Journal archives for August 2022

August 11, 2022

Goldenrod Leaf Bunch Galls

Here is a pictorial identification guide with some quick distinguishing characteristics.

They're not all caused by Rhopalomyia soldiaginis, despite what the iNaturalist computer vision says. Galls early in development can be tough to identify from photos with confidence.

"Leaf bunch" here refers to galls composed of leaves clustered together on a goldenrod shoot. Because the modified structure is usually derived from a bud it also makes sense to call these "bud galls". Please see my catalog of gall-formers on goldenrods for references, more information, and other types of galls on goldenrods.

This covers most of eastern North America north of Mexico, with the caveat that most of the literature on these organisms is focused on the Mid-Atlantic USA. Southeastern states, for example, probably have other, undescribed species. Regional biases could also affect the host-mapping given here. For example, Eurosta soldiaginis, the goldenrod stem ball-gall fly, jumps hosts across its large geographic range; maybe some of these flies do that, too.

On Solidago species (true goldenrods)

Rhopalomyia solidaginis

A gall-midge. Induces tufted mopheads of leaves at the shoot apices of several goldenrod species, including the most common species in eastern North America - Solidago altissima. S. altissima is often the dominant goldenrod species in old-fields, meadows, etc. If you're like me and you grew up in the eastern US, it's the one with the yellow pyramid of flowers that you picture when you hear the word "goldenrod". This midge also hits Solidago canadensis and Solidago rugosa, which are common species in the same environments as S. altissima. All three host species have hairy stems - but check this at mid-plant height, not among the flowering branches - and by summer they have dropped their basal leaves. The gall's leaves are messy at first glance, but actually organized into one or a few distinct rosettes, each rosette with leaves getting thinner toward the center. Hidden from view, in the center of each rosette, perched above the stem, not hollowed out within it, is a tiny insect chamber. These chambers take the form of soft-walled, white, translucent cones.
R. solidaginis multiple rosettes example R. solidaginis vertical section R. solidaginis single rosette example

Rhopalomyia capitata

Another gall midge that makes mopheads at the top of goldenrod shoots. Similar to R. solidaginis, but on a different host - Solidago gigantea. Like the hosts of R. solidaginis, this host is another tall goldenrod species that produces pyramids of yellow flowers. It generally prefers wetter places than the hosts of R. solidaginis, but it can also be found growing next to them. S. gigantea, unlike the hosts of R. solidaginis, has smooth stems at mid-stem height. It also has distinctly fatter flower-heads, overstuffed with disc florets. You can usually find at least some flower heads with over ten disc florets - this is very rare in the other Solidago hosts, which typically max out around six. The gall itself has leaves that are typically not organized into discernible rosettes, instead resembling a putting green of leaves of even, short length. These are surrounded by a few wide leaves that sheath the overall structure. There can be many insect chambers distributed haphazardly among the short leaves. In cases where there are fewer insect chambers than is typical, the gall can look less like a putting green and more like a disorganized mess. The insect chambers are soft-walled, white, translucent cones, very similar to those of R. solidaginis. As with R. solidaginis, they are not visible unless you peel off the surrounding leaves.
R. capitata typical gall R. capitata peeled R. capitata versus R. solidaginis

Procecidochares atra

Unlike the others on this list, these galls are not the work of a gall-midge, but rather a Tephritid fly (one of two groups of flies known as "fruit flies"). This fly modifies the growth of either an apical bud (that is, at the tip of the main goldenrod shoot), a cluster of side-buds off the main stem, or both bud types. Each developing fly is contained by a solid mass of highly modified leaves. The bases of the modified leaves form a thick-walled cup- or vase-like structure around a hollow central cavity. This cavity is the insect chamber. It is usually much larger than the inhabitant, a stout larva or an ellipsoid pupa. The leaves that form the cup eventually separate at their tips, but initially appear shingled together like an artichoke or a Brussels sprout. As the gall grows, the leaf tips grow also, eventually recurving and splaying outwards, giving the appearance of a rosette. At this stage, especially on a terminal bud, the galls can have a superficial resemblance to Rhopalomyia solidaginis galls. However, P. atra galls have leaves that do not get very thin towards the center, and there is usually a semi-open cavity visible at the center. When multiple rosettes are present, only one (if any) is ever attached to the very apex of the stem, pointing upwards. The others are attached below, and point outwards. When a terminal bud gall is present, it is often much larger than the accompanying side-bud rosettes. The host range of this species is not well-characterized, but it appears to include goldenrods along with other aster family members. There may be multiple cryptic species within "P. atra".
P. atra axial bud galls P. atra apical bud gall with axial galls clustered below
P. atra apical bud gall vertical section P. atra lone apical bud gall P. atra developing axial bud galls P. atra galls on Solidago bicolor

