The Nearctic-Caribbean species Leptotrachelus dorsalis (Fabricius, 1801): Larval descriptions with a diagnosis of immature Ctenodactylini and natural history notes on the genus and tribe (Coleoptera, Carabidae)

Abstract Adults and larvae of Leptotrachelus dorsalis (Fabricius), the Sugarcane Savior Beetle, live in association with grasses, the larvae in the appressed leaf axils. Both adult and larval Leptotrachelus dorsalis eat larvae of the Sugarcane Borer, Diatraea saccharalis (Fabricius), and perhaps other insects living in the confines of the leaf sheaths of that and other grass-like species. The geographic range of Leptotrachelus dorsalis extends from Kansas in the west to the Atlantic seaboard, north as far as Ontario, Canada and south to Cuba; it is an eastern species of North America and the Caribbean. Larval character attributes that are shared with a related ctenodactyline, Askalaphium depressum (Bates), provide a preliminary basis for characterization of the immatures of tribe Ctenodactylini.


Keywords
Escarabajo Salvador de la Caña de Azúcar, Louisiana, comensalismo, caña de azúcar, Saccharum officinarum L., Taladrador de la Caña de Azúcar, Diatraea saccharalis (Fabricius) introduction Leptotrachelus dorsalis (Fabricius) Lindroth (1969), adults (Fig. 2) are found at the borders of pools and ponds where the vegetation is tall and rich, e.g., in cattail (Typha latifolia L.) swamps ( Fig. 1) with tufts of Carex rostrata Stokes, Menyanthes sp., Solanum dulcamara L., and others. Adults take cover within the leaf axils of Typha latifolia stems. Adults are attracted to lights. In sugarcane, they are found within the canopy (top quarter) of the plant, the region of the sugarcane plant where the Sugarcane Borer, Diatraea saccharalis (Fabricius) lays its eggs and where the neonate larvae become established behind the leaf sheaths of elongating internodes. Larvae of L. dorsalis are rarely encountered below this region, as young Sugarcane Borer larvae are also rarely encountered in association with the lower mature internodes.  Erwin and Medina (2003) wrote: "Van Emden (1942) described the larva of Leptotrachelus dorsalis (Fabricius), Ctenodactylini, the only known larva of the tribe until now. However, at that time "Colliurina" and "Ctenodactylina" were regarded as subtribes of Colliurini (Colliurini is now classified as an unrelated clade, Odacanthini), thus van Emden inadvertently gave a single combined description for both tribes, and then provided in a key the means of which to separate what he regarded as subtribes, but with very few important features listed. Van Emden provided illustrations for many other genera in his important contribution to knowledge of carabid larvae, but did not provide any for these particular ctenodactyline and odacanthine groups." Now that we have reared specimens (all stages except egg), we realize that Van Emden's description is not that of purely Leptotrachelus dorsalis individuals. Thompson (1979) summarized van Emden's description, but did not add anything new to it, did not provide illustrations of any Ctenodactylini, nor did he sort the mixed characters of the two tribes. He did provide illustrations (heads of L 1 and L 3 and cerci) of Colliuris pensylvanicus L. along with a reasonably complete description. These tribes, Ctenodactylini and Odacanthini, however, are now recognized as not being especially closely related (Erwin 1991).
Bionomics of Leptotrachelus dorsalis are discussed in a separate paper (White et al. in press). Here we provide larval and pupal descriptions of the Sugarcane Savior Beetle and notes on the taxonomic complementarity of Leptotrachelus and Askalaphium larvae as a beginning in understanding the immature forms of Ctenodactylini.

specimens and methods
Specimens were initially obtained from experiments to determine economic thresholds for Sugarcane Borer in new sugarcane cultivars (White et al. 2008).
Descriptive and larval preparation methods follow those suggested by the classic carabid larval method paper of Bousquet and Goulet (1984), particularly their coding system for setae and pores in L 1 larvae, and their description format, believed to be ancestral in carabids. Additional setal and pore positions were discovered in Ctenodactylini (Erwin and Medina 2003) and were designated in a sequence that follows the methods suggested by Bousquet and Goulet (1984). However, we have not attempted that here since we do have the first instars. Those subsequent designations in Erwin and Medina (2003) may be merely accessory setae common in later instars. All Leptotrachelus dorsalis immature specimens illustrated here are from populations in the environs of Houma, LA. They were reared in the laboratory from eggs laid by wild captured adults in sugarcane plantings. Many additional larvae were hand collected in the leaf axils of standing sugarcane, as well.

