Bugs in the garden

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).

milkweed leaf beetles mating on swamp milkweed-

These milkweed leaf specialists look a little like ladybird beetles but are much larger (about twice the size), and have larger black spots.

milkweed leaf beetle mating on swamp milkweed-

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.

labidomera larva-just hatched-

A small clutch of milkweed leaf beetle eggs had just hatched (egg cases still remaining).  This is the first of four instars of larval development before pupation.

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.

milkweed leaf beetle larva-small size-

I found only solitary individuals of milkweed leaf beetle larvae on the plants. This was one of the smallest — probably a second instar larva. Compare its size with that of the unopened flower to gauge how small it is.

labidomera larva-mid size

A doubling of size occurs at each larval instar stage. This individual might be a third larval instar, with its much more ovoid abdoment, and more prominent head and thorax segments (compared to the individual in the previous photo).

milkweed leaf beetle larva-pre-pupa-

This individual was almost as big as an adult and is probably a fourth instar, almost ready to drop to the ground and pupate in the soil or litter near the roots of the milkweed plant.

milkweed leaf beetle larva-pre-pupa-

A fourth instar milkweed beetle larva is still mobile enough to turn the corner at the tip of a very pointed milkweed leaf.

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.

3 thoughts on “Bugs in the garden

  1. What an informative post – thanks very much for all this detail, Sue! I actually had no idea there was such a beetle or such a phenomenon. Yikes! So, is this beetle also responsible for reduced numbers of monarchs, I assume?

    • No impact on the Monarch butterfly larvae really, especially if there is plenty of leafy material for all. The beetles are entirely herbivorous, and in addition to munching on leaves, they may munch on the roots of the milkweed.

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