When one is a practicing naturalist, one must always be willing to say two things. One, “I don’t know.” And two, “Hm…I guess I was wrong.” Y’see, Mother Nature is always ready to send you down the wrong path by making some identifications tricky. And, let’s face it, we can’t all be experts at everything. In fact, as a friend of mine once put it, I don’t consider myself to be an expert at anything, for an “ex” is a has-been, and a “spurt” is a drip under pressure. So, I’m admitting here and now that I stumbled and fell on the ID of the makers of the cottonwood galls posted 24 October. In fact, thanks to a recent post I read elsewhere, these cottonwood galls are caused by the Poplar Vagabond Gall Aphid (Mordwilkoja vagabunda). So, let’s revisit this post and set the record straight.
The clue I should’ve seen right away is that the galls made by the eriophyid mite occur along the affected stem as well as at the tip of the branch; those created by the Poplar Vagabond Gall Aphid (henceforth referred to as the vagabond) are located only at the tip.
The vagabond actually uses multiple hosts over its lifetime. Its life begins as a black egg, which the female has laid either in old galls or in the crevices of the poplar’s bark. Apparently the females have a preference for trees that already have galls on them, a likely indication that these trees are good hosts.
Eggs remain in situ throughout the winter. When spring rolls around, they hatch and the tiny wee nymphs migrate to the tips of their twigs, where new growth is starting to emerge. Here they pierce the tender plant tissue and commence feeding, sucking out the plant’s juices as only aphids can. It is this feeding action that ultimately results in the creation of the distorted hollow “thing” that would’ve normally been new leaves. The nymphs move into this newly formed gall and take up residence while waiting to mature.
Maturation comes with summer, and the now fully-grown adult aphids leave their snug home for greener pastures. While confirmation is still in the wings, scientists think that these aphids possibly spend their summer feeding on the roots of certain grasses. Fast-forward to autumn, and the aphids fly back “home”, taking up residence once again within the hollow chambers of the gall.
Some mating must take place here, because soon wingless females are born inside the gall, and by early November the males appear. But perhaps these two generations are produced via parthenogenesis (females reproducing without the aide of males – it’s more common than you think). I haven’t found any data to confirm either mode of reproduction. Regardless, once we have these wingless females and the males, mating takes place (again?) and little black eggs are once more laid in old galls and the crevices of the tree’s bark. The cycle continues.
When the rumply galls are first formed, they are green, no doubt the result of their original goal in life of being leaves for the tree. By winter, however, they have turned brown and are quite hard. These galls persist on the trees for many years, becoming obvious to the eyes of the curious as autumn claims the tree’s leaves.
The next time you are walking near some cottonwoods, or quaking aspens, or some other member of the poplar family, keep your eyes peeled. You might even bring a gall or two inside for the winter and see if anything emerges come spring (I’d suggest keeping it in a jar, with a lid). Gall watching can be a fun and interesting experiment for the whole family.