Squash vine borers (SVB) are maggots of the SVB moth that bore into the stems of cucurbits (squash, cucumbers) and work their way up the stem. You can tell you have borers when your plant suddenly wilts. Check the stem about an inch and a half up from the ground and you will see a small hole with sawdust around it where the borer entered.
Once the borer is in the squash, the plant is at great risk. If you catch it early enough, you can slit the stem with a razor knife from the entrance hole up to where the borer is and destroy the borer with the knife blade or a wire. Then bury the slit stem under soil to keep it covered. This may or not be successful, but if you do nothing, the plant will die. If the plant can’t be saved, pull it and destroy it. You can replant squash in early July after the SVB threat is over.
The best method is to prevent the SVB from laying eggs near your squash, or provide a physical or biological barrier. If you can cover your plants with floating row cover, that keeps the moths away from laying eggs near or on the plant. But the cover has to be removed when the plant starts flowering so pollinators can get to the flowers.
The SVB moth does not look like a moth but more like a wasp or bug. It emerges from a cocoon in the soil the end of June or beginning of July (but remember we are way ahead of average on degree days because of the mild winter and spring). These moths fly during the day and are very good fliers like a wasp. They stand out because of their coloring so keep a lookout for them and let others know if you see one in the garden (squish it first, then let us know).
The borer itself is a big, fat, ugly thing, shown here in this cross-section. These pictures are courtesy of the University of Minnesota Extension (see http://www.extension.umn.edu/distribution/horticulture/M1209.html for their advice on SVB management).
Here is a video that suggests using cardboard tubes to prevent the borers from physically reaching the stem and boring into the plant.
Other suggestions are to wrap aluminum foil around the stems at the soil level and several inches up. Another, more elaborate technique is to inject the squash stems with a BT solution, inoculating the insides of the stems with BT, which will kill the borers when they try to enter the stem.
