Journal archives for May 2018

May 07, 2018

The Dubious Records of the Dubious Tiger Moth (Spilosoma dubia)

Of all the species of Spilosoma in the United States and Canada, none seems to be more misunderstood in online records than Spilosoma dubia. Between BugGuide, iNaturalist, Moth Photographers Group, and BAMONA, I have seen dozens of misidentified photos labeled as S. dubia, and about one real S. dubia for every ten misidentifications. For example, as of May 2018, the BAMONA page for S. dubia features photo records of Norape ovina, Hyphantria cunea, Spilosoma congrua, and Artace cribrarius, and one or two photos that may be S. dubia, but not one definitive dubia photo in the bunch.

Why is this species so often misidentified? I’d say one main reason is lack of understanding of the species’ range and abundance. S. dubia has two disjunct populations in North America. One population occurs in the Canadian Zone in boreal forest from Alberta east to New England and the Maritime Provinces. This population occurs as far south as northern Pennsylvania, where it is quite rare. The second population occurs along the Gulf Coast, from eastern Texas through Florida, and northward along the Atlantic coastal plain to North Carolina. The species is therefore absent from most of the eastern United States, occurring only in the far northern and far southern parts of the region. (Dale Schweitzer, personal communication). I’ve personally collected the species in coastal North Carolina and the Upper Peninsula of Michigan, and the series of specimens from the two populations are indistinguishable to my eye. Unfortunately, many photos of similar white moths with black spots were posted under “Spilosoma dubia” on BugGuide throughout the 2000s, and those records made their way onto the Moth Photographers Group maps at the time. That led people to believe that this was a common and widespread eastern species, which led to numerous misidentifications on BAMONA, and ultimately to a range map in the new Peterson Guide to Northeastern Moths that shows the species occurring throughout the eastern US. Now that this over-inclusive range has become so widely accepted by moth photographers, photos are frequently posted to iNaturalist identified as S. dubia that are 500+ miles out of range.

So, how does one actually identify a “real” Spilosoma dubia? Well, there are five eastern species to consider before arriving at a definitive ID of “S. dubia”, and seeing the abdomen of the specimen is essential to making an ID. If you just have a photo of the moth with its wings folded up, you probably won’t get a definitive ID.

  1. Spilosoma congrua: forewings range from pure white to moderately speckled with black. The abdomen is pure white. Front legs are yellow-orange
  2. Spilosoma latipennis: forewings are almost always pure, silky white. The abdomen is pure white. Front legs are bright pink (hence the common name).
  3. Spilosoma virginica: forewings are pure white or have a few black spots. The abdomen has bright yellow-orange sides and a row of large black spots down the back of it. Without seeing the abdomen, these could be mistaken for S. latipennis or a lightly marked S. congrua.
  4. Hyphantria cunea: forewings range from pure white to very heavily covered in black. This species is generally noticeably smaller than the Spilosoma species. The abdomen is white on the sides (never any yellow), but usually has a row of small black spots along the top of the abdomen. There is also a difference in the spurs on the front legs, which is usually not visible in photos.
  5. Spilosoma dubia: forewings are almost always moderately speckled in black, basically identical to a well-marked S. congrua. However, the abdomen has yellow along the sides and black spots along the top, like S. virginica. There are long white scales that overlay the yellow section of the abdomen though, causing the yellow to not stand out or be as bright as in S. virginica. I often describe S. dubia as a “dirty” version of S. virginica for this reason. Those white scales rub off over time, leaving the yellow more noticeable. For comparison, here is a S. dubia with the white scales on the abdomen worn off, making the yellow stand out more: and here is a S. dubia with the white scales still intact, obscuring the yellow coloring: The yellow is still visible in both specimens though. The front legs of S. dubia are yellow, just like most of the other species (except S. latipennis).

So essentially, if you are just going by forewing pattern, S. dubia is identical to a well-marked S. congrua, and extremely similar to a moderately marked H. cunea, and could easily be mistaken for either of those species. Only by examining the abdomen can you see the differences that separate these three. Somehow, it seems to have become common practice online to identify any Spilosomina specimen with moderately-to-heavily speckled wings as “S. dubia”, despite there being two other much more likely identifications for such specimens. If you are outside the two known ranges of S. dubia, it’s almost certain you have S. congrua or H. cunea, and even within its range, it’s usually not as common as the other two options.

In conclusion, there are a handful of definite S. dubia photo records online, which clearly show the dirty yellow on the abdomen, and all are from Texas-coastal North Carolina, or from the far northern Canadian Zone. There are many more supposed S. dubia records online from other parts of the United States, and not a single one of them shows the abdomen to confirm the ID. These records should not be trusted unless more information can be supplied. If you live in the area outside where S. dubia is known to occur, and you would like to find a specimen to expand the species’ known range, then be sure you’re getting photos of the top and sides of the abdomen when you encounter possible dubia specimens. Otherwise, your records are likely to remain “ddubious” forever.

Posted on May 07, 2018 04:09 PM by paul_dennehy paul_dennehy | 3 comments | Leave a comment


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