That outburst leaves Jack almost speechless, so Spike's almost gone by the time he manages to come out with even a "Fuck you!" to yell after him - barely even aware of the irony of such a remark given what they were talking about mere minutes before.
He's half-relieved, half-worried by Spike's departure. On the one hand, he can relax and run inside to get away from the damn wasps, but on the other, who knows where Spike's going? That's not a concern that's going to grow any less as the day (if you can call it that when it's pitch dark outside) wears on.