Tribe Ctenodactylini
(The following is based on larvae of Askalaphium depressum (Bates) and Leptotrachelus dorsalis (Fabricius), the only confirmed described larvae in the Tribe). Erwin and Medina 2003, and Fig. 14 herein for illustrations of A. depressum) Head and body depressed, markedly so in A. depressum, much less so in L. dorsalis. Head wider than prothorax in A. depressum, coequal in width in L. dorsalis. First instars of A. depressum unknown, in first instar of L. dorsalis frontal piece with long U-shaped row of short stiff setae likely used as an egg burster. Frontale slightly produced medially, toothed or shallowly bilobed. Neck slightly to moderately constricted, short and broad, cervical groove and keel distinct. Mandible with inner edge of blade and posterior margin of retinaculum moderately serrate (3 rd instar). Maxilla with inner lobe present, unisetose, seta 2× length of lobe in A. depressum, absent and devoid of seta in L. dorsalis. Labium markedly produced medially and unisetose. Antennomere 3 with small tubercule laterad near apex (not a hyaline bulb). Segment IX with two stout curved setae ventrally; urogomphi non-segmented, multi-nodose, infuscated. Pygopod with marked triangular patch of setae postero-ventrally.
Description of pupa. See Fig. 13. Typical of known carabid pupae, not many of which have been illustrated and described. Note the exceedingly setiferous ocular area of head and cerci.  (Fabricius). A dorsal aspect B ventral aspect. Abdominal segments A-7 through A-10 and cerci and pygidium. Scale line equals 0.5 mm.
Characteristics of Ctenodactylini larvae. Erwin and Medina (2003) showed that van Emden (1948) mixed attributes of two unrelated tribes, Odacanthini (Colliurini) and his concept of Ctenodactylini. They found that L 3 members of the genus Leptotrachelus contain the following larval attributes found in larvae of A. depressum: epicranial suture short; cervical groove and keel present; maxilla with inner lobe present; neck not severely constricted; urogomphi nodal, yet not segmented. They located the collection of L. dorsalis larvae that van Emden studied (collected at Oxford, Indiana) in the NMNH, however, they are not in very good condition. They also confirmed his observations and added that second and third instars lack a pencillus and the terebral blade is micro serrate, but refrained from making a more detailed comparison until better specimens were discovered. These observations are now testable with the wealth of material found by the junior author in Houma, Louisiana. Our understanding of the structural attributes of the larval stages for the carabid tribe Ctenodactylini is now progressing. Erwin and Medina (2003) also studied some poorly preserved specimens of Odacanta melanura L. in the NMNH collection. These specimens differ from ctenodactyline larvae in that the mandible has a single seta pensillus and the maxilla lacks an inner lobe, other features are not discernible. Undescribed larvae of some genera of ctenodactylines were also found in the rotten stems and leaf axils of species of the plant genera Heliconia L. and Calathea G. Mey in low wet places in Amazonian Ecuador and Perú. Adults of Ctenodactyla Dejean occur on species of these plants at night. Further discovery is necessary and subsequent documentation is required to define more accurately the Ctenodactylini, and to explore the patterns that must link these beetles evolutionarily and ecologically to the plants on which they live and the food which they eat.

Conclusions
The hypothesized "home reed," Typha latifolia L., as a microhabitat for this commensal species of carabids is classified in the Poales, Typhaceae. This reed, commonly called bulrushes or cattails, is an obligate wetland species and has been found in a variety of climates, including tropical, subtropical, southern and northern temperate, humid coastal, and dry continental up to 2300 m altitude in North, Middle, and South America. However, we point out that species of Leptotrachelus are known to occur commonly as adults on the culms of marsh grasses such as Panicum dichotomiflorum (Leptotrachelus dorsalis : Steiner 1984) in Maryland, USA, and Paspalum sp. (Leptotrachelus spp. : Erwin 1991) in the western Amazon Basin. There are currently 40 described species of Leptotrachelus in the Western Hemisphere, all with proximity to the spread of sugarcane plantations. In these plantations, there are often water-filled ditches with bulrushes and this is the likely source of the beetles that invade the standing canes; it is also a refuge for the beetles when the sugarcane fields are harvested in the fall and winter and resulting residues are burned off, a usual practice. They maintain their populations there in the Figure 13. Pupa of Leptotrachelus dorsalis (Fabricius). A dorsal aspect B left lateral aspect C ventral aspect. Scale line equals 1.0 mm. bulrushes and reinvade the sugarcane ratoons in the spring, or after the next planting cycle. This makes them excellent biocontrol agents; however, broad spectrum pyrethroid insecticides sometimes used by farmers and predation by the red imported fire ant, Solenopsis invicta Buren, can severely reduce beetle numbers. specimen preparation and general collection assistance, Young Sohn who provided the excellent larval illustrations, Charyn Micheli for literature research and critical review of the manuscript, as well as Karolyn Darrow for her assistance with rendering and arranging the many illustration plates and photos; all four individuals are part of the important technical staff of the Department of Entomology, Smithsonian Institution. Lourdes Chamorro (USDA Staff in NMNH) also provided a critical review of the manuscript. Funding for this study was received from the American Sugar Cane League of the U.S.A., Inc. and publication costs borne by the National Museum of Natural History, Smithsonian Institution. We also thank two excellent anonymous reviewers who contributed much to the final product, although we are responsible for any errors that might still be maintained in the final product.