Subject: Vanessa cardui Migration during Fall 1991 & Spring 1992

From: [email-masked] (R D. Bayer)
Date: 7 Dec 1996 22:49:20 GMT

Last year, there were some postings in this newsgroup about Painted 
Lady (Vanessa cardui) southerly flights, and David McCorkle wrote
that he had seen southerly flights at Monmouth, Oregon, in 
October 1965 and on 9 July 1973.  Our observations of migrating 
V. cardui are detailed below.

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                FALL SOUTHERLY FLIGHT (September 1991)

     On 4-5 September 1991 at Newport, Oregon, we saw a southerly
flight of V. cardui along with some smaller orangish skippers,
which we are reasonably certain were Woodland Skippers (Ochlodes
sylvanoides).

     On Sept. 4 at the Hatfield Marine Science Center (HMSC), Bayer 
counted an average of 2.0/minute flying south (range 0-5) across an 
imaginary 50-100 ft east-west line during 11 1-minute watches between 
0919-0930 Pacific Standard Time (PST), and an average of 1.2/minute 
(range 0-4) flying south during 10 1-minute watches during 
1030-1045 PST.  During this time, only one was seen flying north; most 
were flying within 6 ft of the substrate and were very hard to catch 
with a butterfly net.  Bayer estimated about 75% of these were 
Painted Ladies, and the remainder were the skipper.  

     On September 5, some Painted Ladies were only flying south, but 
there did not appear to be as many as on the previous day.  

     The southerly flight may have occurred before or after these 
dates, too.  On Sept. 1, Morse found Painted Ladies and Red Admirals,
(V. atalanta), to be common at the Newport Reservoir, and on 
Sept. 14-15, they were still common at the Reservoir (both) and HMSC 
(V. cardui), but there was no apparent migration.  Red Admirals did not
appear to take part in the Sept. 4-5 migratory flight.

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                 SPRING NORTHERLY FLIGHT (April 1992)

     Morse first observed the northerly flight at the HMSC on 
24 April 1992, when "hundreds or thousands" of Painted Ladies were
flying approximately north with the wind, which at the time was from
the SSW at 10 mph, gusting to 16 mph.  There were a number of
instances of apparent chases, where one or two Ladies pursued a third in
the direction of the migration.  These may have been courtship flights. 

     On April 25 at the HMSC, Bayer counted an average of 1.2/minute 
(range 0-3) flying north across an imaginary 50 ft east-west line 
during 15 1-minute watches during 1445-1500 PST; only four Painted 
Ladies flew south during these watches.  Most were flying less than 
6 ft high, with a few up to 10 ft high.  

     At about 1500 PST on April 25, Morse counted 57 Painted Ladies along
about 0.4 mi of the HMSC Nature Trail; he noted that they did not seem
to be as abundant as on April 24, nor did they appear to be flying in a
uniform direction.  Rather, they seemed to be resting and foraging on
vegetation.  Most flights seemed to be short hops between resting spots 
or flowering plants.

     Painted Ladies continued to be numerous at HMSC at least through 
May 1 (Morse was out of town for the next two weeks and made no local
observations, though he did see rather tattered looking Painted Ladies 
in the Okanogan Valley near Brewster, Washington).  Beginning on 
June 14, Painted Lady larvae were numerous at HMSC, particularly on
Tree Lupine (Lupinus arboreus).

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                           RATE OF PASSAGE

     These directional flights could easily be overlooked by unaware 
observers because the rates of passage were so low.  For example, 
our rates fall into the "Extremely Thin," "Very Thin,"  or "Thin" density 
categories given on p. 103-104 in C. B. Williams' book 
_Insect Migration_; Williams wrote that such low densities would be missed
by most people.  Similarly, in Bayer's experience, rates of passage for 
butterflies or dragonflies of about 5/minute or more across a 50 ft line 
are necessary before casual observers can begin easily detecting 
such flights.  

     Our rates of passage are similar to those posted in this newsgroup 
in October 1995 by David McCorkle for his July 1973 southerly flight 
at Monmouth (83/10 min across a 150 ft east-west line, which 
translates to about 4/minute for a 75 ft line).

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Range Bayer (PO Box 1467, Newport, Oregon 97365; [email-masked]) 
and Terry Morse (Hatfield Marine Science Center, 2030 S. Marine Science
Drive, Newport, Oregon 97365; [email-masked])
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