Tuesday, April 13, 2010

Monarch Caterpillars in Phoenix!

When I visit the Desert Botanical Garden I always look for monarch butterflies or caterpillars first. As our temperatures begin to warm, the morning hours are the best time to catch a glimpse of butterflies nectaring, especially on lavendar. It is also the ideal moment to spot caterpillars hungrily munching on milkweed. In the morning, caterpillars usually crawl and feed at the top of the plants so they are easy to spot. Every week brings something new.

On Friday I walked through the Herb Garden and ran into Denny Green taking photos of monarch caterpillars on Tropical Milkweed, also known as Bloodflower, Asclepias curassavica. We counted seven larvae on two plants. One milkweed in a shady location had two larvae, another in a sunnier spot had five. The caterpillars were very large, 5th instar. With our warm temperatures by now they have likely formed a chrysalis. All photos in today's post are by Denny Green unless otherwise noted and used with his permission.

It was just two weeks earlier that these same milkweed plants were filled with queen caterpillars. While queen and monarch caterpillars have similar coloring, there are striking differences.

Queen caterpillars have three sets of filaments, one pair at each end and one pair near the middle of the larva. Often you can see a red tinge at the end of the filament.





In comparison, monarch caterpillars have two pairs of filaments, one set at each end.













When I spotted the monarch caterpillars, a few questions popped to mind. Could these caterpillars possibly be from eggs laid by the faded female monarch butterfly I saw on March 15th? This was almost three weeks later, longer than normal for their growth stage. During this time period some of our temperatures in the Phoenix area were much cooler, with a few lows in the upper 30's and low 40's. While now a warm 85, I couldn't rule out the possibility since monarch larvae grow only between the temperatures of 52.7 and 91.4 degrees.

Did one or more monarchs lay these eggs?  It is interesting to note that all seven caterpillars are about the same size, so they were likely from the same female monarch. Only one seemed to have a smaller body, but the filaments were as long as the others, one of the indicators of instar size.

The coloration of a monarch caterpillar gives us clues to whether it is hot or cold outside. Look at this picture of a monarch caterpillar on Desert Milkweed, Asclepias subulata, taken by Laura Miller last Fall. Temperature readings at that time were hovering in the middle 90's with mid-60's to 70's at night. When it is warm, the caterpillar adapts with wide bands of white, and thinner bands of black and yellow. In this case the black is almost pencil-thin. When it is hot many of us like to wear light colors to keep cool. The monarch larva does the same.


In comparison we can see how thick the black and yellow stripes are in the monarch caterpillars at the Desert Botanical Garden last week. The darker coloration helps them keep warm by absorbing more of the sun's heat during a cool time of year - just like we are more comfortable wearing darker colored clothing on a chilly day.

How wonderful to see monarch caterpillars in the Phoenix area, the first sighting of Spring!

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