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.

Specialists

Isn’t it strange that a plant that contains so many nasty chemicals (e.g., cardiac glycosides), as well as rubbery latex so alkaline that it can permanently scar the cornea of one’s eye, has so many insects that specialize on it?

But here they are — the amazing milkweed fauna:  lepidopterans, bugs, and beetles, consuming every part of the milkweed plant from its roots to its seeds — all seen in the backyard this summer.

milkweed-monarch butterfly larva

The familiar Monarch butterfly caterpillar, munches away happily undeterred by the milky latex exuding from the leaves and stems of the plant.

milkweed tussock moth larvae

The less familiar Milkweed Tussock Moth larvae — there were so many caterpillars on this particular milkweed plant, they completely defoliated it.

milkweed tussock moth larva

The tussock moth larvae grows some very long tufts and is not quite so gregarious when it’s older.

milkweed bug adults and nymphs

Milkweed bugs (true bugs — Hemiptera) are usually found on milkweed plants that have formed seed pods. They lay a clutch of bright yellow eggs on one of the pods, and the nymphs develop through five molts into adults by feeding through the pod wall on the seed endosperm.

aphis-nerii-cornicle-and-braconid-wasp

Yellow aphids collect on milkweed stems and pods, but feed on the sugars passing through the plant’s phloem vessels, not the seeds. Small wasps (left center) parasitize the aphids by laying their eggs on the host.  Aphids are actually true bugs (Hemiptera), although these non-winged individuals don’t appear very bug-like.

milkweed beetle-Tetraopes sp

The Red Milkweed Beetle is a member of the long-horned beetle family. They lay their eggs near the ground, and the larvae burrow into the roots and develop and overwinter there to emerge as adults the following spring.  Like the monarch butterfly larvae, milkweed beetles incorporate the milkweed’s poisonous chemicals into their own bodies, becoming distasteful to their predators.

milkweed-leaf-beetles-mating

Milkweed leaf beetles are members of the very large leaf-beetle family. They eat the leafy greenery, but the larvae are also known for consuming each other — their cannibalistic tendencies reduce competition for food in their local area!

Isn’t it ironic that in producing poisons to ward off herbivores, the plant becomes more attractive to specialist herbivores also trying to avoid predation?