Stem Galls on Goldenrods (and friends): Gnorimoschema Moth Galls
These are my notes on gall-making Gnorimoschema moths in North American north of Mexico. Much of this is simply a summary from William E. Miller's A Comparative Taxonomic-Natural History Study of Eight Nearctic Gnorimoschema that Induce Stem Galls on Asteraceae, Including Descriptions of Three New Species (Lepidoptera: Gelechiidae) (2000). Miller studied these galls over the course of several decades. In his monograph, he emphasized the importance of physical characteristics of gall construction, particularly the architecture of their exit-holes, for taxonomic differentiation. He also described the biology of several species in detail, performed several interesting experiments in host-choice and gall construction, and directly observed larval behavior during gall exit construction. Unreferenced table entries are from Miller (2000). Name spellings and acceptance follow those in the checklist by Lee et al. (2009), with exceptions indicated.
Gnorimoschema galls versus other goldenrod stem galls
Gnorimoschema is one of several insect groups that can induce stem-swelling galls on goldenrods. Others include Eurosta solidaginis, a tephritid fly that induces the commonly-observed "ball gall". Its galls are nearly spherical, even during enlargement, unlike Gnorimoschema galls, which range in shape from vertically-symmetric ellipsoids to pyriform (exaggeratedly pear-shaped) swellings. Other groups include (1) gall-midges, which usually induce smaller galls, typically with knobby extrusions, usually not horizontally-symmetric, and (2) Epiblema moths, whose stem-boring larvae induce "galls" that are basically rudimentary swellings - usually with buckling, vertical scars on the outside. These occur high on the stem and are typically accompanied by much plant branching. In the instances where they are noticeably vertically asymmetric, the narrower section usually points down. In contrast, Gnorimoschema gall exteriors are similar in texture to the surrounding stem tissue, and if vertically asymmetric, the narrower section points up.
Biology
(At least for the first ten entries - mostly eastern taxa) Gnorimoschema larvae usually enter the host plant stems at the apex of an elongating shoot. They tunnel down a few centimeters, boring through pith, then reverse direction and initiate a gall a few cm above their lowest tunnel. Larval entry usually deforms the leaves at the terminal bud into characteristic shapes, so, at least for most species, the presence of a larva may be discerned before it begins to induce its gall. The larva pre-bores its (eventual) exit hole prior to pupating. It excavates a tunnel out of the gall cavity, usually somewhere in the upper third of the gall (but see G. gallaespeciosum) to the outside, but stops just before breaking the plant epidermis. Some exits remain capped with this thin remnant of stem epidermis and the larva's exit construction is complete. In other species, the larva additionally constructs a "bung" - a stopper-shaped object that seals the gall exit hole from the outside. After the bung is complete, the epidermal covering sloughs off, exposing the outer surface of the bung to the outside world. Its geometry means that it can be pushed out easily from within by the emerging adult moth, but cannot be pushed-in. Bungs were initially thought to be composed of aggregated silk, but in most species they are actually composed of combination of silk and a hardening larval exudate, perhaps ultimately derived from plant resins (Miller 2000).
Gnorimoschema gallaesolidaginis larvae are one of few lepidopterans that are able to feed on plants without inducing the host's jasmonate signaling response (Tooker et al. 2008). Fascinatingly, they are able to suppress this host defense system while simultaneously inducing the host to increase production of nutritive lipids, including the very lipids that are biosynthetic precursors of jasmonic acid (Tooker & De Moraes 2009). Wow! They still manage to get parasitized, though.
Gnorimoschema species and their galls
| moth | host | gall location | gall form | exit hole | notes |
|---|---|---|---|---|---|
| Gnorimoschema gallaesolidaginis | Solidago altissima | stem, about mid-height and lower | hard ellipsoid spindle | upper third flush white bung |
Host-mismatches on S. gigantea are suspected but are extremely rare (Nason et al. 2002; Miller 2000). Mismatches may be detectable by the nature of the exit hole: When third-instars were experimentally transferred from their original galls to empty galls on S. gigantea they still produced white bungs in their new home (Miller 2000). There are also doubtful records from S. missouriensis and S. nemoralis. Miller did not differentiate between S. canadensis and S. altissima; it is possible that this and other members of Solidago ser. Canadense may also serve as hosts. |
| Gnorimoschema jocelynae | Solidago gigantea, S. ulmifolia?, S. rugosa?? | stem, about mid-height and lower | hard ellipsoid spindle | upper third recessed brown bung |
