Down with the thistles

Canada thistle (Cirsium arvense), an invasive species from Eurasia, is common in this area.  However, so too are its enemies.  Even though the thistles have died down for the winter, I have found several Canada thistle bud weevils (Larinus planus) in the yard.  These weevils seem to be seeking out spots where they can stay sheltered until the spring.

A Canada thistle bud weevil (Larinus planus) sheltering on the lavender bush.

Another of the thistles' enemies has found a very cozy way to spend the winter -- within galls on the thistles' own stems. 

A gall on a Canada thistle stem.

As I was clearing dead plants out of the garden, I came across a couple of dried thistle stems interrupted by these large galls.  I was curious to see what, if anything, was still inside.  Cutting open one of the galls, I found several very odd larvae.

Inside the gall there were several larvae of the Canada thistle stem gall fly (Urophora cardui).

The larvae did not have anything resembling legs or even heads.  They also failed to show any sign of life.  Thus, I initially thought that I must have cut them in half.  However, on closer inspection, I determined that they were all whole.  Although strange to me, this form is normal for Canada thistle stem gall fly (Urophora cardui) larvae at this time of year.  In the fall, the larvae finish growing and suspend their development.  Only in the spring will they pupate and then emerge from the gall as adult flies, ready to parasitize a new generation of thistles.

A tall tail

How do wasps get their eggs inside other insects? First, the female wasp must find its host insect. This task may not seem difficult when the host is large and out in the open. However, many wasps parasitize insects that live deep within plants. The hosts of the wasp featured in this video are burdock seedhead moth (Metzneria lappella) larvae.

* To see this video in high definition (1080p), you may need to (1) click "YouTube" to watch on the YouTube website and (2) change the settings at the bottom of the video screen.

To detect the moth larvae feeding within burdock flower heads, the wasp (Agathis malvacearum) uses its sensitive antennae. Once a host has been located, the wasp pierces through the flower head and into the larva with its long ovipositor (from Latin, the "egg placer"). Although the ovipositor resembles a giant stinger, it is used to inject an egg rather than a painful dose of venom.  In fact, the stingers of other wasp and bee species are modified ovipositors; thus, only the females are capable of stinging.

This flower isn't big enough for both of us

Even when there isn't a crab spider lurking in ambush, a flower can be a rough spot for a bee.  Nectar and pollen are often in short supply and competition for these limited resources can be intense.  Bees start foraging very early in the morning (as long as it is warm and clear), but this is not enough.  Once a bee finds a rewarding flower, it must defend against newcomers that would poach its hard-sought prize.

Two bumble bees (Bombus spp.) competing for space on a flower kick at each other.

The coneflower (Echinacea sp.) pictured above wasn't big enough for the two bumble bees (Bombus spp.), which kicked out at each other until one abandoned the flower.  To see the full action of another bumble bee kick-boxing match, watch the video below.

* To see this video in high definition (1080p), you may need to (1) click "YouTube" to watch on the YouTube website and (2) change the settings at the bottom of the video screen.

The painted lady in distress

At the butterfly garden, monarchs weren't the only butterflies to find their favorite plants.  Painted lady butterflies (Vanessa cardui) had also been attracted to the garden and were busy drinking nectar from the many flowers.

Painted lady butterfly (Vanessa cardui) drinking nectar from a coneflower (Echinacea sp.).

Whereas monarchs specialize on milkweed, painted ladies prefer thistle (in fact, they are also known as 'thistle caterpillars').  Even when small, the caterpillars can be located easily due to the white silk 'tents' that they weave around themselves.  Once a caterpillar has eaten enough thistle, it forms a chrysalis and begins its metamorphosis into a butterfly.

Painted lady chrysalis (Vanessa cardui) hanging on a thistle, surrounded by silk webbing.

However, the tent does not protect the chrysalis from all enemies.  I found one chrysalis swaying in a gust of wind.  I waited until the wind died down so that I could get a better picture.  Surprisingly, even when the wind stopped, the chrysalis was still shaking violently.  Looking more closely, I saw that there was a small wasp sitting on it.

Painted lady chrysalis (Vanessa cardui) being parasitized by a chalcid wasp.

It was the touch of this wasp, a parasitoid of the painted lady, that was causing the chrysalis to shake.  If the wasp succeeded in laying its eggs despite the shaking, then in a few days it will be a new generation of wasps, not a butterfly, that emerges from this chrysalis.


Explore some more:
Charlotte Rhoades Park and Butterfly Garden