This comes form Joe Lampl's site:
"Squash Vine Borer Prevention Through Timing
Knowing when squash vine borers are active in your region is the first step. In the North, squash vine borer moths are laying eggs in late June and early July. In the South, they emerge in May and there can be up to two generations per year and moths may continue to lay eggs through mid-August. The West Coast is least impacted by squash vine borers, where they are uncommon.
In the South where winters are mild, plant squash as early as possible (as soon as the soil warms to 60° Fahrenheit) so you may harvest before squash vine borer is active — or at least before borers kill the plants. Northerners can wait until late July to plant squash, after the borers have come and gone".
