Willow pinecone gall: what extension services say

This page describes what the named university pages 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.

What it looks like

The University of Minnesota Extension garden diagnostic page lists the willow pine cone midge gall (Rhabdophaga strobiloides) as a light green, pine cone-shaped growth on the end of a twig where a bud is normally found. It says the gall grows to about one inch long, and that old galls turn light brown and stay on the tree through winter. Ohio State University Extension (Joe Boggs, October 15, 2019) says the structure closely resembles a pine cone, complete with false seed scales, and that it is only found at the tips of branches because it grows from a terminal bud. New Mexico State University Extension (Marisa Thompson, December 7, 2019) describes the same thing on coyote willow as small cones that are attached firmly, harder than the bagworm cases the writer had collected, with a small white larva about the size of a grain of rice in the middle.

What makes it

All the pages name a gall midge, a small two-winged fly in the family Cecidomyiidae. Ohio State says the gall houses, feeds and protects a single fly larva deep inside, in a long central chamber wrapped in layers packed with down-like fibre. It adds that the larva spends the winter fully grown, which is why the gall stays on the twig. A 1987 paper in the journal Oikos, as Ohio State reports it, found the larvae survive deep freezes by loading their bodies with glycerol. New Mexico State quotes a Colorado State University Extension insect and disease handbook: the larvae feed on the end growth, stop the stem from lengthening, and the bud stays bud-like but swells as the larva develops in a cavity in it. The larva pupates in early spring.

How the gall starts, as the sources tell it

Ohio State says the adults come out just before bud break, and that a female lays an egg into a terminal bud with her egg-laying organ and also puts chemicals into the wound. It says where the chemicals come from is not well documented and the process is poorly understood, but it is known that the chemicals turn plant genes on and off in the bud tissue so the growth follows a pattern that is specific to the gall maker. The gall first looks like a dense cluster of curved young leaves, then passes through a ball stage, then lengthens into the cone. The University of Colorado Boulder science magazine (Jeff Mitton, March 24, 2017) says females lay eggs on willow tips in April and that the larvae burrow into the twig and feed in the gall through winter before emerging as adults in the spring that follows.

Where it is found and on which willows

Ohio State says reports of the galls run from New England to California and north into Alaska, and that the literature lists a number of willow hosts, though the writer has only found them on black willow in Ohio. New Mexico State saw them on coyote willow lining a dry streambed near Abiquiu. The Colorado Boulder piece describes about 20 percent of the willows it saw in Rocky Mountain National Park carrying the galls, some shrubs with dozens and most with none, and says a single sandbar willow in central Washington State may carry up to 1,000 galls. Sandbar willow and coyote willow are the same species, Salix exigua, in the sources' names.

Whether the sources call it harmful

Ohio State says the galls cause no apparent harm to overall tree health, and the writer adds an opinion that they add ornamental value. The same page cites a 1984 paper in Ecological Entomology reporting that twigs with a gall at the tip become thicker than twigs without one, which the paper reads as the midge steering the host's resources to its young. New Mexico State quotes the state entomologist, Dr. Carol Sutherland, as advising, in reports on piñon and oak gall samples, that there is no need to control them because the gall makers are not harming the host. The Colorado piece notes that ants, aphids and midges tend to be found on the same willows, and reports a Western Washington University study in which galls were more common on plants that also had ants and aphids.

Look-alikes Minnesota lists on willow

On the same Minnesota diagnostic page, three other swellings are named: crown gall, a round to irregular woody growth at the soil line or on roots; willow witches' broom, a cluster of many small weak shoots from one point on a branch, linked to a phytoplasma; and poplar gall borer, a swollen twig where larvae feed under the bark, with branch tips that die or break above it. The wider group is on the tree galls page, and the broom is on the page about witches' brooms.