I haven’t seen any Monarch butterflies yet this year, and the milkweeds are just about to flower. But the Milkweed Leaf Beetles are more abundant than ever. They just love the pink-flowered swamp milkweed (Asclepias incarnata). Last week I found adults crawling all over the tops of the plants near the unopened flowers (which they also devour in addition to the leaves).
Males follow or ride on females as she munches on flowers or leaves. He guards the female from other potential suitors, until she lays a batch of eggs on the underside of the milkweed leaves. Then he may wander off to find another female. Both sexes may mate many times during their short summer lifespan, which ensures a lot of gene flow in a population.
After seeing adults in the garden for several days now, I examined the underside of the milkweed leaves this morning to see if there were any larvae present, and sure enough all stages of larval development were present.
The eggs are orange, which is warning coloration that should deter egg-sucking predators. But they are often attacked by syprhid (hoverfly) larvae, who apparently tolerate the milkweed poisons (cardiac glycosides) just fine.
Larvae may cannibalize each other at this stage, so that only a few survive to mature. In fact, female milkweed leaf beetles may also cannibalize the offspring of other females, perhaps to reduce the competition for their own progeny on that plant.
There are reports on the web of milkweed leaf beetle infestations that completely denude their milkweed hosts of leaves and flowers, although this seems to occur primarily in a climate where milkweeds grow year-round. The population of these beetles has definitely increased from a rare sighting of one individual several years ago, to finding many individuals in the backyard almost any day. I hope this doesn’t mean I’ll have to manually remove beetles at some point — I already get enough practice doing that with Japanese Beetles.