Beetles are often pretty good botanists, and when it comes beetle botanists there are none finer than species in the family Chrysomelidae. Members of this family are commonly called “leaf beetles” because… well, they are usually found on leaves, and with nearly 40,000 known species (and probably many more still unknown) it is one of the largest animal families on the planet! In fact, LeConte & Horn (1883)—the fathers of coleopterology in the United States—surmised that the function of leaf beetles “is to hold the vegetable world in check by destroying … the leaves”!
Here in Missouri we have 351 species and subspecies of leaf beetles (Riley & Enns 1979, 1982), the vast majority of which specialize on a limited range of host plants. Most restrict themselves to feeding on plants within the same family, and some to just a single plant genus or even species! Such specialization does not necessarily make a species rare—western corn rootworm (Diabrotica virgifera) develops almost exclusively on corn (Zea mays), yet it is one of the most abundant leaf beetles in the state, and among non-pest species the dogbane leaf beetle feeds almost exclusively on common dogbane (Apocynum cannabinum) yet is one of our most commonly encountered leaf beetles. There are, however, many other species of leaf beetles in the state that are rarely seen. Almost always they are also extreme host specialists, and there is no better example of this than species in the genus Phyllobrotica.
Eighteen species and one subspecies of Phyllobrotica are known to occur in North America (Riley et al. 2005, Gilbert 2009), of which five have been recorded in Missouri (Riley 1979, Riley & Enns 1979). With one glaring exception (see below), all of the species for which host plants are known restrict their feeding to one of two closely related genera in the mint family (Lamiaceae)—Stachys for most of the western U.S. species, and Scutellaria for most of the non-western U.S. species (Farrell & Mitter 1990). Of the five species in Missouri, only P. limbata has been encountered with any regularity; Riley & Enns (1979) recorded 61 specimens from six widely scattered counties in Missouri, many of them observed on S. ovata or S. lateriflora. A second Missouri species, P. physostegiae, now also is encountered reliably in southwestern Missouri; however, it wasn’t even described until 1979 due to earlier confusion with the enigmatic P. antennata (apparently still known only from the type collected in Tennessee) (Riley 1979). Prior to this, only a handful of specimens were known, three of which had been more recently collected by Rev James Sullivan of St. Louis on plants in the genus Physostegia (also in the mint family). Followup collections turned up large series of beetles on this plant at several locations in southwestern Missouri, and the species was formally described (Riley 1979). Farrell & Mitter (1990) suggest the unusual host is an example of isolated host transfer due to the unusual natural history of P. physostegiae, which along with its sister species is unique in the genus in that it inhabits dry prairie habitats rather than wet bottomlands. Species of Scutellaria inhabiting dry prairies are often annual and more unpredictably available than those inhabiting more mesic habitats, which could have favored broadened host range or shift by the ancestral P. physostegiae population to a related, chemically similar perennial host such as Physostegia (insects typically use volatile plant chemicals, in addition to vision, as informational cues for recognizing their host plants—Visser 1986).
The three remaining species of Phyllobrotica in Missouri—P. circumdata, P. lengi, and P. nigritarsis—all continue to be among the rarest beetles in the state. The first was never even collected in Missouri until Rev. Sullivan collected 8 specimens—all on S. incana—in a few eastern counties in Missouri in the late 1970s (recorded as “P. discoidea” in Riley & Enns 1979). As far as I can tell, no online images of this species exist, despite it being the most widely distributed species of the genus in North America (Farrell & Mitter 1990, Riley et al. 2003). The second species, P. lengi, was known from Missouri by just four specimens collected in the late 1800s (Riley & Enns 1979) until Rev. Sullivan collected a small series on S. parvula in east-central Missouri in 1988. Like P. circumdata, apparently no online image of this species exists as well—until now… the image above taken of one of the specimens in that small series, which Rev. Sullivan graciously gifted to me shortly after collecting them. The third species, P. nigritarsis, likewise was also known from Missouri by only four specimens—also collected in the late 1800s (Riley & Enns 1979)—until Rev. Sullivan collected a small series in association with S. parvula in east-central Missouri in 1987. Unlike the previous species, however, a single online image does already exist for this species at BugGuide, and the image below—again taken from a specimen in the small series kindly gifted to me by Rev. Sullivan—adds a second.
[Incidentally, both of these photos were taken for a new book by Rev. Sullivan that has just been published—more on that soon!]
Are there additional species of Phyllobrotica in Missouri? Possibly! Phyllobrotica decorata has a known distribution almost as broad as P. circumdata, including several states surrounding Missouri (Illinois, Iowa, and Kansas), and two other eastern U.S. species—P. stenidea and P. vittata—have been collected as far west as Indiana (Riley et al. 2003). There is also the enigmatic P. antennata from Tennessee. Targeting plants in the genus Scutellaria wherever they may be found growing will likely turn up these species, if they occur here, or at least provide additional records for the other species already known from Missouri.
Farrell B. D. & C. Mitter. 1990. Phylogenesis of insect-plant interactions: Have Phyllobrotica leaf beetles (Chrysomelidae) and the Lamiales diversified in parallel? Evolution 44(6):1389–1403 [preview].
Gilbert , A. J. 2009. A new species of Phyllobrotica Chevrolat, 1836 (Coleoptera: Chrysomelidae) from California, USA, with notes on the western United States species. The Pan-Pacific Entomologist 84(4) :269–279 [abstract].
LeConte, J. L. & G. H. Horn. 1883. Classification of the Coleoptera of North America. Smithsonian Miscellaneous Collections 26(507):1–567.
Riley, E. G. 1979. A new species of Phyllobrotica Chevrolat (Coleoptera: Chrysomelidae) from the prairies of southwestern Missouri. The Coleopterists Bulletin 33(3):331–335.
Riley, E. G., S. M. Clark & T. N. Seeno. 2003. Catalogue of Leaf beetles of America North of Mexico. The Coleopterists Society Special Publication No. 1, 290 pp.
Riley, E. G. & W. R. Enns. 1979. An annotated checklist of Missouri leaf beetles (Coleoptera: Chrysomelidae). Transactions of the Missouri Academy of Science 13:53–83.
Riley, E. G. & W. R. Enns. 1982. Supplement to an annotated checklist of Missouri leaf beetles (Coleoptera: Chrysomelidae): new state records and host plant associations. Entomological News 93(1):32–36 [full text].
Visser, J. H. 1986. Host odor perception in phytophagous insects. Annual Review of Entomology 31:121–144 [pdf].
© Ted C. MacRae 2019