Pachybrachis atomarius (F. E. Melsheimer, 1847)

Barney, Robert J., LeSage, Laurent & Savard, Karine, 2013, Pachybrachis (Coleoptera, Chrysomelidae, Cryptocephalinae) of Eastern Canada, ZooKeys 332, pp. 95-175 : 113

Pachybrachis atomarius (F. E. Melsheimer, 1847) Habitus 1 ; Map 1 ; Figures 1c , 5a , 8d

Cryptocephalus atomarius F. E. Melsheimer, 1847: 170.

Pachybrachis infaustus Haldeman, 1849: 262.

Pachybrachys atomus Bowditch, 1909: 319.

Recognition.

Body largely fuscous, dull, mottled with many yellow spots ( Habitus 1 ); elytral puncturation dense, confused discally, more or less arranged in rows towards rear and sides; face of males predominately yellow; pygidium entirely black, convex ( Figure 8d ); male size small: length 1.85 ± 0.07 mm, width 1.01 ± 0.03 mm.

Distribution.

The distribution in eastern Canada is restricted in southern Ontario to remnants of the Carolinian forest ( Johnson 2012; Shelford 1963). In Québec, the distribution is isolated from the main distribution area ( Map 1 ). The unique specimen available was probably collected on the Eardley Escarpment, which is a warmer refugium created by cliffs of the Laurentian Highlands oriented southwards ( Brunton and Lafontaine 1971).

Material examined.

ONTARIO: Essex Co., Ojibway, 7.VIII.1943, S. D. Hicks [6♀, CNC]; same data, except 8.VII.1943, S. D. Hicks [1♀, CNC]; Point Pelee, 9.VII.1931, W. J. Brown [5♂ 1♀, CNC]; Roseland, 24.VI.1944, S. D. Hicks [1♂, CNC]; Lambton Co., Walpole Island [1♂ 1♀, CNC]; Norfolk Co., Normandale, 4.VI.1931, W. J. Brown [1♂ 1♀, CNC]; Turkey Point, 8.VII.1931, W. J. Brown [1♂ 4♀, CNC]; Walsh, 10.VI.1931, W. J. Brown [1♀, CNC]; Ontario Co., Fisher Glen, 12.VI.1931, W. J. Brown [2♂ 6♀, CNC].

QUÉBEC: Pontiac Co., Old Chelsea, 12.VII.1961, J. R. Vockeroth [1♂, CNC].

Host plants.

No plant association records were available from specimens examined, and Barney et al. (2011) did not report any either. Clark et al. (2004) presents the known literature, but since adults were usually swept from vegetation, these records cannot automatically be interpreted as real host associations.

Comments.

Pachybrachis atomarius is one of Fall’s (1915) Group C species that have "great variation in the degree of (elytral) maculation." In spite of the extremely variable elytral mottling, ranging from heavily speckled with yellow to almost entirely black, Pachybrachis atomarius males are relatively easy to identify by the combination of the predominately yellow face ( Figure 1c ) and entirely black, convex pygidium ( Figure 8d ). The entirely black, convex pygidium character also permits identification of singleton females.

Although Pachybrachis atomarius is a typical eastern North American species distributed from Manitoba to Oklahoma to Atlantic states ( Riley et al. 2003; LeSage 1991; Barney, unpublished data), in Ontario it is restricted to the Carolinian Zone in the southernmost part of the province. Its presence in Québec is considerably disjunct from its main distribution area, and this isolation is due to the warmer microhabitat of the Laurentian Highlands cliffs of Eardley Escarpment, which are fully exposed southwards and harbor similarly disjunct insects and plants ( Hall 1991; Layberry et al. 1998; NCC 2011a).

Since Pachybrachis atomarius has not been collected in eastern Canada over the last 50 years, it is likely extirpated from the eastern Canadian fauna.

Figures

Habitus 1. Dorsal habitus of Pachybrachis atomarius. Scale bar, 1 mm.

Map 1. The known distribution of Pachybrachis atomarius in eastern Canada. Carolinian Zone in dark beige; EE - Eardley Escarpment.

Figure 1. Eyes: a close eyes, Pachybrachis pectoralis b normal eyes, Pachybrachis peccans c normal eyes and yellow face, male Pachybrachis atomarius d remote eyes, Pachybrachis hepaticus.

Figure 5. Tibial spurs: a absent from front leg, Pachybrachis atomarius b minute on front leg, Pachybrachis spumarius c large on front leg, Pachybrachis calcaratus d small and pointed on middle legs in most species.

Figure 8. Pygidium: a largely yellow, Pachybrachis bivittatus b with well-defined yellow spots, Pachybrachis cephalicus c with faint reddish spots, Pachybrachis spumarius d black, Pachybrachis atomarius.