Wednesday, November 10, 2010
Monarch Caterpillar Sign
After a summer drought of monarch caterpillars I found this chunky one on the Asclepias physocarpa (aka Family Jewels) that was planted as a curiosity this summer. You may be able to see how the leaf the caterpillar feeds on dangles loosely due to its petiole being partially severed. The caterpillar cuts the leaf attachment which reduces the latex flow in the leaf as it is eaten. This tactic is undoubtedly beneficial to the caterpillar's survival but it is not clear whether it is consumption of the latex, the defense chemicals, the stickiness of the latex or some unknown factor that the caterpillar seeks to avoid to improve its chance for growth and survival.
Apparently (see charts in this article) species of asclepias have notably variant amounts of both latex (sticky milky fluid) and cardenolides (a chemical defense against herbivores). I have had a horticultural and/or aesthetic interest in growing more varieties of milkweeds (currently only have two: A. physocarpa and A. curassavica) but now I also want to pursue this line of planting as a way of observing any differences in monarch caterpillar survival on various species.
The A. curassavica goes through phases of lush growth and blossoming alternated with periods of die-back. It will be interesting to see if the other milkweed species also do this (I expect they do) and if the phases will coincide among the species.