emerges from the brownish egg and bores into the stem of the plant.

Safe inside the stem of the plant the little grub eats away and grows. It feeds 4 to 6 weeks then crawls out of the stem, burrows 1 to 2 inches into the ground and pupates.

Safe in its cocoon, the insect spends the winter until it has received enough heat. One of the trigger to get it to emerge is the number of hours above a certain temperature. That way if the weather is really cold and nasty our little bug is snug and safe until the weather improves.

In Northern areas there is only one generation of borers but further South there can be 2 per season.

Trapping the Adult Insects

yellow bowl

The adults are attracted by yellow. I expect this is because squash blossoms are yellow. Apparently they can be trapped by placing yellow bowls with water.

I don't have a yellow bowl but I found a yellow flower pot so I placed it in a plastic bowl and filled with water for about an inch. Let's see if this attracts anything. I will try this and report back.

NOTE: After almost 3 weeks of yellow trapping has not produced a single insect. Either it does not work very well OR I don't have many borers around.

There are also commercial yellow sticky traps. These attract a number of different pests.

Pheromone lures and traps are also available. These traps are coated with insect communication chemical and mimic the real insect. This tricks the bugs to the trap where they get stuck on the sticky surface. Traps like that exist for many different insects including Japanese beetles, clothe moths and others. They are completely non toxic BUT the pheromone might attract insects from all around to your trap, who might not have come otherwise. Useful if you want to see it they are around though.


The larva eats the inside of the stem and leaves a sort of sawdust that can be sometimes seen at the base of the plant. This burrowing and eating away of stem causes the plant to wilt.

At first the squash will only wilt in strong sunlight but gradually it will droop even in cooler days. If there is only one larva then the plant might make it but if there are several then the plant can collapse and die.

This has happened to my zucchini one year. The inside of the stem near the ground was completely rotted out.

What can you do to prevent damage

This insect larva is difficult to get rid of once they are inside the squash.

Commercial operations will spray insecticide targeting the adult beetles starting at the end of June beginning July. The adults are only active for a few weeks, 6 to 8 weeks. This will kill the emerging adult and just hatched larva but does nothing for the grubs which are already inside the stem.

If you have plants that have grubs you can try killing the grub by poking their entry hole with a wire. If all other things fail, get a sharp knife and slit the stem lengthwise. Look for creamy white grubs with dark brown heads. Kill them but try to limit the damage to the plant. There might be more than one grub per plant. Once that's done clean up any frass and sawdust on the plant and mound some clean soil around the cut. You hope the plant can recover and make some more roots.

If the squash plant has already started to vine you might try putting some clean soil over the stem at nodes, and hope it makes more roots to help support the plant and not need the old damaged stem so much.

You need to keep your plant as healthy as possible. If you suspect and infestation pay particular attention to the watering and fertilization of your squashes. A fast growing plant is better equipped to fight and grow new roots than a malnourished underwatered one.

Some plants like zucchini will produce fruit early in their growth and you might get an impressive crop before the grub gets the best of them.

Prevention is the best way to avoid squash vine borer.

Covering the new plants with plastic mesh well tucked in will keep the adults from coming in and laying their eggs. This only works if there were no squash in the same area the previous year. If you had squash and pumpkins in the same area the year before you might well have cocoons ready to hatch so putting mesh will only trap the insects inside the mesh, where it wants to be!

There are insecticides that can be used and they are effective if you can spray while the adults are around. I avoid pesticides if I can.

Encouraging parasitic wasps in your garden will help control the moth. This means a source of pollen for the adults and no insecticide. Some predator wasps can be bought and released in your garden.

This year has been very dry and I have a shortage of my usual helpers. The wasps are not very numerous. I guess there is not a lot to feed them. The adults eat from flowers, only the wasp larva eats bugs.

One trick that might work is planting a kind of squash that the borer really likes such as hubbard, and hoping that they will attract most of the insects and keep your other squashes bug free. If you find your bait squash gets infected then it's not so painful to pull it out and dispose of it.

If you live in an area that has a long growing season, it might be possible to start another crop and set it out after the adults moths have gone. There are some varieties of zucchini and squash that are faster maturing than others.

The usual advice of rotating crops and not planting the same plant in the same spot applies as does getting rid of all the debris in the fall, and turning the ground over fall and spring to eliminate as many cocoons as possible.

I've heard of people putting a tarp or clear plastic over a piece of ground and just letting the sun warm it up. This either kills whatever is under the tarp or causes the insect to emerge too soon and die. I think that would be an easy experiment to run before planting the baby squash. This is called soil solarization. Here is my page on the process

In order to minimize the buildup of borer population it's very useful to till the soil as soon as the harvest is complete. This helps kill any pupa which has migrated to the soil ready to emerge in the spring.

Some Nematodes can attack the Squash vine borer.

Nematodes are a class of roundworm many of which are parasitic. Some are very effective in controlling garden pests. They are purchased as live worms and mixed with water. This is sprayed onto the plant stem and around the soil.

In some trials Steinernema carpocapsae or S. feltiae applied to the stem and soil provided control of squash vine borer similar to a conventional insecticide.

15 Million Live Beneficial Nematodes Hb+Sc+Sf - Kills Over 200 Different Species of Soil Dwelling and Wood Boring Insects.

Insecticide Control

Because the insect is either comfortably safe underground or snuggly eating away in a squash stem, the timing of any insecticide application is critical for success. You want to catch the critter as it emerges from the ground up to the time it burrows into your squash stem.

Some Insecticide residues must be present when the egg hatches so larvae contact or feed on residues before or as they enter the stem.

Several active ingredients are approved for organic production in the US. These include azadirachtin (neem), neem oil, kaolin clay, geraniol, thyme oil, pyrethrins, and spinosad. Research is spotty as to effectiveness and also depends on concentration.

Treat any insecticide with care, just because they are approved does not mean they are harmless. Pyrethrin is poisonous to fish and bees so it's better to spray in the evening. Keep pets and people away when you spray.

I try to be accurate and check my information, but mistakes happen.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine