Squash Bugs vs. Squash Vine Borers: How to Tell Them Apart

Squash Bugs vs. Squash Vine Borers at a Glance

Squash bugs are flat, brownish-gray insects about 5/8 inch long that suck sap from leaves, so their damage starts as yellow speckles and ends as brown, crisp leaf edges. Squash vine borers are the larvae of a wasp-like moth, and they feed inside the stem, so the first sign is a vine that wilts suddenly with sawdust-like frass at its base. If the trouble is on the leaves and you can see insects or eggs, think squash bugs. If the trouble is in the stem and you see no insect at all, think borer. The two need different treatments, and one bed can host both.

	Squash bug	Squash vine borer
What it is	A true bug that sucks sap	A moth whose caterpillar bores into stems
Adult	Flattened, mottled brownish-black, 1/2 to 3/4 inch	Clear-winged moth, orange and black abdomen, 1 to 1.5 inch wingspan
Eggs	Shiny bronze, in clusters of about 20 under leaves	Tiny, brown, flattened, laid singly at the base of the plant
Damaging stage	Nymphs and adults, on leaves	Larva, hidden inside the stem
First symptom	Yellow spots or white stipples on leaves	Sudden wilting of a vine
Giveaway sign	Egg clusters and grouped gray nymphs	Hole in the stem with sawdust-like frass
Speed	Builds over weeks	Plant can die if untreated
Main fix	Hand-pick, remove eggs, trap under boards	Row cover, stem wrap, stem surgery

How to Identify Squash Bugs

Adult squash bugs are 1/2 to 3/4 inch long, mottled brownish-black and somewhat flattened. University of Maryland Extension describes them as about 5/8 inch long with the triangular plate behind the head that marks a true bug. They give off a strong odor when crushed, and adults can be found gathered on plant stems at soil level.

The eggs are the easiest stage to recognize. They are about 1/16 inch across, shiny, and bronze or reddish-orange to brown, and laid in clusters of about 20 on the undersides of leaves, often along the center vein. Eggs hatch after about 10 days.

Newly hatched nymphs have pale green bodies with dark heads and black legs. Older nymphs turn gray. They feed in groups and pass through five growth stages over four to six weeks before becoming adults, so a plant with one stage on it usually has several.

How to Identify Squash Vine Borers

The adult is a moth that most people take for a wasp. Maryland describes a fat, black, clear-winged moth with orange legs and an orange and black abdomen, and Wisconsin Extension gives the wingspan as 1 to 1.5 inches.

Females lay tiny, brown, flattened oval eggs one at a time at the base of the plant. They hatch seven to 10 days later, and the larvae burrow into the vine immediately. From then on the pest is out of sight.

The larva is a wrinkled, white to cream caterpillar with a dark head, up to 1 inch long at maturity. You will only see it if you open the stem. What you will see from outside is frass, the light brown to greenish-white droppings that look like wet sawdust and pile up at the entrance hole.

The Damage Each One Causes

Squash bug damage is a leaf problem that develops gradually. Feeding leaves small white dots or yellow spots. Over time the spotted leaves look tattered, then turn dry and brown or black along the edges. With large numbers, leaves yellow and die, and Wisconsin Extension notes that the feeding can also cause wilting. Pumpkins and squash are the preferred hosts, followed by gourds and melons.

Borer damage is a stem problem that seems to arrive overnight. A runner or a whole vine wilts quickly because the larva is feeding within the stem, and Wisconsin Extension gives midday wilting as the first symptom. Maryland Extension states that the entire plant dies if left untreated. At the base of the wilting stem, look for the entrance hole and the frass pushed out of it.

A quick field check settles most cases. Turn over several leaves and inspect the crown for egg clusters, nymphs and adults. Then examine the base of each main stem for holes and frass. Do both checks every time, even after you find the first pest.

Wilting has other causes too. If leaves show holes or chewing instead of speckles, the leaf damage identification chart covers the likely culprits.

Timing: When Each Pest Shows Up

Squash bugs spend winter as adults under plant debris, soil clods, rocks, log piles and firewood, then move onto squash plantings once the new season starts. Wisconsin sees one generation a year, and Maryland may see two. With eggs taking about 10 days and nymphs four to six weeks, several stages are usually present on a plant at once.

Squash vine borers overwinter as pupae about 2 inches under the soil. The moths emerge on a regional schedule: mid-May to mid-June in Maryland, and late June and July in Wisconsin, when about 900 growing degree days (base 50) have accumulated. There is one generation a year in the north and up to two farther south. Larvae feed inside the vine for 14 to 30 days, and Maryland notes they can feed for up to 4 weeks.

Those numbers explain why borers are so hard to catch. With only seven to 10 days between egg and a larva inside the stem, protection has to be in place before the moths fly. Squash bugs give you more time, since their nymphs take four to six weeks, or 28 to 42 days, to mature in the open where you can reach them.

How to Control Squash Bugs

Squash bugs are manageable when you start early and stay with it.

1. Check leaf undersides regularly once plants are up. Tear out the bit of leaf holding an egg cluster and destroy it, or crush the eggs in place.
2. Hand-pick nymphs and adults, and crush them or knock them into a container of soapy water.
3. Lay a board on the soil near the plants. Bugs hide under it overnight, and you can destroy them in the morning.
4. Spray nymphs directly with insecticidal soap, neem or horticultural oil. Maryland Extension rates these as effective when sprayed directly on nymphs, so aim for the clusters of young ones. See the insecticidal soap label guide and neem oil versus horticultural oil for product selection.
5. Use row cover on young plants to prevent egg laying, and remove it when flowers open so pollinators can reach them.
6. At the end of the season, remove all plant debris. Maryland calls this step essential because it takes away the adults’ winter shelter.

For gardeners who use conventional insecticides, Wisconsin’s treatment threshold is one egg mass per plant during flowering. Trellising vines and rotating crops also help.

How to Control Squash Vine Borers

Prevention carries most of the weight with borers, because the larva is protected once it is inside.

1. Cover plants with floating row cover from planting until flowering. Our comparison of row covers for pest protection covers the weights to use.
2. Wrap a collar of aluminum foil around the lower stems to block egg laying.
3. Watch the stem bases for frass. When you find it, make a vertical slit upward from that point, remove the larva, and mound soil over the wound so the vine roots there.
4. As an alternative to cutting, Maryland notes that Bt (Bacillus thuringiensis) can be injected into the stem at the wound.
5. Where sprays are used, Wisconsin recommends two to three treatments, 5 to 7 days apart, during the three-week egg-laying period.

Variety choice is the simplest long-term answer. Butternut, Tromboncino and cushaw squash are resistant, and yellow crookneck is less susceptible than zucchini. If borers take your zucchini every year, growing one of the resistant types alongside it protects part of the harvest. A fuller prevention calendar is in squash vine borer prevention.

Quick Recap

• Squash bugs: flat brownish bugs 1/2 to 3/4 inch long, bronze egg clusters of about 20 under leaves, speckled leaves that turn brown and crisp.
• Squash vine borers: a wasp-like orange and black moth, a cream larva up to 1 inch long inside the stem, sudden wilting and sawdust-like frass.
• Borer eggs hatch in seven to 10 days, so covers and stem wraps must go on before moths fly in late spring or early summer.
• Hand-pick and trap squash bugs, spray only the nymphs, and clear debris in fall.
• Butternut, Tromboncino and cushaw resist borers.