The goldenrod is
full of tenants

The larva takes the apical bud and upward extension stops. Leaves continue to be produced, but with no stem to distribute them they stack into a dense rosette three to five centimetres across — a green flower made of leaves.4 Growth often resumes from a lateral bud, so a bunch gall is frequently found partway up a stem with a sharp kink beside it, not at the top.

It is bivoltine, and its two galls do not resemble each other. The spring generation, in April and May, produces an inconspicuous swelling with a single small chamber that is easy to walk past. The summer and autumn generation, in August and September, produces the conspicuous rosette, built of two to five chambered subunits clustered at the shoot apex.4

Where the insect spends the winter is not agreed. The podcast's own show notes describe autumn larvae burrowing into underground rhizomes to overwinter, with a second brood raising the visible bunch galls from mid-June.1 The taxonomic revision instead places the spring galls in April–May and the summer galls in August–September, without a rhizome stage.4 These two accounts cannot both be right, and this guide does not resolve them.

The rosette is dense, persistent and structurally complex, and it houses insects that could not otherwise live on that stem. More than one source calls the species an ecosystem engineer, in the sense a beaver is one.2

One further oddity: broods are reported to be single-sex — one female's offspring all male, another's all female. No adaptive explanation has been established.

Hosts hereS. altissima · S. canadensis · S. rugosa

Like Rhopalomyia solidaginis, this fly produces two structurally different galls in a season, and the two are catalogued separately because a person in a field would not connect them.2

Recorded from S. altissima, S. canadensis, S. rugosa, S. nemoralis and S. odora in this region.2 Beyond host records and a short note on its biology published in 1998, very little about this fly is easy to find.

With more than three thousand observations it is the fourth-commonest goldenrod gall on the continent, and almost nobody who knows the ball gall knows this one. That gap between what is abundant and what is famous runs through this whole subject.

Hosts hereS. altissima · S. canadensis · S. nemoralis · S. odora · S. rugosa

A large terminal leaf cluster, bivoltine like R. solidaginis, recorded here from S. canadensis, S. gigantea, S. rugosa and S. leavenworthii.4

Its larvae are heavily attacked by parasitoids — heavily enough that rearing the midge itself is difficult, because what emerges is usually something else.4 In many galls, parasitoids and inquilines are reared more often than the gall-maker.

Felt described a further midge, Rhopalomyia inquisitor, from a single male and a single female, and later speculated that it lived as an inquiline inside R. capitata galls. The 2009 revision looked and found no sign of it there, suggesting instead that it induces inconspicuous conical galls on leaves that Felt simply missed.4 Galls within galls do occur in this system.21

Hosts hereS. canadensis · S. gigantea · S. leavenworthii · S. rugosa

A female lays a single egg into the apical bud of a young shoot in mid-spring. The larva hatches, bores down into the stem, and the plant answers by building a sphere of its own tissue around it — roughly 20 mm across, thick-walled, with the larva alone at the centre in a single chamber.2

How the larva compels this has a partial answer. Cytokinins — plant hormones that drive cell division — occur at high concentration in the larva, and were shown to be synthesised by the insect rather than merely accumulated from plant tissue.9 Immunolocalisation later placed both cytokinin and auxin almost exclusively in the larval salivary glands.10 Whether that is the whole mechanism is not settled; a 2023 genomic study examined, and largely discounted, a contribution from the fly's Wolbachia endosymbiont.11

The gall is a battleground with an optimum in the middle. Small galls let the wasp Eurytoma gigantea reach the larva with its ovipositor. Large galls attract downy woodpeckers, which open them in winter. Across sixteen populations and four generations, the two pressures push in opposite directions and larvae in intermediate galls survive best.76

Then it freezes. The larva overwinters as a third instar and is freeze-tolerant: it permits roughly 65% of its body water to convert to extracellular ice, protected by glycerol, sorbitol and trehalose, and survives to about −35°C.1213 In spring it pupates, and the adult escapes by inflating a balloon-like bladder on its head — the ptilinum — to burst the thin cap of tissue the larva cut the previous autumn.

The plant pays for this. Galled ramets produce fewer flowers, and the seed they do set germinates less successfully; stem height and existing rhizomes are reduced. Notably, the number of new rhizomes produced is not.29 The damage is real and it is modest — which is roughly what you would expect of a parasite that has been on this plant a long time.