Host-mismatches on S. altissima are suspected but are extremely rare (Nason et al. 2002; Miller 2000). Mismatches may be detectable by the nature of the exit hole: When third-instars were experimentally transferred from their original galls to empty galls on S. altissima they still produced dark bungs in their new home. S. ulmifolia may serve as an occasional host (Miller 2000). Miller almost never mentions the ubiquitous S. rugosa, which is similar to S. ulmifolia. I think it's likely that at least some of his records from S. ulmifolia actually refer to this host species. Nason et al. found strong evidence for host-associated genetic differentiation, but were somewhat equivocal as to whether the genetic distance was enough to qualify S. gigantea-raised moths as a distinct species from G. gallaesolidaginis, rather than a host-race or semispecies. The adults are nearly identical. Lee et al. (2009) recognize this species. |
| Gnorimoschema salinaris | Solidago sempervirens, S. juncea, , S. ulmifolia?, S. rugosa?? | lower stem, near ground | hard ellipsoid or pyriform spindle | upper third flush brown bung with interior white cushion (but see notes) |
Classically known from S. sempervirens only, Miller assigned galls and adults reared from galls on S. juncea to this species. The entry in this table reflects his observations. He also reports that it may occasionally form galls on S. ulmifolia. Miller almost never mentions the ubiquitous S. rugosa, which is similar to S. ulmifolia. I think it's likely that at least some of his records from S. ulmifolia actually refer to this host species. I do wonder whether his inland galls on S. juncea are truly conspecific with G. salinaris; one photo of a gall on S. sempervirens shows a light-capped exit hole rather than a brown-capped one (Patterson 1915). S. juncea and S. sempervirens were once thought to be closely related; they are now in different sections (Semple & Beck 2021). I think it is likely that members of Solidago sect. Maritimae other than S. sempervirens may also serve as hosts, particularly the barely-distinct S. mexicana. Miller performed gall-transplant experiments with S. juncea-raised larvae. When moved into emptied galls on S. altissima they still produced their characteristic dark bungs with white cushions, the bung exterior surface flush with the surrounding plant epidermis. |
| Gnorimoschema septentrionella | Symphyotrichum novi-belgii, S. pilosum, S. lanceolatum | stem, mid-height or sometimes quite high | hard ellipsoid or pyriform spindle | upper third no bung exit difficult to discern |
Fyles (1911) reared the type specimen from "Aster junceus Ait.", which POWO lists under S. novi-belgii, but this may also have referred to what is now S. boreale; I'm not sure. Miller reports it from many other asters, all of which are now in the genus Symphyotrichum. This includes S. praealtum, but this is probably a misidentification because this collection is from Upper Michigan, outside the range of S. praealtum. In my experience S. praealtum is very difficult to distinguish from the widespread and polymorphic S. lanceolatum. |
| Gnorimoschema gallaespeciosum | Solidago rigidiuscula | very low on stem | hard ellipsoid spindle | middle third no bung |
Miller described and reared this species from "Solidago speciosa" in Minnesota; based on his site description this was probably the newly-resegregated S. rigidiuscula, but it is conceivable that other Solidago species in subsection Erectae could also serve as hosts, including S. speciosa in the strict sense. |
| Gnorimoschema gibsoniella | Solidago rigida, Symphyotrichum pilosum | very low on stem | hard ellipsoid or pyriform spindle | upper third no bung |
The type specimen was reportedly reared from Solidago rigida in Manitoba, but Miller was unable to find any Gnorimoschema galls on that species, despite searching extensively near the type locality (and elsewhere in the upper Midwest). He did rear adults from galls on "Aster spp." (elsewhere he specifies mostly "Aster pilosus") that he identified as G. gibsoniella. He reports this gall from MD, IL, MN, and MI. Solidago rigida does not resemble Symphyotrichum pilosum. I find it hard to believe that anybody would mistake the two, even without flowers present. Maybe this species is a prairie-Astereae oligotroph? |
| Gnorimoschema busckiella | Symphyotrichum patens | lateral stems | spindle? |
no bung |
Known only from the type series - NJ. |
| Gnorimoschema subterraneum | Symphyotrichum ericoides, S. chilense | ground-level on stem | spindle? | Type from "Aster multiflorus" - this may actually refer to another Symphyotrichum species; Miller assigned it to S. ericoides. Powell & Povolný (2000) ascribed moth galls on S. chilense to this species in West. | |
| Gnorimoschema gallaeasterella | Solidago flexicaulis, S. caesia, S. uliginosa, Eurybia divaricata, Symphyotrichum cordifolium | near apex of stunted stems | hard ellipsoid spindle | conflicting descriptions | This one is a confusing mess. Kellicott's original description (1878) gave E. divaricata as the host, but the accompanying illustration looks more like Solidago flexicaulis, as pointed out by Brodie (1909a). Brodie found many galls similar to the illustration on S. flexicaulis, as well as some on the related S. caesia (1909b). He was able to rear adults only from S. flexicaulis, which he identified with Kellicott's description of adult G. gallaeasterella. He also reported that these galls were being harvested en masse as a quack medicine, branded "Fitt Apples", by an enterprising local crank. He gave a provisional name, ceasiella, for the S. caesia-derived galls in case those would prove to be the work of a distinct species. Busck determined that moths reared from both S. flexicaulis and