Pieris Butterflies Over Time

Mustard White (Pieris oleracea)
May
(poor worn thing)

Mustard White (Pieris oleracea)
July

Mustard White (Pieris oleracea)
August

Mustard white butterflies (Pieris oleracea), which are native, have been extirpated in many parts of Vermont, but not in my woods. They are plentiful every year. They give me a chance to observe their wing color. As the season progresses, their wings change colors. 

“The population’s wings will change over the course of the year,” [Associate Professor of Biology Andrew] Stoehr says. “It takes many days for their wings to develop so they are trying to predict the weather weeks in advance. During those caterpillar stages, they’re receiving information about the temperature.” 

As ectotherms, they rely on sunlight and temperatures to function. As a caterpillar and chrysalis, the insect is monitoring the weather so it can develop the most comfortable pair of wings, which are designed to soak in the preferred amount of heat. (1)

That information was written about Pieris rapae, the Cabbage white butterfly that comes from the non-native "cabbage worm" that gardeners hate.  I have always noticed changes in the Mustard white's wings and have been trying to document changes in color over the years. I want to know why they change. It's a simple question, but a difficult job.

(Professor Stoehr began The Pieris Project to track changes in wing color attributable to climate change. It's a community scientist thing but you have to submit dead specimens. I do not do dead unless I find them dead and I don't find them dead. If you find them dead, you may want to mail them off to Stoehr.)

Cabbage White (Pieris rapae)
May

Cabbage White (Pieris rapae)
August (raised)

Cabbage White (Pieris rapae)
September

Cabbage White (Pieris rapae)
October

Bugguide describes my P. oleracea thus (emphasis added by me):

Mostly white, with two forms. The spring form has well-defined lines of dark green-gray scales along the veins on the hindwing underside, and the summer form lacks these scales. It is similar to other Pieris species. (3)

Even though the two butterflies are in the same genus does not mean that P. oleracea responds to temperature changes as P. rapae does. I added the emphasis in the above quotation because that sentence has always made me nuts: What exactly is similar in other Pieris species? I need more information. What other reasons are there for butterfly wings to change color during one season? I see the changes in the Mustard whites but are the changes caused by temperature as in the Cabbage whites? Do gray scale covered veins warm a butterfly better than having no gray scales?

 References:

1. "Researcher Finds Environmental Clues on Butterfly Wings;" Tim Brouk; October 9, 2019
2. The Pieris Project
3. Species Pieris oleracea; bugguide.net;  Joel Kits, Steve Nanz, Beatriz Moisset, David Ferguson; 2005

Photo Sunday by
Noah Clark

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