Although the beetles I photographed for my springtime Acmaeodera post are among the smaller buprestids occurring in Missouri, they are by no means the smallest. That honor belongs to the curious little genus Mastogenius, measuring only around 2 mm in length and, thus, looking for all intents and purposes like little black dots. Slightly larger, but still smaller than our smallest Acmaeodera, are members of the tribe Trachyini. Adults in this group exhibit a highly derived morphology compared to other groups of jewel beetles – flat, compact, and wedge-shaped rather than the elongate, cylindrical form more commonly associated with the family. This seems in part due to their unique larval habits – mining within the leaves of their host plants rather than boring through the wood. Three genera in this tribe occur in the U.S.¹, all of which are found in Missouri. These include: 1) Taphrocerus, which mine the leaves of sedges (family Cyperaceae); 2) Pachyschelus, which mine the leaves of herbaceous plants in several families – primarily Fabaceae; and 3) Brachys, which mine the leaves of hardwoods, chiefly oaks (Quercus). It was two species in this latter genus (out of three that occur in Missouri) that I encountered a couple weekends ago at Reifsnider State Forest in Warren County (noted for its high quality example of a mature white oak forest).
¹ A species in the Old World genus Trachys was introduced to North America from Europe and is established in New Jersey.
Brachys ovatus is the largest of the three species, usually measuring a little more than 5 mm in length. In addition to size, it can also be distinguished from Missouri’s two other species of Brachys by the dense row of long hairs occurring along the apex of the last abdominal sternum. For those of you who prefer not to have to look at the underside of its butt, the white-margined band of bronze pubescence before the apex of the elytra and longitudinal rows of bronze setae in the basal half of the elytra are usually sufficient for distinguishing this species. Brachys ovatus is a common associate of oaks throughout Missouri during spring – I have collected it on ten of Missouri’s 21 oak species, including both ‘white oaks’ and ‘red oaks’. Despite its common occurrence on oak and the frequent reference to it in the literature as a leaf-miner of oaks, few reliable rearing records exist to document the range of hosts it actually utilizes. There are older reports of this species mining the leaves of other hardwoods such as beech (Fagus), elm (Ulmus), hickory (Carya), and hornbeam (Carpinus); however, the veracity of these reports is questionable, and they may refer only to incidental adult associations.
Brachys aerosus is another commonly encountered species. This is a highly variable and hard-to-define species, but in general it can be recognized by the basal region of the elytra largely lacking pubesence and with a purple, blue, or green luster, and by the predominantly gold to bronze pubescence covering the apical area of the elytra. Adult length is generally from 3 to 5 mm – somewhat smaller than B. ovatus, and differing also in that it is commonly associated with a variety of hardwoods besides oak. In Missouri, I have primarily collected it on oaks and elms. Literature reports – mostly old and unreliable – record as larval hosts many other hardwood genera such as chesnut (Castanea), beech, hazel (Corylus), hickory, hornbeam, linden (Tilia), poplar (Populus), and even such unlikely genera as huckleberry (Vaccinium) and grape (Vitis). Because of its variability and the broad diversity of hosts with which it has been associated, this species is suspected of acutally being a species complex. The late George Vogt spent many years making careful observations with reared material in an effort to determine species boundaries and their host associations. Unfortunately, Vogt passed away before publishing his observations, and his eccentric record keeping with cryptic notes (Anderson et al. 1991) makes it unlikely that they ever will be published. It will take some enthusiastic sole to repeat his work and publish it before we can ever know the true identity of the species hiding under this name.
A third species in the genus, Brachys aeruginosus, is smaller than either of the two above species – generally measuring only 3 to 4 mm in length. This rather uncommonly encountered species is most similar to B. aerosus in appearance but can be distinguished, in addition to its generally smaller size, by the predominantly light gold to silver setae that cover the apical area of the elytra. As with the two above species, it is most often associated with oaks but is occasionally collected on other hardwoods as well. Whether it utilizes species beside oak for larval development is unknown. I hope to find and photograph this species in the near future.
Anderson, D., C. L. Bellamy, H. A. Howden, and C. Quimby. 1991. George Britton Vogt (1920–1990). The Coleopterists Bulletin 45(1):93–95.
Copyright © Ted C. MacRae 2009
2 thoughts on “Brachys on oak”
Your mention of Pachyschelus has me hoping you have information on a species I have observed as an adult in northeast Illinois. Early in the season I have found adult Pachyschelus purpureus feeding on leaves of wild geranium (Geranium maculatum). Late in the season I have found them feeding on bitternut hickory leaves (Carya cordiformis). It’s really odd to have an insect feeding both on an herbaceous and a woody plant, and to be so specific (at least locally) on both. The gap between these observations has me thinking the species is bivoltine (unless, of course, we’re talking separate sibling species). It makes sense, given their small size and flattened shape, that the larvae are leaf miners. Do you have information to enlighten me on any of this? I’m especially interested in any larval host plants. Thanks!
Hi Carl. Pachyschelus purpureus is restricted to wild geranium and is active only during early spring. No species of Pachyschelus have been recorded utilizing hickory (or any other deciduous tree for that matter). The only other species in the genus known from Illinois is P. laevigatus, which is black rather than blue and lacks any pubescent spotting; however, it is restricted to Lespedeza. Another species that does resemble P. purpureus somewhat is P. nicolayi. That species is blue like P. purpureus but is a little smaller and lacks the pubescent spotting – it breeds in Apios americana and Wisteria frutescens and is active later in the season than P. purpureus. This species has not yet been recorded from Illinois but may occur there. Perhaps you are observing this species on wisteria vines that have colonized a hickory tree, thus intermixing their foliage. I would be most interested in seeing some specimens to confirm their identity for this possible new record.