The foxes set fire to the stacked grain (waiting to be stored) and the standing grain (waiting to be harvested). Afterward, they leave the grain fields and the winnowing grounds and enter the olive orchards and the vineyards, setting also them ablaze. These were the very things the Philistines were getting ready to harvest. So the damage is extensive, occurring at a crucial point in the agricultural cycle, and will cause all sorts of problems for the Philistines’ food supply. It is also worth noting that Dagon, the Philistine god, is a grain deity. Samson has directed his attack to the realm over which the Philistines thought their god was sovereign.
5 And when he had set fire to the torches, he let the foxes go into the standing grain of the Philistines and set fire to the stacked grain and the standing grain, as well as the olive orchards.