Squash bugs are a hassle for growers of zucchini, pumpkins and other cucurbits, but there are steps that can be taken to manage and control this challenging pest. My guest this week is entomologist and pest management expert Diane Alston, and she’s here to share strategies to reduce squash bug issues.
Diane is a professor and the head of the biology department at Utah State University, where she has worked for 32 years. She started there as the extension entomologist for the Utah Extension and later became the extension’s Integrated Pest Management (IPM) coordinator and also co-directs the Utah Plant Pest Diagnostic Laboratory.
Diane grew up in Southern California, about 30 miles east of Los Angeles, in a family that gardened. She was always interested in plants and that led to her interest in insects that feed on plants. Diane attended the University of California Riverside to study botany and then North Carolina State University, where she earned both a master’s degree and a doctorate.
The Difference Between a Bug and an Insect
Every bug is an insect but not every insect is a bug. Diane explains that there is a group of insects called “true bugs,” and the squash bug is one of them. These are insects that have piercing-sucking mouthparts. Most use their mouthparts to feed on plants, but there are predatory bugs as well, such as the assassin bug.
Bugs use their proboscis (an elongated sucking mouthpart) to pierce plant cells and feed in the phloem tissues. “They’re directly tapped into the vessels of the plant where the nutrients are flowing,” Diane says.
The plant takes up water and nutrients from the soil, photosynthesizes, and moves those nutrients around to produce its fruits. The squash bug taps into the plant and takes out high-quality nutrients from the sap.
How to Recognize Squash Bug Damage
Squash bug damage appears as little yellow lesions on leaves, stems and vines. Those spots will turn darker and grow in time. The edges of the leaves start to get crispy, especially as the days get hotter and the plants are stressed. The plants are trying to take up water for photosynthesis and respiration but struggle because of the damage.
Squash bugs will also feed on the fruits themselves, causing lesions. If the population of squash bugs is high enough and the plant is stressed enough, it may suddenly wilt as well.
It was previously thought that squash bugs inject a toxin as they feed, causing wilt, but Diana says rapid wilt is just a result of heavy feeding that breaks xylem tissues, disrupting water flow. “Basically, it’s like getting a puncture in your house,” she says.
The Difference Between the Squash Bug and the Squash Vine Borer
The squash bug and the squash vine borer are much different pests, though they share a taste for squash. The squash vine borer is not a bug but a moth, and it’s the moth’s larva that does the damage.
The squash vine borer is not present west of the Rockies, though Diane was introduced to the pest during her time in North Carolina. She said that squash vine borer damage can cause sudden wilting of a plant the same way that squash bug damage does. The thing to look for to be able to tell the difference is a hole at the base of the plant on the main vine or stem. That is a bore where the squash vine borer larva, or caterpillar, entered the vine or stem to feed.
If the problem is squash bugs, the signs to look for are the bugs themselves. They often hang out at the base of the plant where the old leaves are touching the ground. Lift up the leaves and find brown-to-gray adult squash bugs.
Crops Squash Bugs Love to Feed On, and Crops They Are Less Likely to Attack
Squash bugs, as the name applies, are most commonly found on squash — but that’s not all.
Straightneck, crookneck and yellow squash are all targets, and zucchini and pumpkins are favorites. Squash bugs may also be found on cucumbers and winter squash, though the damage tends to be less extreme. Cantaloupe and watermelon may also be attacked, though that is even less common.
Black Zucchini is said to have fewer squash bug problems than traditional zucchini, and newer varieties tend to have fewer issues than older ones. Hubbard squash has a thick rind that resists squash bug damage.
Cucurbit Yellow Vine Disease
Cucurbit yellow vine disease is a bacterial disease found in the Midwest and the Eastern United States that squash bugs can spread as they feed. The bacteria will clog the plant’s vascular system, causing leaves to yellow and leaf edges to curl. The lower stem will turn yellow and the roots will rot. Affected plants should be removed and disposed of — not composted.
Remove Old Vines to Reduce Squash Bug Populations
Squash bugs are difficult to control because they overwinter as adults, Diane says. If old squash vines are left in place between growing seasons, the squash bug population will persist in the plant litter. She recommends pulling up all of the old vines and hot composting them to reduce the survival of the bugs. If your compost pile does not get hot, you should dispose of the vines off-site.
