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- The Scourge of Squash -
Squash Vine Borer and Squash Bugs
- About this time of year, two different insect pests begin to plague many squash and pumpkin fields. Despite differences in their appearance and life cycle, both squash vine borers and squash bugs share a similar preference for squash and pumpkins over other cucurbits. Also, control of both pests depends on proper monitoring and timing of insecticide application.
The squash vine borer grows up to an inch long and looks like a white, fat, wrinkled grub, with a brown head. It bores into and feeds inside squash and pumpkin vines causing them to wilt. To determine if wilting is caused by squash vine borer, rather than disease, growers should look for entrance holes in the stem and piles of “sawdust” like frass nearby. If a grower splits the stem, he will find the squash vine borer itself, unless it has left the vine to pupate in the soil.
The adult squash vine borer is a clear-wing moth, and looks quite different from typical moths. Like the name implies, a clear-wing moth has transparent hind wings. The squash vine borer’s front wings are greenish brown and its hind legs are covered with a fringe of brownish red hairs. The moth is about 5/8 of an inch long with a wingspan of 1 to 1 1/2 inches. Unlike most moths, which are active at night, it flies, feeds, and lays eggs during the day. In fact, if you encounter a squash vine borer moth flying in a squash field, it is easy to mistake for a wasp.
In most of the Great Lakes region, one generation of squash vine borer occurs each year. Pupae overwinter in the soil and emerge as adults in late spring to early summer. Female moths lay eggs singly at the base of a plant beginning around 1,000 degree days (base 50). After hatching, larvae burrow into the stem. Once inside, they feed protected from insecticides; they must be controlled between the time they hatch and the time they burrow into the vine. After feeding for a month or more, mature larvae leave the plant and burrow into the soil to pupate and to spend the winter.
To detect squash vine borer, growers are advised to scout their squash and pumpkin fields beginning at 900 degree days (base 50). At that time you can often see adults flying in zig-zag patterns along the base of plants, looking for a place to lay eggs. Growers should also look for entrance holes and frass piles in vines that are beginning to wilt. Another way to monitor adult flight is with a pheromone lure and trap. The lure contains a scent that mimics that of the female moth and attracts males to the trap. Because it attracts males, the trap does not actually control squash vine borers, but it does tell growers when the moths are flying and eggs are being laid.
Fortunately, squash vine borers are effectively controlled by a few well-timed sprays directed at the base of the plant where the eggs are laid. Once adult moths or larvae are found, two to three insecticide applications spaced five to seven days apart will kill the larvae after they hatch from the eggs but before they bore into the vines. Remember, any larvae already in the vine will not be affected by the insecticide. A number of materials, including Asana, Capture, permethrin (Ambush, Pounce), and endosulfan (Thiodan, Phaser) are registered for control of squash vine borer in squash and pumpkins. Pyrethroids (Ambush, Pounce, Capture, Asana) are highly toxic to bees, and should be used with caution or avoided if plants are in flower.
Squash vine borer is often most abundant in fields that have had problems in previous years. Because squash vine borers spend the winter as pupae in the soil, crop rotation, destroying crop residue, and plowing or disking the soil in the fall or spring can kill some of the overwintering population and reduce problems the following year. Adult moths are fairly mobile, so fields adjoining last year’s infected squash or pumpkin fields can also be heavily infested.
Squash bugs, in contrast, are quite different from the squash vine borer in life cycle, type of feeding and type of damage. Both adult and immature squash bugs feed on foliage using their beaks to suck plant juices from leaves and later from fruit. Squash bugs can damage squash and pumpkins throughout the growing season, although early summer is the critical time for control.
Adult squash bugs are grayish-brown, up to 3/4 of an inch long and flattened above. Their head is extremely narrow, compared with their body, making them look pointed. Like other true bugs, they have a diamond-shaped area at the rear of their body (which is dark brown in squash bugs) where the membranous parts of their front wings are folded. Also, like other true bugs, their mouth is formed into an elongated beak used for piercing and sucking.
Squash bugs overwinter as adults in crop debris and in field borders and become active as the temperature warms early in the spring. They are attracted to cucurbits, especially squash and pumpkins. Research from Oklahoma indicates that overwintering squash bugs prefer older, larger plants, when given a choice. Seedlings can be severely damaged by the overwintering adults, and should be treated with insecticides if plants are wilting and squash bugs are found on the underside of leaves. Eggs, which are laid in rows on the underside of leaves, are reddish-yellow when laid and later turn a metallic-bronze. After hatching, nymphs are pale green, but turn brownish-gray as they mature. Young nymphs feed in clusters on the underside of leaves.
Later in the season when the plant canopy becomes dense, squash bugs can be extremely difficult to control because they feed under the leaves, out of reach of insecticide applications. In addition, research from Oklahoma indicates that plants grown on plastic mulch have increased squash bug problems, probably by providing the bugs with more areas protected from insecticides. It is essential for growers to control the young nymphs, which are less mobile and more susceptible to insecticides than adults or older nymphs, BEFORE the canopy closes. Growers should check the underside of leaves during the early flowering stage of the crop. If more than one squash bug egg mass per plant is found, treatment is recommended. A number of effective insecticides are registered for control of squash bugs on squash and pumpkins.
If populations are not controlled early, they can build under the protective cover of the plant canopy and, when foliage dies back in the fall, squash bugs may feed on and damage the fruit. Large numbers of squash bugs feeding on squash and pumpkins in the fall can cause cosmetic damage and even collapse of the fruit. Again, to prevent this, it is important to control populations early in the season.
As with squash vine borer, removal of crop debris and plowing or disking fields may kill overwintering insects and reduce problems next year.
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