The ladybird and the cocoon of doom

About 10 days ago, while cleaning one of my stick-insect cages, I noticed an adult 7-spot ladybird Coccinella 7-punctata with a silky cocoon stuck between its legs. Ladybirds don't make cocoons like that - what could it be? Some small invertebrates can be truly difficult to identify, but this one was, for once, straightforward as it is a common parasite of several ladybird species, Perilitus coccinellae, a wasp in the family Braconidae. But, to be sure I put the cocoon in a hatchery and waited...

Perilitus coccinellae cocoon, approx 6mm long

An adult P. coccinellae (a female - males are unknown) did indeed emerge and provides a good opportunity to look at a common, but often overlooked parasite.

 

Adult P. coccinellae - note the wing venation and the needle-like ovipositor with the abdomen curled under the body

Adult P. coccinellae - a closer look at the ovipositor

Adult P. coccinellae - see how the abdomen is curled under the body

The wasp has an interesting life history (as parasites often do). Although ladybird pupae are sometimes targeted, adults are usually parasitised. The wasp taps the ladybird with her antennae then curls the ovipositor forward and begins to tap and probe for weak points, laying a single egg into the ladybird along with trophic ('nutritional') cells which absorb food from the host so the wasp larva can feed on them once it hatches. It does not feed on the ladybird directly, although the diversion of nutrients to the parasite ipacts on the host's reproductive development and fat storage. After reaching its 4th instar, the larva chews its way out from the underside of the ladybird's abdomen, having immobilised it, possibly by affecting the host's motor neurones. It then forms the cocoon which is attached to the ladybird's legs. The ladybird is alive at this point which means that the wasp is protected by its warning colouration and reflex bleeding defence. The wasp then pupates, emerges as an adult and seeks a new host as soon as its wings are dry. The ladybird usually dies within about a week, either of starvation or fungal disease. Some host ladybirds do regain the use of their limbs although most are greatly weakened and die soon after. In winter the cycle stops for a while as the wasp larva hibernates within a host ladybird.

So, a bad end for the ladybird, but an interesting tale - if you'd like to read it in more detail, try Majerus (1994) which contains plenty more interesting ladybird info.

Reference

Majerus, M.E.N. (1994). Ladybirds. HarperCollins, London. [New Naturalist series]