intimate relationships: a tale of moss, sumac, and a clever aphid’s gall
AH, THE RELATIONSHIPS! Alongside the mossiest patch of my lawn, a vigorous cutleaf staghorn sumac grows. I use that plant in several other spots in the garden—both the plain green Rhus typhina ‘Laciniata,’ and also the gold-leaf cultivar called ‘Tiger Eye.’
But this particular sumac, the one beside the mossy lawn, always gets colorful, pod-like galls in high summer, as if it were festooned with tiny bright balloons, mostly greenish but some red and appropriately heart-shaped (above). My other sumac plants, not so much.
And I never knew why.
I knew it was an aphid—specifically the native sumac gall aphid, Melaphis rhois—who created the gall, typically found on staghorn sumac or smooth sumac (Rhus glabra). My friend Charley Eisemann had told me that on a walk several falls ago. It doesn’t do any damage, though some leaflets may color up or fall off a little early. The insect is just making a nursery on its host plant to rear its offspring, a relationship that’s been going on like this for at least 48 million years.
What I didn’t know Until Joe Boggs of Ohio State wrote about it recently was the critical moss connection—a case of what’s called an alternate host (not just sumac, in summer, but moss in the offseason). His blog post got me reading all manner of geeky sumac gall literature.
Apparently females lay an egg on the underside of a sumac leaf, which causes a gall to start forming—creating a nursery, complete with food, for its offspring. That single aphid resulting from the deposited egg then produces asexually within the gall—by parthenogenesis—so that many clones of itself eventually also fill the gall.
If all goes to plan, the gall splits in later summer, when the released winged females drop onto or fly off to the nearby moss–the alternate host–and establish the next asexually reproducing colonies. In spring, aphids from the moss recolonize the sumac, and the cycles repeats.
Well, except at my place sometimes things get interrupted when someone (chipmunks? birds?–not sure as I haven’t caught anyone in the act) breaks open the galls and systematically has at the contents, the way I might eat a piece of fruit and discard the skin. Lots of empty gnawed-open galls on the ground beneath that one sumac at the moment here, along with ones that seem to be faring better, presumably filled with teenage aphids by now.