Is Your Squash Bored to Death?

comments (6) August 7th, 2008

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The wing span of the adult female squash-borer moth is 1 in. to 1-1/2 in. At bottom, eggs (left) and larva (right).
Out-patient surgery. If you suspect borers, carefully slit the stem and remove the larva with your knife.
The wing span of the adult female squash-borer moth is 1 in. to 1-1/2 in. At bottom, eggs (left) and larva (right).Click To Enlarge

The wing span of the adult female squash-borer moth is 1 in. to 1-1/2 in. At bottom, eggs (left) and larva (right).

Photo: Susan Carlson

Say you’re new to gardening and you’re growing squash for the first time. One day you notice the plants are suddenly wilting. If watering doesn’t help, you might ask neighbors or friends for advice. But what if you are without nearby friends who can take a look, or those who do can’t explain what they see? Here are some basic steps to take in figuring out what’s wrong, then deciding on a logical pest-control strategy. While our discussion here applies to squash vine borers, a similar approach can be taken with other pest problems.

Work from the symptoms.
Often the sight of the pest itself alerts you to a potential problem. But just as frequently, it is a symptom you notice first, like the squash plants suddenly wilting or collapsing. Watering doesn’t help, so you wonder what to do next and whether the problem will spread to other squashes.

Then you closely examine the main stem of the plant, or the portion of the vine that has died back, and see one or more holes with a greenish, moist, sawdust-like material on the ground and within the hole. Guessing that something has entered the stem, you slit open the stem near the hole. Aha! Inside you find a brown-headed, fat, white larva, about 3⁄4 in. long. There is more of the wet sawdust material, which by now you realize is the excrement or frass from the insect.

Find out the bug’s name.
You have a key to all the world’s literature about this insect once you know its name. A common name is good, but the Latin name is better because there will be no confusion about which insect you are dealing with.

A gardening friend or neighbor might be able to tell you it’s a squash vine borer. Or call your local cooperative extension office, describe the damage and the larva, and no doubt you’ll learn both the common and the scientific name.

Then look up the pest by name or by the damage symptoms on the plant in a good garden book, learn something about the insect’s life cycle, and get suggestions for some non-toxic controls. These books also usually link pests with the plants they damage. Therefore, all you have to do is search under squash, find a description of the damage you see on your plants, and learn which pest is the culprit.

Learn the pest’s life cycle.
Learning about an insect’s biology and life cycle is the key to finding least-toxic solutions for controlling it. In Destructive and Useful Insects, the authors explain that squash vine borers attack many varieties of squash, pumpkin, and other members of the cucurbit family in parts of the United States east of the Rockies from Canada to South America. They winter in the soil, an inch or two below the surface, in tough black cocoons, then emerge as moths in the spring about the time the squash plants send out runner vines. These large, swift, daytime-flying, orange and black, clear-winged moths are named Melittia cucurbitae. They lay small, flattened, brownish eggs on squash stems and leaf stalks, especially towards the base of plants. The young larvae hatch and enter the stems a week or two later. These borers tunnel along, eating the inner tissues of the stem for about a month, then they leave the plant and make cocoons in the soil. In the North, they stay in the soil until spring, but in warmer areas there may be a second generation later in the season.

Now, armed with life-cycle infor­ma­tion, let’s see how we can build a least-toxic control program for squash vine borers.

Start with good garden culture. In general, any insect that overwinters in the soil can be suppressed by using a number of horticultural controls. The first is rotation. By growing squash in a different area of the garden every year, borers emerging from the soil where the squashes were last year will have a certain
distance to go to find them again. Every inch they have to travel exposes them to deadly predators and parasitoids.

Next, good garden clean-up is critical. When plants die back, and at the end of the summer, rake up all debris. Place all squash debris in a clear plastic bag. Tie the top, set the bag in the sun for a week to kill remnant insects, then add the debris to your compost pile.

After raking, turn the top couple of inches of soil so cocoons are brought to the surface. Over the winter, they will be killed by cold temperatures, especially alternate freezing and thawing. They’ll also be exposed to birds, mice, shrews, and insect predators.

In spring, turn over the soil deeply, so any surviving cocoons are left at a greater depth than the usual inch or two. Deeply buried, the emerging insects will have a tough time making it to the surface.

Plant smart and safeguard seedlings.
If you live in a warm region, where two generations of squash vine borers are a threat, it’s worthwhile to stagger your squash plantings over time, assuming some of the plants will have a chance to grow large before the moths lay eggs again. The larger the plants, the greater the chance some vines will survive to produce fruit when borers attack. Along the same lines, pinch back young plants to encourage branching. With more stems, some will be spared attack by borers, and more will survive borer removal.

Adult moths are swift fliers, so they’re tough to catch even if you do spot them in the garden. But you can protect young plants with physical barriers like row covers or agricultural fleece, making sure they are well secured and free from holes.