It lives about fifty weeks inside the gall and perhaps two outside it.

Hosts hereS. altissima · S. canadensis · S. gigantea · S. rugosa · S. ulmifolia

A tapered spindle in the stem, longer than it is wide, occupied by a single tortricid larva.15 It is easily confused with the elliptical gall of Gnorimoschema; the two are compared below.

The species is univoltine and the fifth instar overwinters inside the gall.16 Where Eurosta lets itself freeze and manages the ice, Epiblema refuses. Autumn cold-hardening suppresses its supercooling point to roughly −38°C and floods the body with glycerol — over 2 molar, as much as 18% of total body mass.1617

Cryoprotectant production is fast and temperature-cued: in the laboratory, synthesis begins within six hours of a shift from +16°C to −4°C, reaches half-maximal in thirty hours, and peaks after four days.17

Two galls, two solutions, one meadow. Freeze tolerance and freeze avoidance are not variations on a theme — they are opposite bets about the same January, and both work.

Hosts hereS. altissima · S. canadensis · S. gigantea · S. juncea · S. nemoralis · S. ulmifolia

A moth, not a fly. The caterpillar tunnels several inches downward from the shoot tip, which is why the resulting gall is elongate rather than round — the damage is a line, not a point.

The life cycle is where popular accounts diverge, and the hosts of the podcast that prompted this guide said so on air before landing on a two-winter version. The literature describes one year. Adults emerge in late summer, mate, and lay eggs in autumn on lower stems and dried leaf litter near the base of the plant. The egg overwinters. Larvae hatch in spring, climb, and bore into the new shoots.141

Before pupating, the larva cuts an exit passage to the surface — the only stage with chewing mouthparts — and plugs it with silk and plant material. Miller describes the finished plug, or bung, as honey-coloured at first and whitening with weathering, its outer surface flush with a ring of raised plant tissue.14 The opening is bevelled, widening outward, which is easy to push open from inside and awkward to dig into from outside.

Whether the larva leaves a thin membrane over the passage or plugs it completely is reported both ways in popular sources.1

Bung colour is a trait of the moth rather than the plant, and can be used to separate this species from its relatives.14

Hosts hereS. altissima · S. canadensis · S. gigantea · S. juncea · S. nemoralis · S. ulmifolia

Flat, roughly circular blisters five to fifteen millimetres across, raised slightly on both leaf surfaces, often ringed so that they read as small targets. Colour varies with the host — yellow, grey, buff, or purple with a pink centre and a white margin on S. canadensis; dark grey with a pale margin, blackening with age, on S. juncea. Strikingly different colours can appear on a single leaf.19

The female carries conidia of the fungus Botryosphaeria dothidea in mycangia — pockets on her ovipositor — and inoculates them as she lays. The fungal hyphae, not the plant, form the structure that encloses and feeds the larva; the fungus hardens and blackens as the larva matures and becomes incorporated into the leaf tissue.518

The association is obligate in both directions with respect to successful gall formation: without the fungus the midge cannot develop on goldenrod.18

It is multivoltine, and as the season advances galls appear on progressively higher leaves.520 It is also by far the least host-specific gall former on goldenrod — recorded from 41 goldenrod species in this region and 44 across the continent, against five for the ball gall at either scale.2

Records from many additional hosts exist on iNaturalist without a confirmed insect identification, and some may represent other Asteromyia species.2

Hosts hereS. altissima · S. arguta · S. auriculata · S. bicolor · S. buckleyi · S. caesia · S. canadensis · S. chapmanii · S. curtisii · S. delicatula · S. erecta · S. fistulosa · S. flexicaulis · S. gigantea · S. glutinosa · S. houghtonii · S. juncea · S. latissimifolia · S. leavenworthii · S. mexicana · S. missouriensis · S. mollis · S. multiradiata · S. nemoralis · S. odora · S. ohioensis · S. patula · S. petiolaris · S. pinetorum · S. puberula · S. radula · S. riddellii · S. rigida · S. rigidiuscula · S. roanensis · S. rugosa · S. sempervirens · S. speciosa · S. squarrosa · S. uliginosa · S. velutina