S. caesia galls were in fact G. gallaeasterella (Cosens 1910). He also assigned somewhat different-appearing moths reared from Doellingeria umbellata to this species (1939, and in earlier works), in disagreement with their discoverer, Fyles (1890, 1911), who considered these a distinct species (see next). Busck still maintained that moths reared from Eurybia divaricata were also this species. Miller found a few galls on Symphyotrichum cordifolium, and used these, as well as older descriptions, as the basis for his description. He did not rear any moths of this species, however. He described them as lacking bungs, but earlier descriptions (on other hosts!) do specify a "plug" (color not given) capping the exit hole (Kellicott 1878). McDunnough (1959) compared moths from galls on stems of Doellingeria umbellata against those raised from galls stunting Solidago uliginosa nearby. He concluded that they were definitely distinct, with obviously different female genitalia. He confidently identified the S. uliginosa-derived moths as G. gallaeasterella, despite this being a new host, based on the stem-stunting position of the gall and apparently all aspects of the adult moth appearance. |
| Gnorimoschema gallaediplopappi | Doellingeria umbellata | mid to lower stem | hard ellipsoid spindle | upper third no bung |
See previous; while Lee et al. (2009) still reject this species, every author (other than Busck) who has published on it seems to have recognized it as distinct. Miller (2000) provisionally accepted it, having neither personally observed nor collected it. McDunnough (1959) reared it from Doellingeria and emphasized the adults' morphological distinction from G. gallaeasterella. He also reported galls on Solidago rugosa in Nova Scotia that produced an unidentified Gnorimoschema male very similar to this species. Table entries from description in Fyles (1911), not Miller as in other entries. |
| Gnorimoschema slabaughi | Grindelia squarrosa | main and lateral stems | hard ellipsoid spindle | upper third bung of coarse plant fragments and silk |
|
| Gnorimoschema baccharisella | Baccharis pilularis | twigs | spindle | open when mature (larvae exit gall to pupate) |
|
| Gnorimoschema grindeliae | Grindelia hirsutula | soft, nonpersistent swelling | open when mature (larvae exit gall to pupate) |
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| Gnorimoschema crypticum | Hazardia squarrosa, Isocoma menziesii | soft, nonpersistent swelling | open when mature (larvae exit gall to pupate) |
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| Gnorimoschema octomaculella | Acamptopappus sphaerocephalus, Chrysothamnus spp. | leaves | open when mature (larvae exit gall to pupate) |
||
| Gnorimoschema coquillettella | Acamptopappus sphaerocephalus, Ericameria arborescens, E. linearifolia | leaves | onion-shaped structures from sealed-together leaflets | (larvae exit gall to pupate) | |
| Gnorimoschema ericameriae | Ericameria ericoides | leaves | onion-shaped structures from sealed-together leaflets | (larvae exit gall to pupate) | |
| Gnorimoschema powelli | Baccharis sarothroides | stems | swellings |
References
- Brodie, W. Lepidopterous galls collected in the vicinity of Toronto — No. 2. The Canadian Entomologist 41, 73–76 (1909).
- Brodie, W. Galls found in the vicinity of Toronto — No. 3. The Canadian Entomologist 41, 157–160 (1909).
- Busck, A. Restriction of the genus Gelechia (Lepidoptera: Gelechiidae), with descriptions of new genera. Proceedings of the United States National Museum 86, 563–593 (1939).
- Cosens, A. Lepidopterous galls on species of Solidago. The Canadian Entomologist 42, 371–372 (1910).
- Fyles, T. W. Gelechia gallaediplopappi (n. sp.). The Canadian Entomologist 22, 248 (1890).
- Fyles, T. W. Gnorimoschema gallaediplopappi Fyles and Gnorimoschema gallaeasterella Kellicott. The Canadian Entomologist 43, 135–137 (1911).
- Kellicott, D. S. A new gall moth, and notes on larvae of other gall moths. The Canadian Entomologist 10, 201–205 (1878).
- Lee, S., Hodges, R. W. & Brown, R. L. Checklist of Gelechiidae (Lepidoptera) in America North of Mexico. Zootaxa 2231, 1–39 (2009).
- McDunnough, J. H. On some changes in nomenclature of microlepidoptera, with description of a new species. American Museum Novitates 1–9 (1959).
- Miller, W. E. A comparative taxonomic-natural history study of eight Nearctic species of Gnorimoschema that induce stem galls on Asteraceae, including descriptions of three new species (Lepidoptera: Gelechiidae). (Entomological Society of America, 2000).
- Nason, J. D., Heard, S. B. & Williams, F. R. Host‐associated genetic differentiation in the goldenrod elliptical-gall moth Gnorimoschema gallaesolidaginis (Lepidoptera; Gelechiidae). Evolution 56, 1475–1488 (2002).
- Patterson, J. T. Observations on the development of Copidosoma gelechiae. The Biological Bulletin 29, 334–372 (1915).
- Semple, J. C. & Beck, J. B. Revised infrageneric classification of Solidago (Asteraceae: Astereae). Phytoneuron 2021–10, 1–6 (2021).
- Tooker, J. F., Rohr, J. R., Abrahamson, W. G. & De Moraes, C. M. Gall insects can avoid and alter indirect plant defenses. New Phytologist 178, 657–671 (2008).
- Tooker, J. F. & De Moraes, C. M. A Gall-inducing caterpillar species increases essential fatty acid content of its host plant without concomitant increases in phytohormone levels. Molecular Plant-Microbe Interactions 22, 551–559 (2009).
