Although we don’t think of ourselves as an insect ID blog, some of these requests give us some great opportunities to talk about stuff that’s happening seasonally. Fall is an important time for bugs, because they’re finishing up their lifecycles for the year and are preparing to hunker down for the winter.
We’ve gotten some folks asking about wispy flying insects that resemble flying cotton balls.
Your Name: Jim
Your Bug Question: The air here is filled with tiny white bugs. They look like tiny wisps of cotton. No bigger than a pin head. They are innumerable. What are they? Nashville Tn.
These are woolly apple aphids, which are pests of fruit trees. They typically feed on pome fruits, apples and pears, but also feed on the crabapples that people grow in their yards. Because they feed on common yard plants, everybody sees them about this time of year.
They look like cotton balls because they’re covered in a waxy coating which serves quite a few purposes. It’s waterproof, so it repels the liquid waste that can accumulate underneath aphid colonies…basically shielding the aphids on the lower branches from the poop which falls from aphids on the upper branches. It might also protect them from frost, keep parasitic fungus from contacting the cuticle and infecting the aphid, protect it from predators, or even act act as a kind of fur coat.
We’re not actually entirely sure what they use the wax for. It’s probably a multipurpose survival coat.
…but we do know why they’re flying around at this time of year.
Aphids are really interesting critters. During the year they reproduce without males, a process called parthenogenesis, and give birth to pregnant daughters. Each aphid is like a tiny Russian nesting doll, with the next generation already growing within the previous generation.
So there’s no need for males most of the year.
However, during the fall something interesting happens. An entire generation of aphids is born with wings, and takes flight. In addition to this, the aphids give birth to male aphids who then go on mating flights. This is the only time of year when male aphids appear.
Aphids spend most of the year on one host plant, and in the fall they disperse to another. On this second host, they’ll lay eggs which will last over the winter and hatch in the spring. They’ll hang out here for awhile, and then move back to the original host during the summer.
The woolly apple aphid lives on apple trees most of the year, but it’s migrating to Elm trees at this time of year. Those cottony little aphids flitting through the air right now could either be males or females, and they’ll lay eggs on Elm and die at the end of the season. Their sole purpose is to find an Elm tree and lay their eggs on it, they don’t even have mouthparts. During the spring, they’ll produce another generation who will again sprout wings and fly to apple trees to begin the cycle again.
The lifecycle is really complicated so I took the liberty of summarizing it in a diagram below, simplified from Sandanayaka & Bus, 2005.
These aphids will reproduce rapidly, and can reach numbers high enough to stunt the growth of trees. The nutrients they remove from trees are bad enough, but their poop also encourages the growth of sooty mildew which uses their poop for nutrients.
So they’re a pest of apples, and probably harass the crabapples growing in your yard as well. They’re harmless, and don’t bite people. Instead, it’s a really cool event that happens once every year.