The problem with row covers, however, is pollination. Squash blossoms are pollinated by insects, usually bees. So uncover squash plants when the female flowers open, and let the bees do their work. Of course, the minute you uncover the plants they are vulnerable to pests. But, by the time the female flowers have started to open, your squash plants should be large enough to withstand some borer damage.

Protect plant stems from borers. When you uncover the plants for pollination, provide some protective barrier for their main stems. Moths like to lay eggs toward the base of squash plants, and damage to the main stems is most likely to cause fatal injury. Wind nylon stockings or strips of aluminum foil around the stems. A large paper cup, with sides split and bottom removed, can be fit upside down around the stem. In all cases, the barriers should start just below soil level and rise at least 6 in. up the stem.

Show no mercy when you catch them.
An obvious control is to kill the pests by hand. When you see eggs glued to a stem, scrape them off with a dull knife or crush them. If you see a suspicious hole littered with frass, slit open the stem directly above it and carefully remove the borer with a knife or wire. Take care to injure the walls of the vine as little as possible. Then immediately cover the cut area of the stem with soil, and the vine will probably survive.

Since many squashes can put out new roots from nodes along the stem, bury a few long trailing runners. That way, if there is dieback due to borer damage on some stems, you will have new plant sections already rooted.

Biological pesticides are an option.

Insect-eating nematodes can be used to kill squash vine borers. A solution of the nematodes is injected into plant stems with a syringe when the holes of the borers are first seen. There are three species of insect-eating nematodes available commercially. Ask suppliers to recommend which species will be effective on squash vine borers and how much you’ll need to get the job done.

Last but not least, you may wish to inject squash stems with Bacillus thuringiensis kurstaki, or Btk, the bacteria that kill many caterpillars. Follow directions for mixing up the material right before use, just as you would for a spray.

Start by injecting Btk into the main stems within an inch or so of the soil, about the time blossoms start to open. Repeat the injections in 10 to 14 days. Your goal is to bathe the inside of the hollow stems with the Btk solution. Any borers present will eventually die after eating stem material coated with Btk.

Good bug reference books

Destructive and Useful Insects, 1992, C.L. Metcalf and W.P. Flint. McGraw-Hill Book Co., New York City.
The classic text for students of entomology. We turn to it first when starting a search.

Chemical-Free Yard & Garden,
1991, A. Carr et al. Rodale Press, Emmaus, PA.
A sophisticated text written for the home gardener with a great deal of excellent information on the use of beneficial insects.

Rodale’s Garden Problem Solver, 1988, J. Ball, Rodale Press, Emmaus, PA.
Includes many innovative solutions to pest problems.

Gardener’s Guide to Common Sense Pest Control,
1995, Olkowski, Daar, and Olkowski, The Taunton Press, Newtown, CT.
Our own book, and another good reference on least-toxic methods for solving pest problems.

by William Olkowski
August 1998
from issue #16


posted in: pests, squash

Comments (6)

Webhound writes: You say to "inject" the Btk into the stems. What device do
you use to do that? An insulin syringe? or what?
I was under the impression that you needed an Rx from a physician to purchase syringes. Please explain. Thanks.
Posted: 11:08 pm on September 1st
PaulMartin writes: How active are the borers, I have had problem the last two years. My understanding is the flying adults are only around a couple weeks is that true.
Posted: 2:10 pm on June 22nd
CINDYH writes: Squash bugs are not the same as squash borers but can wreak havoc on the plants and fruit. They are stink bugs. I pick them off and crush them (hold your nose). When watering they will run up the vine and you can catch them. Wear gloves as they are prickly too. I try to minimize wetting the foliage as this invites more trouble. Mine are multiplying like crazy. They lay brown eggs in a cluster underneath leaves. Then hatch by the hundreds. Good luck.

Will try the radish suggestion. Thanks.
Posted: 11:36 am on June 22nd
maggieray writes: Could you please tell me if squash borers and squash BUGS are one in the same. They sure don't look like what has been gobbling up my plant. These bugs are grey and there are hundreds of them. this is the first year we have plant squash and pumpkins and both of them (planted in different places a long way apart) are filled with these things. I would appreciate help from anyone on this. Thank you.

Posted: 9:08 am on June 22nd
mcfar73 writes: I started growing the vining type up a trellis. Makes it much easier to find the pests: Borers and Squash Bugs. Starting the 2nd week in June, start examining each day for signs of the moth/larvae from the borer. I fought them off last year by spraying liquid 7 at the base every 7 days for 3 weeks, starting the 2nd week in June. I prefer not to have to use chemicals, because of the damage to beneficial insects. However, these borers are tricky little things--I don't spray the squash, just the vine close the ground. You have to be diligent with this if you want it to work--I also plant twice as many vines in anticipation of losing some. Will try radishes this year--will plant today among them!
Posted: 2:27 pm on June 7th
KKufahl writes: One sure fire way to keep squash borers out is to plant radishes along with the squash. The radishes naturally deter the borers and taste great too. I have been doing this for years and have never had any trouble with borers.
Posted: 10:21 am on March 13th
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