Identification
Adult: forewing pure white or marked with transverse rows of black spots; antennal shaft white; inner face of pro-femora yellow-orange; hind tibiae with both pre-apical and apical pair of spurs; abdomen white
[adapted from description by Gerald Fauske]
Range
Southwestern U.S. north to southern Canada, east to Atlantic.
Season
adults fly from April to August; longer flight season southward
Food
Larvae are general feeders on herbaceous plants, including dandelion (Taraxacum spp.), plantain (Plantago spp.), and pigweed (Amaranthus spp.)
Life Cycle
Larvae have two color forms
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eggs; newly emerged larvae; larvae at different stages; older larva of one color form; older larva of the other color form; cocoon; adult
Remarks
Note: This species is very often confused with Spilosoma virginica and Hyphantria cunea where ranges overlap. When a view of the abdomen is not possible, one of the key clues as to which species, lies with the appearance of the legs: in particular, the tibiae and tarsi of the forelegs. The key to using this characteristic to separate these three species is as follows:
S.congrua; tibiae and tarsi will be solid white laterally, bordered by solid black medially along the interior of the tibiae/tarsi with a clear demarcation and no banding on tarsi.
S.virginica; tibiae and tarsi will be strongly white with complete or partial black banding along the tarsi.
H. cunea; tibiae and tarsi vary – in immaculate specimens the tibiae/tarsi is often completely white/pale, while in heavily marked specimens, the tibiae/tarsi are completely black. In other lighter marked specimens, tarsi may appear banded and most similar to S.virginica, but will only appear so laterally with solid black/dark medially. When dealing with this variation, it’s typically described as H.cunea having dark tibiae/tarsi, the tarsi banded with white, verses S.virginica having mostly white tibiae/tarsi, the tarsi banded with black.
Where an egg mass is present, another clue is available. The egg masses of S.virginica are typically yellow, of S.congrua typically white, and those of H.cunea, a very pale green.
See Also
Adult:
S. virginica has yellow markings on the abdomen, while
S. congrua's abdomen is pure white. [comment by Paul Dennehy]
Print References
Covell, p. 66, plate 13 #1
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