Twisted gnarly leaves on my pretty Nyssa are the handiwork of a tiny gall-making insect.
The range of color in Nyssa’s autumn display is hard to beat.
Some call it tupelo, others call it black or sour gum, but I call it drop-dead gorgeous, Nyssa sylvatica. Of all the trees in the forest, I think this one takes home first prize in autumn with shades of scarlet, orange, yellow, and green swirled together in a raucous mix. A common native of eastern North American forests, this beauty is almost pest free and thrives in managed landscapes.
One summer’s day while enjoying my Nyssa’s deep glossy leaves, I was miffed by several gnarly leaves at the tips of branches. Along the margins of said leaves were numerous yellowish-white crescent shaped galls. Galls are abnormal plant growths often associated with an insect, mite, nematode, or microbial pathogen. To get a closer look at this aberration, I plucked a few leaves and dissected the galls under the lens of a powerful microscope. The galls were hollow pockets packed with hordes of tiny yellow sucking insects known as phylloxerids, close kin to woolly aphids we met on beech trees and alder branches. These suckers are tiny, about a millimeter in length. Within each gall several of these gals were surrounded by dozens of pill-shaped translucent eggs, offspring produced asexually, without contributions from males.
Early in the growing season, phylloxerids induce small crescent-shaped galls along the margins of leaves.
How do they make galls? Well, galls form when an invading biotic agent, in this case a tiny insect, takes control of the genetic machinery of the undifferentiated cells in the developing leaf. Compounds released by the insect as it feeds tell the cells of the leaf something like this, “don’t expand to form a normal flat leaf, curl over along the margin and form a nice hollow pocket, a home where I can lay eggs and my children and I can dine unseen and unmolested by hungry predators.” How clever is that! Pretty clever indeed, but one small problem exists. During autumn when my Nyssa drops its gnarly leaves, surely these leaf-bound gall-dwellers do not fare well in the soil. Where do the tiny phylloxerids go?
Early in the season, marginal galls created by the phylloxerid twist and distort young leaves at the tips of Nyssa’s branches. Within each gall are female phylloxerids laying dozens of pill-shaped eggs. Phylloxerids are strange looking creatures. Between the first set of legs in the center of the body are sucking mouthparts used to initiate gall formation and to remove sap from the leaf’s cells.
Up in the treetop, flocculant tufts of wax mark the location of tiny insects. Are these overwintering phylloxerids that will lay claim to my Nyssa next spring?
In a remarkable treatise on all things phylloxerid published some 116 years ago, Theo Pergande of Cornell University described the overwintering stages of the phylloxerid. He observed tiny wax covered phylloxerids in protected locations on the bark of the Nyssa. This week I scaled my Nyssa and several meters up in craggy folds along the trunk and in rough patches of bark were small white tufts of wax housing tiny sucking insects, presumably the overwintering generation of phylloxerids. While the exact molecular mechanisms by which gall-makers control their plant-hosts remain largely unknown, hundreds of species of insects and mites have discovered astonishing ways to alter the plants they live on and in during millions of years of intimate association.
Acknowledgements
We thank Drs. Fredericka Hamilton and Gary Miller of USDA for help in identifying the tiny creature featured in this episode. “North American Phylloxerinae Affecting Hicoria (Carya) and Other Trees” by Theo Pergande, and the amazing ‘Aphids on Worlds Plants’ website were used as references for this episode.