When the flowers of autumn are gone, when even the asters are fading fast, where is a wasp or bee or butterfly to go for sustenance? The answer may surprise you, and turn your concept of what is a "pest" on its head.
Highly active, flying insects like bees, wasps, flies, moths, and butterflies rely on high carbohydrate resources like flower nectar to fuel their vigorous lifestyle. When those normal floral resources are no longer available, the insects must travel down other avenues. Fallen, fermenting fruits are one solution. The high sugar content of an apple, pear, or peach beginning to rot does not go overlooked by yellowjackets and paper wasps in particular.
Pomes and other fruits are not, however, the answer to autumn insect nutrition. The overwhelming majority of sweet, sugary carbs are provided by other insects, namely aphids and scales. At this time of year, aphids in particular are feeding on plant sap in earnest, and excreting copious amounts of liquid waste called "honeydew." Infested trees are literally dripping with honeydew, and a great diversity of other insects are drawn to this equivalent of the corner bar.
Many aphid species are also transitioning to alternate host plants for the coming winter. This is why you see so many aphids on the wing, landing on your plate at the tailgate party, and otherwise providing a tiny but prolific nuisance to outdoor activities. The aphids thus need their own fuel, and it takes a ridiculous amount of plant sap to yield that result. Xylem and phloem are notorious for being nutrition-poor, so sap-sucking insects cycle those products rapidly through their digestive systems. Liquid honeydew comes out as fast as sap is going in.
Social wasps tend to dominate the scene at fall aphid colonies. Because the wasp colonies are winding down, if not finished altogether, their paper palaces have emptied totally and there are now vastly more individual wasps out in the field than there were earlier in the year when many workers were inside the nest feeding the larvae, building new cells in the comb, and engaging in other housekeeping chores. With no purpose left to serve except their own individual survival, worker yellowjackets might qualify as unstable or mentally-ill were they human beings.
Meanwhile, flies, the normal prey of many social wasps, are free of worry from the purposeless wasps and fearlessly rub shoulders (humeri?) with them at the aphid honeydew banquet. The aphids themselves are still vulnerable to flower flies (family Syrphidae) that lay their eggs in the colonies. The fly larvae that hatch eagerly feast on the aphids, along with lady beetle larvae and lacewing larvae.
Here in my Colorado Springs, Colorado neighborhood, ornamental conifers seem to be real aphid magnets. The trees are no doubt at least a little weakened by their circumstances of planting, isolated from other trees in soils that are not always compatible; and maybe (probably?) minus the symbiotic fungi they need to help them get their own complete nutritional requirements.
Scale insects, too, afflict these pines, firs, and spruces. Scale insects are relatives of aphids, but are even more sessile, often covered in a hard, waxy shell secreted by the insect. The "lump" is thus a living lid over the insect that created it. Like aphids, scales secrete honeydew as a waste product. If you are unaccustomed to recognizing scale insects, it is easy to be perplexed by the wasp and fly activity. Even butterflies and moths will be flitting around inexplicably.
So, the aphid colony is a bustling place at this time of year, the last epicenter of "bug" action before the leaves finally fall and the killing frosts finish off those insects still commanding our attention. They still survive, of course, hidden from view, often in life cycle stages we would not recognize as insects until spring returns them as such.
Enjoy this last hurrah of bugdom. You can easily approach the buzzing horde without fear, so intent are they on feeding. Worry not of stings, though be careful where you reach and step. This is no season to be barefoot to be sure.