Journal archives for March 2017

March 05, 2017

ID Guide 1: Distinguishing Two Very Similar Armyworm Moths


I tend to write up identification notes for myself as reminders on how to separate similar species...and then most often file these in a drawer somewhere until someone happens to ask about the same ID challenge. So, I'm going to attempt to begin posting a series of really miscellaneous Identification Guides to various tough moth identification challenges that I have encountered in Central Texas. With this limited geographic focus, some/many of these may not be relevant nor sufficient for ID help in a wider area, but hopefully they will offer starting points for consideration. All images are my own unless otherwise noted.

Two Similar Armyworm Moths

The Gray-streaked (or Unbarred) Armyworm (Spodoptera albula) and Southern Armyworm (Spodoptera eridania) occur across much of the southern U.S. They are superficially very similar. Both are paler and more uniformly patterned than a couple of their common relatives, the Fall Armyworm and Yellow-striped Armyworm.

After examining a series of photographs taken in the Austin area in October-November 2012 as well as available online images and specimens at the University of Texas Insect Collection, I offer the following information on how to separate these with some small details visible in specimens or close-up photographs.

Typical pale adults of the two species share these overall general characters:

(1) Overall pale cream to whitish-gray ground color.
(2) A small reniform spot which is fairly well marked by black spots or smudging.
(3) A subterminal zigzag band of brown edged on the outer side by white.
(4) Usually have a post-median row or arc of small dark dots on each vein (sometimes absent/obscure).
(5) Pale puffy grayish-white thorax with several thin dark longitudinal lines.
(6) Both have typical “Spodoptera” forelegs with puffy gray pantaloons (heavily scaled tibia), pale gray first tarsus, and darker, banded or white-tipped 2nd-5th tarsi.

Here are examples of the typical pale adults of each species:
Spodoptera albula_5605 Spodoptera eridania_7109.JPG
Above: Gray-streaked Armyworm Below: Southern Armyworm

Here’s what I’ve figured out for distinguishing pale adults of the two species. (See note below about dark morph [female?] Southern Armyworm Moths.)

Gray-streaked Armyworm Moth:
Spodoptera albula_5605_marks

White margin of subterminal brown band: Note that the forward points of this zigzag band (highlighted by white) are inbetween the veins, particularly the point I’ve highlighted, and the point where the white touches the outer wing margin is on a vein rather than inbetween the veins.
Post-median band of dots: This row of dots, when present, is typically less than half way from the reniform spot to the outer wing margin, usually about 1/3 to 2/5 of the way, i.e. closer to the reniform spot than the outer margin.
Claviform spot: This elongate spindle-shaped gray spot is almost always present just on the basal side of the small orbicular spot (cream-edged brown dot).
Basal wing dash: Best viewed from the side. Most Gray-streaked Armyworm Moths have a long black dash from the base of the forewing. This dash is gently curved downwards (towards the forward wing margin), ending near the claviform spot. A shorter thin dark line may or may not be present on the thorax*. When paired with this dark line on the thorax, the dash looks partially doubled as in this individual.

  • This field mark is emphasized in a number of published guides and online keys, but it is so variable and subject to wear that it should be used only in conjunction with the other marks. In particular, it can be difficult to distinguish a single dark line (thoracic? or base of wing?) when viewed from directly above. A shadow of the thoracic “epaulet” (tegula) may also show on photos illuminated from above.

Southern Armyworm Moth:
Spodoptera eridania_7109_marks

White margin of subterminal brown band: The forward points of this zigzag white band are on the veins, particularly the point I’ve highlighted, and the point where the outermost white touches the outer wing margin (that is, the distal point of the dark brown band) is inbetween veins rather than on a vein.
Post-median band of dots: This row of dark dots on the veins, more often present in this species, is typically at least half way from the reniform spot to the outer wing margin, and thus a bit closer to the outer margin.
Claviform spot: The elongate gray spot of the Gray-streaked Armyworm is absent in this species.
Basal wing dash: Most Southern Armyworm Moths have only the thin dark line on the thorax, making this a single shorter dark line. This is best viewed from the side; see note on this field mark under the Gray-streaked Armyworm, above.

A darkly streaked form (female?) of the Southern Armyworm Moth is more distinctive:
Spodoptera eridania_6832_dark
Southern Armyworm Moth, dark form

[Originally prepared for Austin Butterfly Forum, November 12, 2012; revised Dec. 2012, Mar. 2017.]

Posted on March 05, 2017 16:59 by gcwarbler gcwarbler | 4 comments | Leave a comment

Numerologists Field Day?

I just had an odd moment when uploading this Common Oak Moth for March 1 (= 3/1):
I was refiling that image in my usual folder for “iNat - Moths” when I got a warning that “There is already a file named ‘Phoberia atomeris_0927'. Do you want to replace it?” I hadn’t uploaded this image previously so I went on to investigate why there was a duplicate.

It turns out I had photographed and uploaded an image of this same species on February 15, 2016, just over a year ago…and it happened to end up with the same sequential file number from my camera.

What are the odds of that?

Well, the numerologist in me recalled that my camera cycles through 4-digit file numbers so one thing that this tells me that in the interim 1 year and 13 days, I had taken precisely 9,999 digital images with my little point-and-hope camera. (It would be an even 10,000 but it doesn’t use the file number “0000”.) If I were to take just one picture of this species randomly during any file number cycle, the odds would have been 1 in 9,999 of ending with the same file number. Yet over the past year, I’ve probably documented over a thousand different species with my camera. Even taking multiple images of most species, the odds of ending up on the same file number with the same species would seem astronomical.

But of course, the Common Oak Moth is fairly routine at my porchlights and I try to document the species at least once each month in which I encounter them. It turns out I had taken exactly 13 (!!) images of the species in the interim year+. So the odds of me randomly landing on “0927” with a Phoberia atomeris might be better stated as about 1 in about 770 (that is, pretty close to 10,000 ÷ 13).

Oh, by the way, that file number “927” can be written using only the digits “1” and “3”. It can be factored by prime numbers as 3 x 3 x 103 which can be rewritten:

927 = 3 x 3 x ((3 x ((3 x 3) +1)) +13)

If you were curious, neither of the iNat observation numbers above (5,228,931 or 2,702,960) is a prime number. The prime factors of the latter number include the number 13, that is:

2 x 2 x 2 x 2 x 5 x 13 x 23 x 113

If you divide the latter observation number by 13, you get 207,920 which is an anagram of 6 of the 7 digits of the previous number after dropping the numeral “6”.

My favorite numbers? Why, 7 and 13, of course. My wife and I were married on July 13.

I’m not makin’ this stuff up. If you read all the way through this journal post, your new bumper sticker should read, “NUMBERS HAPPEN!”.

Posted on March 05, 2017 15:01 by gcwarbler gcwarbler | 1 observation | 4 comments | Leave a comment