Asphondylia monacha

This midge forms "Death Star" spherical complexes made of several mini-rosettes of small leaves. Galled plants have the overall appearance of a shaggy lollipop. It primarily hits Solidago juncea - "early goldenrod" - a host species whose largest leaves are the lowest ones and whose stems are strictly hairless (sometimes there are a few hairs among the flowering branches, but often these are smooth as well). It has a preference for drier or rockier sites than the hosts above, but can also be found among them. At the center of each mini-rosette within the gall is an insect chamber that is a stiff, hard-walled, greenish cone made of about two modified leaves, stuck together as if by dried glue. They are not always visible without peeling away the surrounding leaves. The insect chambers have a white fungus lining the walls. Occasionally, the mini-rosettes are not organized into a spherical complex, but instead are distributed along the stem, clustered where stem-leaves attach, or dispersed among the flowering branches. A. monacha can hit a number of other host species, including Solidago erecta and Solidago uliginosa, and probably others.
A. monacha typical gall cluster A. monacha mini-rosette and insect chamber A. monacha dispersed rosettes

Asphondylia solidaginis

This midge makes two types of galls. The first are "leaf-snaps" on Solidago altissima and other goldenrods with hairy stems and narrow "triple-veined" leaves. Leaf-snaps are leaf blisters that connect two leaf blades together, forming a round, hard compartment that contains the insect. These are conspicuous and are often posted to iNaturalist. Bud galls, the second type produced by A. solidaginis, are less frequently noticed. These galls are much smaller than those made by R. solidaginis. They always occur as a single rosette, with leaves that get shorter, but not much narrower, toward the center. As with several other Asphondylia bud galls, the insect chamber is a stout, stiff, hard-walled, greenish cone made of about two modified leaves, stuck together as if by dried glue. The inner walls are lined by white fungus tissue. The insect chamber is prominently visible in the center of each rosette without peeling away the surrounding leaves.
A. solidaginis typical bud gall among flowering buds A. solidaginis typical apical bud gall A. solidaginis leaf-snap galls and apical bud galls

Asphondylia rosulata

Similar to A. solidaginis in that this species also makes both leaf-snaps and bud galls, but on a different host, Solidago rugosa, which has very hairy stems and net-veined leaves, the veins of which often appear sunken compared to the surrounding leaf tissue. Unlike the hosts of A. solidaginis, you cannot make out three unambiguous "main veins"; instead the veins branch away from the main vein in the familiar way of most leaves, with successive side-veins getting progressively smaller towards the leaf apex. Insect chambers as in A. solidaginis. There tends to be less of a distinction between leaf-snap galls and bud galls in this species; some leaf-snap galls appear to arrest shoot growth (see third photo) in the same manner as an apical bud gall.
A. rosulata apical bud gall with few surrounding leaves A. rosulata apical bud gall with a rosette of surrounding leaves A. rosulata leaf-snap gall terminating a shoot

Asphondylia silva

Similar to A. solidaginis bud galls, but hits the distinctive host, Solidago caesia. This host is a woodland goldenrod species with distinctive arching stems. Its flowers are distributed along the stem at points of leaf attachment, rather than in a pyramidal cluster at the end of the stem. The gall is not conspicuous, and sometimes appears only as a bare insect chamber accompanied by a few shortened leaves. I have found some apical bud galls on S. caesia in northern Michigan that have a more elaborate rosette of surrounding leaves. Otherwise similar to the bud galls of A. solidaginis.
A. silva apical bud gall from the Type specimen locale A. silva-like apical bud gall with a rosette of surrounding leaves A. silva-like apical bud gall lacking surrounding rosette

Asphondylia pumila

*Similar to *A. monacha, but on Solidago patula **- an angular-stemmed host with wide, sharkskin-rough leaves that likes wet soil.

Unnamed Asphondylia species

Other Asphondylia-like galls have been reported from Solidago sempervirens, Solidago odora, Solidago nemoralis, Solidago rigida, and probably several others. Some may be caused by A. monacha, others may be the result of distinct species. All have fungus-lined inner insect chambers at the center of rosettes of modified leaves that do not become linear-thin towards the center. Charley Eiseman has a fascinating entry on the S. sempervirens-galling midges here.

