In most areas, yellowjacket colonies are either dying out or are becoming extra obnoxious, or both. In late summer, a yellowjacket colony is at its largest (1,000-4,000 workers), but its social structure is breaking down. Next year's queens and males are produced and workers begin dying off.
You've heard that the reason yellowjackets become so aggressive and "in-your-face" late in the season is because they begin foraging for sugary foods rather than insects as they have fewer developing larvae to feed. While no one really knows why yellowjackets undergo such a noticeable personality change in the fall, one yellowjacket expert rejects the change in diet explanation, saying that yellowjackets routinely forage on both carbohydrates and proteins throughout the season.
Apparently much of the change in yellowjacket behavior has to do with social problems within the nest as allegiances change and conflicts increase in late season. The current queen's influence begins to wane and she will die soon. There's fighting among the workers as some of them are developing into next year's reproductives and dealing with that whole hormone thing. Workers begin demanding regurgitated food from the remaining larvae, sometimes killing them in the process. According to the expert, yellowjackets are losing control as their society is collapsing.
How concerned should you be about fall calls for yellowjacket control? After all, the wasps will be dead soon and new queens won't re-use the same nests next year. Nevertheless, there are some good reasons to eliminate yellowjacket nests, even late in the season.
In more northern states, yellowjacket colonies begin to decline in July or August, but some remain active throughout September. In southern states, reproductives may not even be produced until after September. Yellowjackets nesting in building voids can remain active well into December. In very warm regions, colonies might not die off at all.