Figs 35–54, 57, 60, 63
Mythimna decolor Walker, 1858: 1658.
Enargia discolor; Smith 1900; misspelling.
Cosmia discolor; Dod 1905; Dod 1910; misspelling.
Enargia decolor; Franclemont 1939.
Enargia decolora; Hampson 1910: 239; unjustified emendation.
‡ Enargia decolora ab. mia Strand, 1916: 164; unavailable infrasubspecific name. ‡ Enargia decolora ab. sia Strand, 1916: 164; unavailable infrasubspecific name.
Type material. Mythimna decolor: male holotype. BMNH, examined. Type locality: “Orilla [ sic], West Canada ” [Orillia, Ontario, Canada].
Diagnosis. Enargia decolor is externally most similar to and broadly sympatric with E. infumata. Specimens with little dark shading can also be similar to E. fausta, particularly females of both species. About 90% of E. decolor specimens can be recognized by one or more of the following external characters: reniform spot lacking pronounced dark scaling at base, or if dark scaling present, not darker than color of adjacent medial line; claviform often present as dark dash; well-marked specimens with pronounced hindwing medial line; male antenna slightly serrate, not prismatic. Internally, in males the corona extends only halfway along the ventral margin of the valve (2/ 3 in E. infumata and E. fausta, Figs 55, 56), and the vesica cornuti and aedeagus are larger (compare Fig. 60 to Figs 58 and 59). Females have a longer ovipositor and corpus bursae (Fig. 63).
Distribution and biology. Enargia decolor has a boreal-transcontinental distribution, occurring across the Canadian boreal plain and then southward through the western cordillera at higher elevations, where it is presumably limited by the availability of trembling aspen and possibly other poplars. Records for examined specimens range from northernmost British Columbia (Ft. Nelson) and south-western Northwest Territories (Ft. Smith) east to New Brunswick; also reported from Nova Scotia ( Ferguson 1954), Ohio ( Rings et al. 1992) and New York ( Forbes 1954). In the western United States, specimens were examined from western Montana, Idaho, Washington, Oregon, Nevada, Utah, Wyoming (Albany and Carbon counties), western Colorado, New Mexico (Grant Co.), and Arizona (Graham Co.). Notably, I have not seen any specimens from the Rocky Mountain front ranges of Colorado, where it would be expected to be widespread if there is a continuous distribution southward into New Mexico / Arizona. As discussed under Remarks, the populations from west of the Rocky Mountains south to New Mexico / Arizona may represent a distinct species.
The larvae prefer Populus tremuloides, and Prentice (1962) also reported a small number of larval collections from Betula papyrifera, Populus balsamifera, Salix sp., Populus grandidentata Michx. and Alnus rugosa (Ait.) Pursh. Since this species has mostly been correctly identified, Prentice’s larval host records are also probably mostly correct. Larvae can reach high population densities, causing local defoliation of P. tremuloides ( Wong and Melvin 1976). Th e balsam poplar group have quite resinous buds and leaves at bud break, so these may not be suitable hosts, at least for early instar larvae. McGuffin (1958) gives detailed descriptions including setal maps of E. decolor, but a diagnostic comparison of morphology and biology of larvae to E. infumata and E. fausta is still needed. Wong and Melvin (1974) describe the larvae and larval biology of E. decolor.
Remarks. Enargia decolor as it is currently defined may consist of two species. Specimens from Nevada, Utah, western Colorado, Arizona and New Mexico are on average duskier, and the medial area tends to be the darkest forewing area (subterminal area equally dark in boreal E. decolor); specimens have the markings more obscure overall, often with a pinkish tinge not seen in boreal decolor. Comparison of male genitalia from this region to boreal decolor also suggest a slight difference. Five barcoded specimens from Alberta and New Brunswick exhibited four haplotypes, with a maximum divergence of about 0.26 %; three Utah specimens representing two haplotypes differed between 0.86–1.37 % from the Alberta / New Brunswick material. Additional specimens from key geographic areas ( Colorado, Wyoming, Idaho, Oregon) are needed to fully evaluate the taxonomic status of these populations.