Color-changing Leaf Beetles

The coloration of leaf beetles or Chrysomelidae is fascinating. Their colors do not only range from highly aposematic to extremely cryptic and from opaque to transparent or metallic-reflective, some leaf beetles can even change colors. 

While some species go through reversible changes repeatedly and quickly probably in a reaction to exogenous factors like light, temperature or humidity, others experience one irreversible color change that coincides with their ‘attainment of sexual maturity’ as Knab described it in 1909. He referred to the resulting mature color as ‘nuptial color’.

This color change is distinctly different from the chitinization and pigmentation process  right after eclosion (hatching from the pupa) and usually happens weeks or months later.

Most Chrysomelidae are single brooded and become sexually mature only after they have aestivated or hibernated. The same physiological changes that lead to sexual maturity seem to induce the color change. The resulting new look is so different that the two stages have sometimes been described as different species.

This blog chapter will concentrate  on  two Arizona leaf beetle species from two different sub-families, one cassidine and one chrysomelinae.

The Canyon Ragweed Tortoise Beetle, Physonota arizonae Schaeffer, 1925

For several years, towards the end of the monsoon season in late August, I found the freshly hatched adults among many last instar larvae and pupae on the leaves of Canyon Ragweed. At this time the perennial plant produces several feet-high stems with rich leaves. 

The newly enclosed, and presumably sexually immature beetles have a striking pattern of cream and brown spots. Surprisingly, they blend in well with the scar tissue of ragweed leaves that were damaged by larval feeding. 

On the 3rd of July 2009, before the non-mosoon of that year began, I finally found a single 'mature' individual. I only recognized it by its size and the three spots on the pronotum. It helped that I was actively looking for this type. At this stage, the beetles are metallic brown-green and somewhat camouflaged on undamaged leaves. I assume that this was a mature beetle that had over-wintered because we hadn't been able to find any larvae til then and also none of their very obvious frass pattern.

 Two color forms of  P.arizonae Photo by Rick Williams

Later that year a  friend sent me a picture of the two forms together, taken on September 8th . So the change seems to happen not too long after eclosion, before the winter pause.

Calligrapha serpentine (Rogers, 1854)

Is a very attractive leaf beetle living on Narrow-leaf Mallow (Sphaeralcea angustifolia) Desert Globe Mallow  (Sphaeralcea ambigua), and probably some related Malvaceae species in Mexico and in the US from Texas to Arizona. 

I have found mating adults in the grass land of Cochise County, AZ in early to mid July and in Grant County New Mexico in early August. Their elytra showed a distinct black linear pattern on a vividly iridescent green base color.

On September 4 September 2010 I went back to a patch of Mallows 2 miles north of Lochiel, Cochise Co. where I had seen the beetles last year in late July. I was quite surprised to find bright red leaf beetles with exactly the same black pattern on one plant and only hairy red and black leaf beetle larvae on other Mallows.

 A post by Sheri L. Williamson from Oct. 2009 on Bugguide.net confirmed that the green and the red beetles were indeed the same species. She also connected me to the delightful True Confessions of a Citizen Scientist by S. A. Russel who had followed C. serpentina from the egg to larva, pupa and early red imago and to mature green nuptial beetledom. 

Since this change from red to its total complementary opposite, green, seems particularly interesting, I decided to observe its progress in the five beetles that I had collected.

   September 10. All  5 beetles are still red

The problem was to provide appropriate food because Globemallows are spring flowers in Tucson and have just about no leaves in summer. My beetles had to spend the first three days without food in the fridge and then lived for a couple of weeks on an unusually diverse diet of several Mallow and Hibiscus species that I brought home from field trips to the Tucson Mountains, Mount Lemmon and Kitt Peak.

 September 15. All beetles are showing patches of green

After 5 days one beetle was still mainly tomato-red. The others showed patches of green, mostly on the lateral portions of the elytra. The biggest, roundest beetle (probably one of  the  females) appeared more green than red.

September 16. The big female is nearly all green, one male is still mostly red. 

September 18. All beetles have green elytra but the four smaller ones are still a little paler than the big female. 

September 19. All beetles are bright green

 

The color change had taken about 8 days. It didn't happen gradually, but patches switched from red to green rather abruptly, first in the lateral areas of the elytra and then moving towards the center seam.

I'm still curious about the physiological changes and how the colorshift is correlated with the process of reaching sexual maturity. I did not observe any mating attempts.  Little is known in which form these beetles overwinter. The post-monsoon season is dry and a lot of herbaceous plants are already starting to shrivel up. So my guess is that the adult beetles will enter a resting phase now, and new larvae will grow up during the vegetation seasons of he next year.

Due to the lack of food I had to end my own observations here.