Answer: It sounds to me as though your plants were attacked by squash vine borers. The squash vine borer overwinters as a fully grown larva in cocoons in the soil, 1 to 6 inches deep. It pupates in the spring and the adult (a moth) emerges in June. Moths are active during the daytime and in the evening they rest on leaves. This is different than the behavior of most moths, which are active at night. The moths fly slowly in zig-zags around plants, and lay eggs singly on stems; eggs are usually found on the main stem near the base, but are also found on leafstalks or on the undersides of leaves. Moths are active for about one month.
Eggs hatch in 9 to 14 days. Larvae enter the stem at the plant base within a few hours after hatching from the eggs. Larvae feed inside the stem for 4 to 6 weeks. Fully grown larvae leave the stems and crawl into the soil to pupate. There is usually one generation per year, but a partial or complete second generation is possible.
The stage most susceptible to natural enemies is the egg stage, which is attacked by parasitic wasps. Larval and adult ground beetles (Family Carabidae) can attack larvae of squash vine borer, but do not appear to cause significant mortality.
To help control the pest:
Destroy vines soon after harvest to destroy any larvae still inside stems.
Rototill the soil in fall or spring to destroy overwintering cocoons.
Cover vines at leaf joints with moist soil, to promote formation of secondary roots that will support the plant if the main root and stem are injured.
Rotate your crops so you don't plant squash in the same part of the garden year after year.
Borers can be removed from vines if detected before much damage is done. Examine stems in early summer; once holes are detected, slit the stem longitudinally with a fine sharp knife, remove the borer, then cover the wounded stem with moist soil above the point of injury to promote additional root formation.
Stems can be covered with a barrier, such as strips of nylon stockings, to prevent egg laying.
Catch and destroy the moths, especially at twilight or in early morning when they are resting on the upper side of leaf bases.
Hand-pick the eggs before they hatch.
Best wishes with your squash plants next summer!
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