Posted: 09/25/2022 17:39:21
Modified: 09/25/2022 17:38:33
A student put a leaf on my desk, pointed to several green bumps on its underside, and asked, “What are those green growths?” It was a moment of amazement for the professor, and in this case, I was indeed perplexed. The growths certainly appeared to be galls of some sort. But which ones?
Galls are abnormal growths on plants caused by irritation – often insect-related. A good first step in identifying a gall is to determine what plant it came from. Gallogenic insects are very faithful to their host plant species. Specialization makes sense because there is so much complex chemistry involved in defeating a particular host’s defenses and co-opting its growth mechanisms so that the plant develops the perfect structure to house and feed insect larvae. growing.
The leaf galls on my desk were about the size and shape of a pencil eraser. They protruded from the underside of a hackberry leaf and were distinctly green, in stark contrast to the browning autumn leaf.
These clues revealed that the growths were hackberry nipple galls (Pachypsylla celtidismamma), which form on the underside of developing hackberry leaves in the spring, when the nymphs of psyllid insects (also called lice jumping plants) release chemicals that redirect normal leaf growth to make a personal green miniature yurt. The interior of the tiny house provides all the nutritional needs of the single pale yellow nymph that grows inside. The nymphs grow throughout the summer, and in early September cut exit holes in their galls and drop to the ground.
After a final moult, the newest adults look for places to hibernate. The rough bark of the hackberry offers many nooks and crannies for small adults. Despite the abundance of seemingly perfect overwintering homes for these tiny insects, however, many still make their way into our homes. These very small cicada-like jumping insects are harmless to pets and humans, so their appearance indoors is a minor inconvenience likely to be experienced by anyone with hackberry trees growing near their home.
Hackberry trees provide many benefits to birds and other wildlife. Nutritious berries provide a balanced diet of protein, fats and carbohydrates. In turn, the seeds disperse far and wide in the feces of animals that eat them. Hackberry trees are also home to more than 30 species of butterfly larvae and moths, including the hackberry emperor, which is found only on hackberry trees.
In addition to the hackberry nipple gall makers, four other species of gall-forming psyllids specialize in hackberry trees. Three of these species emerge in early fall and are important food sources for migrating birds. The fourth species overwinters in its galls and emerges in early summer.
All of these various insects attracted to a tree species beg the question: how do trees survive the onslaught? Hackberry trees have co-evolved with a wide range of native invertebrates and seem well suited to meet the challenge. It is not uncommon for a few hackberry branches to be affected by galls while the rest of the tree remains largely unharmed, and there does not appear to be much, if any, damage to the overall health of the trees. .
Saint Michael’s College was fortunate to receive more than 30 Arbor Day Foundation hackberries this year. More than 20 students and staff spent a fall afternoon planting them in the college’s nature area. Maybe in about 30 years, a future student will trick another teacher with a leaf adorned with little green yurts!
Declan McCabe teaches biology at Saint Michael’s College. His first book, “Turning Stones: Exploring Life in Freshwater” will soon be published by McDonald and Woodward. Illustration by Adelaide Murphy Tyrol. The Outside Story is attributed to and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation: www.nhcf.org.