Hackberry nipple gall: what extension services say

This page describes what the named sources say. It does not identify a gall on any particular tree, say whether a tree is healthy, or tell anyone to treat or leave it. Pesticide advice in a source is mentioned only in general terms, and the guide on who may apply pesticides covers licensing.

What it looks like

Texas A&M AgriLife Extension's field guide describes the galls as swellings of leaf tissue, 1/8 to 1/4 inch across, on leaves or leaf stems, and says they can be cut open to show a pale, developing psyllid inside. Nebraska Extension in Lancaster County (Sarah Browning, May 14, 2024) says the gall sits on the underside of the leaf and looks like a small round green pouch. Purdue Extension (Hot News HN-86, March 17, 2014, Timothy Gibb) calls them small, discolored nodes on the undersides of hackberry leaves.

What causes it

All the sources name a psyllid, a tiny jumping plant louse, in the genus Pachypsylla. The Illinois Extension piece (Ryan Pankau, August 2, 2019) names the hackberry psyllid, Pachypsylla celtidismamma, on common hackberry, and says each spring the adults lay eggs on the emerging leaves. After hatching, the young feed on leaf tissue, and the leaf grows abnormal tissue around them, which makes the gall. The psyllid then feeds on sap from inside it for the rest of the summer.

Texas A&M gives the timing: the adults overwinter in bark cracks and crevices, mate in spring, and lay eggs on the underside of expanding leaves. The nymphs hatch and start feeding, which makes the leaf tissue expand into a pouch around them, and the adults come out in the fall, in September in that source. There is one generation a year.

The Missouri Department of Conservation field guide adds that the name covers more than one insect. It says a commonly met group that forms galls on hackberry leaves may include four to seven species long treated as one, with bodies that look alike but galls of different shapes: rounded cones or disks, indented mushroom caps, pudgy doughnuts, or knobby nipples that may be hairy or smooth. It gives estimates of 7 to 13 hackberry psyllid species north of Mexico, says one type makes inflated, blister-like pouches in the leaf, and says the timing of egg laying matters because leaves are suitable only for a brief stage in spring, and if the eggs are laid at the wrong stage the galls may not form properly.

Whether the sources call it harmful

Source	What it says about the tree
Texas A&M AgriLife Extension	Probably no hackberry is free of the gall-forming psyllids. The galls are unsightly and occasionally cause early leaf drop, but do not appear to harm the health of the trees. Management: none, not considered a major pest.
Illinois Extension	The galls and the insects in them have a negligible impact on tree health, and the piece says there is no need for any control of these native insects.
Purdue Extension	Not a serious pest of hackberry, because the galls do very little damage to the leaf. Chemical treatments are seldom warranted.
Missouri Department of Conservation	The galls look odd on leaves and twigs but usually do not hurt the tree.
Nebraska Extension	Names the gall as one of the common galls, and says leaf galls seldom, if ever, cause permanent injury and that control is not recommended or necessary.

The adults in fall

Several sources spend as much time on the adults as on the gall. Missouri and Purdue say the adults, which look like tiny cicadas, gather on window screens in late summer or fall. Purdue says they are small enough to pass through the mesh of many screens and fly to windows again when temperatures warm in late winter or early spring. Texas A&M calls them a nuisance around homes but medically harmless, and Missouri says they are completely harmless to people. Purdue says that to keep them out, closing windows during the short period when they are active, or fine-mesh screens, are the options, and that chemical treatments are seldom warranted. Illinois says any that get indoors die, since a house is not a suitable place to overwinter.

Look-alikes the sources name

Texas A&M says hackberry also carries gall-forming midges, such as the maker of the thorn gall, and that their young are legless maggots or grubs, unlike the psyllid nymphs. It names the hackberry blister gall maker and the hackberry bud gall maker as other Pachypsylla species. Virginia Cooperative Extension (publication ENTO-146NP, June 2, 2022, Eric Day and Theresa Dellinger) lists the hackberry button gall, flask gall, nipple gall, star gall and melon gall as galls made by Pachypsylla species on hackberry. Nebraska names a different growth on hackberry, witches' broom galls caused by an eriophyid mite. Insect and mite galls on other trees are on the tree galls page, and the witches' broom page covers that growth form.