Parasitism

What is a parasite? 

A parasite is an animal or plant that lives on another animal or plant while it is still alive – so for example a flea is a parasite – it lives on its ‘host’ animal, your dog or cat, and sucks its blood. It doesn’t usually kill the host – unless it spreads a fatal disease through the bite – because it is not in the parasite’s interest to do that. If the dog died when the flea bit it, the flea would have to find another host to live on! Fleas and ticks live on the outside of the host animal so they are called ectoparasites (ecto- means outside).

Organisms that live on the inside of animals and plants are called endoparasites (endo- means inside). Butterflies and Moths have  both ectoparasites and endoparasites.

Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum.

 (this was adapted by a Dutchman called Augustus de Morgan from a verse by Jonathon Swift (who wrote Gulliver’s Travels).

Ectoparasites on Butterflies and Moths.

Certain little mites live on the outsides of the insects’ bodies, sucking their body juices through mouthparts rather like ticks do.

It is thought that these ectoparasites do not cause the death of their hosts, although insects with several on them do seem weaker and perhaps more likely to die from other causes.

These tick-like parasites are feeding on the internal fluids of the Large Skipper (on the left) and the Meadow Brown.

Endoparasites in Butterflies and Moths

Many butterflies and moths are attacked by tiny parasitic wasps or flies which lay their eggs in the host – in the egg or the larva or occasionally the pupa – so that when the eggs hatch out, the tiny parasite larvae eat out the insides of the host and eventually kill it. Instead of a butterfly or moth emerging from the pupa, more of the parasites hatch out instead. The parasite larvae leave the main organs of the caterpillars until last so that their dinners on legs do not die before they are ready for them to!

Many of these parasites are particular to single species of butterflies and moths, so if the host insect becomes rare, the parasite will too.

The story of the Holly Blue Butterfly

The Holly Blue butterfly is usually the first of our blue butterflies to be seen on the wing in the spring after they emerge from pupation in March or April. They lay their eggs on holly flowers mainly, but will lay on Dogwood, Spindle or Gorse too.

In the gallery below, you will see a series of photographs of a Holly Blue being parasitised by a tiny wasp called Listrodromus nycthemerus, less than 1cm long.

This parasite Listrodromus nycthemerus only lays its eggs in Holly Blue eggs – it is ‘specific’ to the Holly Blue and cannot survive in any other butterfly or moth. In some years the Holly Blue is extremely scarce, so the parasitic wasp will be scarce too.

Another wasp that lays its eggs on Holly Blues has recently arrived in the UK from southern and eastern Europe and it has a different method of breeding from the first parasite. This tiny wasp is called Cotesia inducta and lays its egg on the very small larva of the Holly Blue not the egg. The wasp larva hatches out inside the Holly Blue caterpillar and eats it. The Holly Blue caterpillar only reaches about one third full size before the wasp larva breaks out of it and spins its own silken cocoon on the leaf beside the caterpillar’s dying body. After 2-3 weeks the tiny wasp emerges from this cocoon.

How do the parasitic wasps affect the numbers of Holly Blues?

Results of a study of Holly Blue larvae in Bedfordshire 1990 – 2005 - many thanks to Richard Revels for permission to use his results and his photos on the gallery page below.

Holly Blue larvae were collected and reared in captivity to see how many of them hatched into healthy adults and how many produced parasitic wasps.This table is also to be found as a printable document on this link.

Holly Blue larvae were collected and reared in captivity to see how many of them hatched into healthy adults and how many produced parasitic wasps.

Use the information on this table to produce line graphs showing how the numbers have changed between 1990 and 2005, using a blue line to show the numbers of healthy Holly Blues hatching each season and a red line to show the numbers of the parasitic wasp Listrodromus nyctheremus. From summer 2004, add a green line for the other parasitic wasp Cotesia inductus.

Now try to answer the following questions

1. What can you learn from the pattern of the blue and red line graphs?
2. In which year did the new parasite Cotesia inductus appear in Bedfordshire?
3. Holly Blue adults emerge from pupas in the spring and the late summer, then lay eggs which hatch into larvae on Holly flowers in the spring and Ivy flowers in the late summer. It may not just be the parasitic wasps that determine how many larvae are found each season. Make a list of all the factors you can think of that might affect how many Holly Blue caterpillars are found by the researchers.

While the parasitic wasps are inside the caterpillars eating away their body organs, they themselves are not safe from attack. There is another tiny wasp which can identify which Holly Blue caterpillars have parasites inside them and can lay its eggs through the body of the caterpillar into the parasite so that instead of the parasitic wasp Cotesia inductus hatching out, another even smaller wasp emerges from the dead wasp grub inside the dead butterfly larva………

These parasites of parasites are called hyperparasites.

The Small Tortoiseshell Butterfly

The Small Tortoiseshell caterpillars have spines on their bodies and nasty tasting chemicals in them to protect them from predation by birds and mammals, but they are attacked by at least two types of Parasites which lay their eggs in them and hatch out before the caterpillars have a chance to pupate. One of these is a parasitic wasp similar to the one which attacks the Holly Blue, but there is also a small fly – a two-winged fly called a Tachinid – which lays its eggs on the Small Tortoiseshell larvae.

Parasitic wasps in Peacock butterfly larvae

The parasitic wasp called Phobocampe confusa lays its eggs in the larvae of Peacock butterflies. The photo on the right shows the adult wasp after it has emerged from its pupal case.

The wasp grubs emerge from the dead caterpillars and form pupae that look like tiny birds eggs.

and another view. Two empty caterpillar skins and two wasp pupae.

The Large White

Many young caterpillar collectors have been very disappointed to find that instead of their Large White larvae climbing up their jam jars and pupating on the lid, the caterpillars gradually grind to a halt in their final stage and masses of yellow cocoons burst out of them. These will hatch into the tiny wasp Apanteles glomeratus – they are so tiny that up to 80 wasps can hatch out of one caterpillar!

This Fritillary larva has been parasitised by a tiny wasp which has laid its eggs in it. These have now completed their larval stage and emerged as cocoons from the caterpillar killing it in the process.

Painted Lady caterpillars have their parasites too

The parasitic wasp larvae have killed this Painted lady larva and have emerged to form the fluffy pupae in a cluster beside the corpse.

The Holly Blue and its parasites

The first part of the gallery of photos below illustrates the stages in the life cycle of the Holly Blue butterfly and the parasitic wasp which lays its eggs inside the larvae of the butterfly.

Many thanks to Richard Revels for permission to reproduce his photos on this page.

The second part of the gallery shows several other caterpillar species with their parasitic wasp cocoons - grateful thanks to Richard Comont for these images.

Click on the first picture then scroll through the gallery by clicking on 'next'.

Holly Blue egg laying.

Holly Blue egg.

Holly Blue larva

Holly Blue Parasitic wasp.

Holly Blue parasite and pupa.

Holly Blue larva with wasp pupa.

Holly Blue parasite with empty pupa.

Ground Lackey larva with parasitic wasp cocoons.

Adonis Blue larva with parasite cocoons.

Adonis Blue larva with parasite cocoons.

Brimstone butterfly dead larva + parasite cocoon.

Brown Argus dead larva with cocoons of parasite.