Rhopalomyia hirtipes

Narrow leaves surround a prominent, potato-like structure. Usually low to the ground. Found on Solidago juncea, but the host is so drastically modified that it's often tough to identify. The "potato" splits open with four slits at gall maturity, exposing a mass of spongy tissue within from which the flies issue. Included here for completeness, given the cluster of surrounding leaves, but the central globular mass makes these unlikely to be confused with other galls.

Dasineura folliculi

This gall-midge makes clusters of deformed leaves at the shoot apex, each organized into a single rosette. Often, several side-shoots are also galled, perhaps by successive generations of midges. The leaves comprising the gall are wrinkled and blistered. Those toward the center are held tightly together, at least at their bases. There are no insect chambers per se - the larvae instead hide among the wrinkles and crinkles and move about freely, feeding at various spots on and around the gall. These feeding spots appear as pale, circular deformations on nearby leaves. The interior of the rosette contains the stunted bud, often rotting away. Sometimes, however, the bud recovers and continues to grow beyond the gall. The Dasineura larvae that induce the gall are orange. Sometimes there are other Cecidomyiid larval residents. Unlike all the above species, the larvae do not pupate in the gall, instead traveling to the soil to do so. Their pupae will not be found among the gall leaves. Published hosts include the familiar species Solidago rugosa and Solidago gigantea, both with flowers arranged in loose pyramids and basal leaves withered by flowering-time. I have personally found D. folliculi-like galls on S. canadensis and S. caesia; there may be other hosts. Not often found, in my experience, on the ubiquitous Solidago altissima.
D. folliculi gall on S. gigantea with many feeding spots on surrounding leaves D. folliculi gall developing on S. gigantea D. folliculi gall on Solidago rugosa

Various Leaf-Tying Arthropods

The larvae of several moths tie and roll goldenrod leaves together, sometimes very conspicuously, involving many leaves. These structures are not galls, however, because the plant does not participate in their formation. One common genus that does this is Dichomeris, but there are several others. Other arthropods may also form similar structures.
caterpillar leaf-tie on Solidago

On Euthamia ("grass-leaved goldenrods", or "goldentops")

These plants are not particularly close relatives to true goldenrods in the genus Solidago, but they are included here because they were historically placed in Solidago, and often inhabit similar sites. Distinguish them from Solidago by their grass-like, linearly-thin leaves with three to five strictly parallel main veins. Their flowers are organized into flattish structures, all held at about the same level, not in pyramids, fountains, or wands. No basal leaves remain at flowering time, unlike in the Solidago species with flat-topped arrangements of flowers.

Rhopalomyia lobata

These rosette galls have white material in the center that eventually opens to reveal clusters of spongy hairs from which flies emerge. Surrounding leaves have very wide bases that narrow abruptly to resume their normal, grass-like shape. There are often irregular bulges and blisters.
R. lobata gall, opening to reveal spongy white internal tissue R. lobata gall, developing with abruptly narrowing leaves R. lobata gall next to A. pseudorosa gall

Asphondylia pseudorosa

Rosette bud galls, often on side-shoots. Leaves are wider than normal at the base, but taper gradually towards the tip. At the center of the gall is a single greenish, conical insect chamber, typical of the other Asphondylia chambers described above. Early in development the galls do resemble little, green roses. Remnants of old galls dot host plants at the end of the growing season, their wide leaves persisting at nodes created by the growth of axial shoots.
A. pseudorosa gall with typical rosebud form A. pseudorosa bud gall terminating a side-shoot A. pseudorosa gall remnant

Dasineura carbonaria

This gall-midge makes clusters of deformed leaves at the shoot apex, each organized into a single, appressed cluster. The leaves comprising the gall are wrinkled and blistered, often with starkly discolored, purple blisters. There are no insect chambers per se - the larvae instead hide among the wrinkles and crinkles and move about freely, feeding at various spots on and around the gall. The Dasineura larvae that induce the gall are orange. Sometimes there are other Cecidomyiid larval residents.

all images by Daniel McClosky, CC-BY 4.0 (please use with attribution)
suggestions, corrections, comments welcome
Posted on August 11, 2022 06:55 PM by ddennism ddennism | 6 comments | Leave a comment

August 13, 2022