The squash bug (Anasa tristis) is a pest to plants in the cucurbit family (squash, pumpkins, cucumbers, watermelon, etc.) and one of the most difficult garden pests to control organically — but it can be done!
Squash bugs (not to be confused with the Squash vine borer) not only damage plant foliage as they feed, they can transmit the cucurbit yellow vine disease (Serratia marcescens) to plants. The bacterium overwinters with the squash bugs that carry it, and then in the spring the bugs infect the new crop. Young seedlings with their first true leaves are more susceptible to infection than older plants, so it is very important to keep squash bugs off your plants at this stage. (More on that ahead.)
Recognizing Squash Bugs and Understanding Their Life Cycle
Squash bug eggs are usually found on the underside of cucurbit leaves between the veins, though can also be found on stems. The bronze-brown flattened, oval-shaped eggs are about 1 millimeter each and laid in loose clusters of about 20.
Generally, squash bugs lay eggs between early June and midsummer, but this will vary depending on region. Ten days after they were laid, the eggs hatch and squash bug nymphs emerge. The nymphs have long antennae and three pairs of legs, and start out light green and just a tenth-inch long. As they molt and progress through five nymphal instars, they eventually grow to a half-inch, and their color changes to gray. The entire nymph stage takes a little over a month.
The adults are five-eighths of an inch long with flat, dark gray, almost black bodies. They have antennae and six legs, like nymphs. Squash bugs have one brood per year in the North and can have two or even three in the South, and it’s only adults that overwinter. In the spring, they emerge to feed and mate, beginning the cycle over again.
Recognizing Squash Bug Damage
Squash bugs use their mouthparts to suck sap right out of leaves and inject their toxic saliva in the process. The toxin creates yellow spots that eventually turn brown and create holes, and the affected leaves wilt, dry up and die.
Mature plants can tolerate more damage than young squash and pumpkin plants can, but it is always a good idea to intervene after spotting squash bug damage.
A cucurbit that is infected with cucurbit yellow vine disease is in trouble. The bacteria will clog the plant’s vascular system, causing the leaves to yellow and leaf edges to curl. The lower stem will yellow and the roots will rot. Affected plants should be removed and disposed of — not composted.
Squash Bug Prevention
If you have had squash bugs before, your chances of getting them again are high — unless you do something to intervene.
Remove the old vines and leaf litter from your garden where adult squash bugs overwinter. This will greatly reduce the population that emerges at the beginning of the growing season.
In a heavily mulched garden, squash bugs will have many places to hide. Plastic mulch (thin sheets of plastic perforated for water to pass through) is an appropriate mulch for squash because it will provide the weed suppression of organic mulch while not providing habitat for pests.
Whether you’ve had a squash bug issue or not, it’s always a good idea to practice crop rotation by mixing up where you plant cucurbits. For instance, where squash were grown the year before, plant nightshades (like tomatoes and peppers), root crops (like radishes and carrots) or salad greens, because these edibles are not susceptible to squash bugs and cucurbit diseases.
Certain squash varieties are far less likely to be attacked by squash bugs than others. These include acorn squash, spaghetti squash, butternut squash and zucchini.
Nasturtium, a vining plant with edible leaves and flowers, is said to repel squash bugs. Interplanting nasturtium with your cucurbits as a companion plant may keep squash bugs away.
Floating row cover installed over squash seedlings will deny squash bugs the opportunity to lay their eggs — at least in your garden. This thin barrier lets light and water through but not squash bugs and other squash pests, such as squash vine borer and squash beetles. However, row cover also excludes pollinating insects, so it should be removed when the plants begin to flower.
You can continue to use row cover until the threat of squash bugs has completely passed if you hand-pollinate. First, identify a female flower to pollinate. You can recognize female flowers by the small embryonic fruit between the flower and the plant stem. Next, identify a male flower — there will be nothing between the flower and the stem — and peel back the petals to reveal the pollen-covered anther. Brush the anther around the stigma of the female flower, and then close the flower with a lightweight clip clothespin to allow the pollination process to complete. My preferred method is to cover the pollinated flower in a nylon mesh netting bag. These bags are both cheap and reusable.
Another hand-pollination method is to take a small, soft painter’s brush and dab the brush onto the anther to collect some of the pollen, and then lightly “paint” the pollen onto the stigma of the female flower.
Squash Bug Control
Handpicking is a tedious but effective — and organic — squash bug control. Inspect young squash plants, especially the underside of leaves, for eggs. Leave the leaves but remove and destroy the eggs. Frequent inspections and egg removal can stop a squash bug problem before it starts.
A piece of sticky tape works well for removal or you can try scraping them off with a credit card.
You can pick off nymphs and adults as well and drop them in soapy water. Some people even use a cordless car vacuum or shop-vac with a narrow attachment in their garden to make the job go much faster.
Insecticidal soap kills squash bug nymphs when directly applied. You can buy insecticidal soap ready-made or make your own by mixing just one to two tablespoons of dish detergent with a gallon of water. Use the mix in a spray bottle with a narrow spray pattern to target the bugs.
Even though insecticidal soap is considered an organic pest control approach, it is non-selective, just like synthetic, inorganic pesticides. It has the potential to kill whatever it comes in contact with, especially soft-bodied insects, including beneficial insects. Insecticidal soap should only be used sparingly and should never be sprayed indiscriminately.
Keep in mind insecticidal soap is far less effective on mature squash bugs, which is one more reason to get an infestation under control right away.
If at all possible, practice manual controls first and resist the urge to use chemical pesticides for squash bug control. The beneficial and neutral insects that these chemicals kill are essential to a healthy ecosystem.
Another common and destructive pest of squash plants is the dreaded Squash vine borer. Check out my post on how to prevent and control the squash vine borer. It can devastate your plants quickly if you don’t catch it in time.
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