The Chemical Signals that Attract Squash Bugs
Cucurbits have certain plant chemicals that attract organisms that want to feed on them, such as squash bugs. Cucurbitacins are a group of volatile chemicals in cucurbits that squash bugs really hone in on.
Squash bugs are very well adapted to finding squash — they have evolved to do so.
IPM Strategies to Prevent Squash Bug Infestations
There are two planting strategies to outwit squash bugs. The first is to plant as early as possible so plants are up to a larger, more resilient size before squash bugs come along. Tiny seedlings will be much more sensitive to squash bug damage.
The second timing strategy is to put out plants later on, after squash that was planted elsewhere in the area has attracted the bulk of the squash bugs. The get the timing right either way, research when squash bugs are active in your area.
Using floating row cover will provide a physical barrier between plants and squash bugs, preventing the bugs from both feeding on plants and laying their eggs. The only issue with using a physical barrier is that the squash flowers need to be pollinated. If squash bugs can’t get in, neither can pollinating insects. One solution is to hand-pollinate, and the other is to remove the covers for a few hours in the warmest part of the morning when bees are most active. The squash bee is a widespread type of bee that is very effective at pollinating various squash.
Egg removal is another important proactive step to stopping a squash bug problem before it begins. Female squash bugs lay 220 to 250 eggs per season in clusters of 12 to 20 each. The slightly elongated copper to bronze eggs get even darker as they get closer to hatching. The eggs are commonly laid right up against the vein of a squash leaf, where the leaf is attached to the stem, and are well hidden to prevent predation from lady beetles and other insects. But if you know where to look, you’ll find them.
Diane recommends smashing those clusters with your garden gloves on. Another method is to remove the eggs with duct tape. Even a credit card can be used to scratch them off, or a lint brush can pull them off just like duct tape does.
After the egg stage is the nymph stage, which progresses through five instars. The nymph stage looks fairly similar to the adults. When they are first hatched they have a red head and antennae. As they grow and molt, their legs become black and the body becomes grayer. Squash bug nymphs tend to stick together and are easy to squish.
The adult squash bugs are physically harder to kill. They can still be squished, but it takes more force. They are more spread out, they fly in short hops, and they scurry away. When you do catch and squish them, their scent glands leave a foul smell like a stink bug does.
Crop rotation is a strategy that Diane recommends for growers on a half-acre farm or larger. That means following a season of growing squash with a different crop that is not susceptible to squash bugs for the following few years. Crop rotation will reduce squash bug pressure and can also reduce disease pressure, such as Verticillium wilt.
Trap crops are another useful tool for commercial growers. If the grower is planting a crop that is sensitive to squash bugs but not a favorite, the grower can plant a trap crop that is very attractive to squash bugs. The primary crop will get relief from squash bug pressure, and the trap crop can be treated with insecticides to knock down the population.
Most commercial growers are using insecticides containing pyrethroids, which are man-made chemicals that are similar to pyrethrins derived from Chrysanthemums. Pyrethroids fortunately don’t last long, which is good news for pollinators.
Getting Some Help from Parasitoid Insects
Tachinid flies are parasitoid insects that lay their eggs around the thorax of squash bugs. When an egg hatches, the fly larva tunnels down into the body of the squash bug, slowly killing it in the process.
Squash bugs stop feeding soon after they are parasitized, so tachinid flies are an effective biological control as far as gardeners are concerned.
Squash Bug Controls Available to Home Gardeners
Kaolin clay is a hydrophobic fine clay that can be applied to plants to physically deter squash bugs. The same product is used by fruit growers to create a barrier film that stops insects.
Horticultural oil is a petroleum mineral oil that’s similar to dormant oil but highly refined. At a 1 percent concentration with water, horticultural oil can be used to suffocate squash bug eggs so they don’t hatch.
Pyrethrin is an organic product that is effective at controlling nymphs, but not adult squash bugs. The adults have essentially a coat of armor that makes it really hard to control them with sprays.
I hope you enjoyed my conversation with Diane Alston. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
How have you overcome squash bug pressure? Let us know in the comments below.
Links & Resources
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Soil3 – Our podcast episode sponsor and Brand Partner of joegardener.com
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