Are newly colonised parasitoids responsible for the decline of the Small Tortoiseshell?

Butterfly Conservation has become concerned that a parasitoid fly that has arrived recently in the UK may be causing widespread mortality to Small Tortoiseshell caterpillars in southern Britain. Numbers of Small Tortoiseshells recorded on transects were extremely low in 2006 and 2007, and the parasitoid is one potential explanation for this worrying trend.

To investigate the effects of Sturmia bella, we are asking volunteers to collect batches of final-instar wild larvae of Small Tortoiseshell and Peacock butterflies this summer, and to rear them in appropriate cages to determine how many die from parasitism. Emerging butterflies can be released into the wild, and any emerging parasitoid pupae should be sent to us for identification. We are hoping to collect data from as many localities as possible across the UK, and we will be linking information on the frequency of parasitism from Sturmia bella and other parasitoids to information from the UK Butterfly monitoring Scheme on trends in butterfly population sizes.

Collect a sample of approximately 30 final-instar caterpillars from each group of caterpillars you find. If there are fewer than 30 individuals then please collect them all. Remember you are not harming the butterfly population as you will be able to release surviving butterflies within a few weeks. However, you should not collect from nature reserves or other protected sites without permission.

To standardise results, for both species we would like you to collect only the large, final instar caterpillars. If you find younger caterpillars near your home, you may be able to return to the same location a later time to collect them.

At the time of collection, please note down:

Rear the caterpillars in appropriate cages under conditions as similar to outside as possible (e.g. in an unheated naturally cool outbuilding or shed; otherwise your coolest indoor room). Even if caterpillars have been collected from more than one group in close proximity, please keep the caterpillars from each group of larvae separately, so that information can be supplied independently for each group.

Caterpillars can be reared on cut young-nettle shoots standing in water in a well ventilated cage (such as a terrarium or net butterfly cage). Please ensure the caterpillars cannot drown themselves in the jar of water either by wrapping kitchen roll tightly around the bunch of nettle stems and pushing the whole lot into the jar, or by plugging the jar with a crumpled plastic bag or rag. Replace the nettle stems when they start to wilt, ensuring that no caterpillars are removed with the old food (also, check any food-plants introduced to the cage for other insects or predators, and remove them before introduction to the cage). Ensure that the cage keeps the caterpillars in, and anything else out!

Alternatively, caterpillars can be kept in airtight transparent plastic boxes with loose nettle stems. If opting for plastic boxes, please ensure that the box is never in direct sunlight, and that any condensation is promptly removed - lining with absorbent paper such as loo roll is essential. The foodplant should be changed regularly (every 2 days is ideal), and the box kept clean to avoid disease. Nymphalid larvae pupate hanging down vertically, and so the container needs to be tall enough to do this so that the pupae do not come into contact with anything below. When the caterpillars are entering the pupal stage, avoid touching or moving them until they have pupated and the pupa hardened.

Make a note of the number of butterflies that emerge, and the number of pupae which fail to produce anything. Emerging butterflies should ideally be released close to where they were collected. Anything else which emerges from the host pupae should be sent to us for identification, along with information about the specimen (see below). In addition to several species of parasitic flies (family Tachinidae), we also expect to find a variety of parasitic wasps, some of which are very tiny.

We are particularly interested in the parasitic flies. If the caterpillar has been parasitized by Sturmia bella a medium sized whitish larva will emerge instead of the butterfly. Usually only one parasitoid larva emerges from a butterfly chrysalis (rarely two), descending from a silk-like thread to form its own dark brown puparium. Each parasitoid puparium or cocoon should be removed and kept in isolation as soon as detected. Sturmia emerges from the host after pupation while the chrysalis is still soft. It often pushes out from beneath the wing buds, which drop back down and then dry "normally". For many chrysalises it is very hard to tell that Sturmia has emerged once the chrysalis is dry. As a result, it is possible that puparia on the floor of the cage may be overlooked, and the host remains may look ‘normal.’ It is therefore important check for anything in the tissue paper on the floor of the cage when it is changed, and look through the frass for parasitoid puparia.

Collect everything that relates to the parasitoid puparia (e.g. thread) as this can help identification, and mail these to us along with the puparia.

(4) Mailing puparia and reporting your results

Please mail any parasitoid puparia to us. We will hatch out the adult flies or wasps, but we should be able to identify them even if they die or emerge before reaching us. The specimens should be double packaged, first in a small container in a way which prevents the puparia rattling and becoming damaged (for example in a short section of plastic tube lightly packed with tissue paper). We can send suitable containers on request. The outer layer of packaging needs to be strong and crush proof, such as a small cardboard box, and the space between the inner and outer packaging layers should be well padded. Please send specimens by first class post to:

Dr Owen Lewis, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS

With your package, you should also report, separately for each larval group:

These are also the email addresses to use if you have any queries at all about this exercise.

Thank you for your help!