Map of Black Witch Moth Migration by Month

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Return to Texas Entomology - Compiled by Mike Quinn

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 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.