Map of Black Witch Moth Migration by Month

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Return to Texas Entomology - Compiled by Mike Quinn



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 PURPLE LINE
 Aug-Oct Records north of the Turquoise-July Line 

 TURQUOISE LINE
 July Records north of the Green-June Line 

 GREEN LINE  
June Records north of the Red Line  

 RED LINE
December through May US Records 

Blue Records - March through July 2004 - Year of a major outbreak


Introduction

The Black Witch Moth (Ascalapha odorata) is a large brown bat-like moth that annually emigrates north across North America with usually a few individuals being recorded in Canada each year.

Methods

The standard method of plotting a species' range is to plot all the records as the Moth Photographers Group (MPG) did for the Black Witch Moth. The MPG map shows a fairly even distribution of the moth with a higher density in the south from CA to TX and FL. Some clustering further north, such as in CO, KY and OH, may be due in part to increased survey efforts.

To tease out more information of a migratory species, one again usually maps all records for the species but color codes the records by date as Journey North did by mapping the Monarch Spring 2012 Migration by two week intervals.

Contrary to these normal mapping protocols, I only mapped the records that occurred north of the previous month(s)'s records. This methodology cuts down drastically on the over 500 BWM records that I would have been obliged to plot using traditional methods. 

I used Blue Markers to show the tremenous number of records from 2004 when this species had a major outbreak. I was able to greatly increase the number of records I received as I worked for Texas Parks and Wildlife at the time and we issued a June 28th press release requesting BWM reports. 

Results and Discussion

The onset of the BWM emigration out of Mexico begins in June of each year and appears to be very strongly tied to the start of the rainy season in Mexico which begins in May and June. While Monarchs cross the Rio Grande starting in March and take two months to reach Canada, Black Witches have been recorded reaching Maine by June 9th, Door Co. Wisconsin by June 16th and Manatoba by June 28th. By the end of June, Black Witch Moths have been recorded in 19 states and two Canadian provinces.

The by-month mapping methodology employed here documents the moth's very significant migration out of Mexico and into the central US and also indicates that almost no Black Witch Moths migrate north out of Florida in June.

The Blue Markers (2004) show a more western distribution, CA west to LA and NE, particularly for June than the Green-June Markers (all yrs except 2004) which primarily range from the Rocky Mountains east. However, the records north of the Green-June Line do show an overall more westerly distribution in line with the 2004 outbreak data. This western distribution is certainly more pronounced than the data show given the many times fewer potential observers living in western North America than in the east.

Nearly all the BWM data I have are from random observations except the following two data sets from locations on the Texas coast north of Corpus Christi.In May and June of 2004, Ray Little conducted regular 25 mile long surveys of Matagorda Island, Calhoun Co. on a 4-wheeler looking for sea turtle nests. He also kept track of BWMs he saw which flew up from the beached sargassum as he drove past. None were see on surveys prior to May 30. Very similar numbers were reported by the Aransas Pass Lighthouse keepers. The keepers reportedmoths perched under north facing eaves of one of the buildings just south of Matagorda Is. in Aransas Co. during 2004 and 2005.

Data from 25 mile patrols on
Matagorda Island, Calhoun Co.

May 30, 2004   2
June   2   6
June   8  10
June 16  18
June 22 12
June 24    8
 
Aransas Pass Light Station on 

Harbor Island, Aransas Co.
 


June 16, 2004 10-15
June 23   8
June 24 12
July 01, 2005
16

Other significant numbers along the Gulf Coast suggesting peak moth passage roughly in line with the numbers above include "dozens" reported on June 15, 2004 further up the coast on Grand Isle in southeast coastal Louisiana by Wayne Keller and a report of "fairly high numbers" on July 2, 2005 at Galveston Island State Park southeast of Houston.

The large  numbers of moth reports that came in over more dispersed areas inland and from people's memories as a result of requests for sightings run in newspapers and other media in in July 2004 in Midland-Odessa, Fort Wort, and Wichita Falls are more difficult to pin point, but consistently showed a mass movement through various parts of Texas primarily in late June through early July.

"Hundreds" were reported within the eye and wake of Hurricane Claudette when that storm hit Port O'Connor July 15, 2003 and "thousands" of Black Witch Moths were reported in the wake of Tropical Stome Cindy after it came ashore at Grand Isle, Louisiana July 5, 2005. These reports are discussed spearately here.

Black Witch Moth numbers generally show peak abundances for two consecutive months for most states that I compiled records for as seen here. Those months vary by latitude until one reaches Canada where nearly all records fall into a July to August time frame.

Interestingly, the lack of winter records for south Texas, as indicated by the absence of Red Markers (December through May records) in that region, does not appear to be a fluke. There's an active network of naturalists in the lower Rio Grande Valley that I'm well connected to, but I know of no BWM records from that region and time period. By contrast, BWMs are reported for every month by Kimball (1965) for southern Florida.

A testament to the observational skills of south Texas naturalists is evidenced by these photos shot in October of mature Black Witch Moth caterpillars from Starr and Hidalgo counties along the Rio Grande. Mature larvae have been reported from all three of Texas' southmost counties, as early as July 16th and as late as Oct. 23rd. The moth probably overwinters in the pupal stage in leaf litter or below ground and only emerges the following year triggered by the late spring/early summer rains.

Reports of large numbers of BWMs across the southwest which are not associated with tropical storms and are out of sync with the moth's initial wave of migrants (as shown in the map above) are probably the result of a second generation of moths in Mexico.

Aug.   8, 2000
"at least 50" near Patagonia, southeast Arizona
sheltering in an old shack
Aug. 12, 2004 - OCR
37 separate reports
San Deigo, Orange, and LA counties in so. Calif.
triggered by Orange County Register article
Aug. 26, 2012 6
Beeville, Bee Co., south Texas
under 8 x 15 ft covered poarch
Aug. 27, 2005 51 Bentsen-RGV State Park, south Texas
under the pavilion roof
Sept   1, 2005
up to 72 Bentsen-RGV State Park, south Texas sheltering under the pavilion roof.





I find no significant evidence of a southbound fall migration in the U.S.
 

Please report additional BWM records to entomike@gmail.com
Please include date, location & county of record.
This insect is rather common across Texas so I'm particularly interested in non-TX records.
If possible, send a mid resolution photo. Thanks, Mike

19 Aug 2012 Mike Quinn / entomike@gmail.com / Texas Entomology