Hello Antonia
Flame 'withers' neighbouring vegetation. For instance, if you have a bonfire in your garden, the grass near the bonfire 'withers'.
Here, the context suggests that 'they' went on the sexual rampage in Woodstock. You could say that they 'cut a swathe' through the eligible inhabitants.
The metaphor seems (at first glance) slightly inaccurate: the vegetation that is 'withered' is not in the path of the 'flame', but to the side; whereas the vegetation in its path is e.g.'devoured'. The latter case would seem to apply to 'their' sexual partners.
MrP