Emperor Moth (Saturnia pavonia) – Complete Care Guide

This guide outlines the full care process for the Emperor Moth (Saturnia pavonia), the UK’s only silkmoth. This species is a favourite for many and can often be seen in the wild - in flight from March-May and caterpillars from April/May until August, depending on local conditions. Adult moths are dimorphic, with males occurring in shades of brown and females in grey, although both have similar patterning with an eye-spot on all four wings.

Egg & Hatchling Care

Saturnia pavonia eggs should be kept in an airtight plastic container to maintain stable conditions - Temperature and humidity are important factors when hatching eggs.

The eggs are typically grey, and their colour and shape remain consistent throughout development. A small dent in the eggs is normal, however a total collapse of the egg indicates they are infertile or have died.

Kept at room temperature, eggs hatch in around two weeks, though they can take up to 3 weeks depending on temperature. Ambient humidity is usually sufficient, best maintained by storing eggs in an airtight plastic container. Open the container once per day to prevent stagnation.

Importantly, do not add host plant leaves before hatching, as leaves release moisture and promote mould growth.

When the eggs begin to hatch, move caterpillars gently using your finger or a paint brush (whichever you are most confident with) to move them to a new container.

Host Plants for Saturnia pavonia

The best host plants for pavonia include a wide range of deciduous shrubs and trees commonly found across the UK:

  • Willow (Salix)
  • Hawthorn (Crataegus)
  • Birch (Betula)
  • Oak (Quercus)
  • Cherry, Plum and other stone fruits (Prunus)

Multiple others are reported to be accepted but these are have produced the best results for us in the past. In heathland the larvae also eat Heather. Also note that you can change the host plant when rearing this species, as they are far less selective than others. A varied diet may even produce larger caterpillars at maturity.

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Fourth Instar Larvae

Two caterpillars in the same stage (L4) displaying distinct colour forms.

Rearing larvae

The larval (caterpillar) stage is straightforward in this species when their needs are met. After moving hatchlings to a new container you can now provide food material. Line the container with paper towel to absorb condensation as this will prevent mould buildup leading to disease in the caterpillars.

Rear larvae in suitably-sized containers for the first and second instars, then move them to mesh cages once they have shed their skin and entered the third stage. It is important to open the plastic containers once or twice a day to refresh the air - do not allow condensation to gather.

Use cut stems in jars to provide food for your larvae once transferred to mesh enclosures. If there are gaps where water is exposed, fill these with tissue to prevent larvae falling in and drowning. This kind of enclosure provides optimal ventilation which this species seems to particularly benefit from. Larvae may drink water droplets if misted occasionally, which is useful if foliage is drier.

The caterpillars will exhibit a variety of colour forms in the fourth and fifth instars with varying degrees of black and in shades of green. They can have either yellow or purple tubercles in the final instar, becoming large and finally turning an orange colour when the larva reaches maturity.

The caterpillar may begin to spin a cocoon among the host plant stems and leaves, or it may wander for a while before selecting a location on the enclosure sides (usually the corners) for pupation. The cocoon has a round shape with a clear opening covered by fine silk, from which the moth will emerge next spring.

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Caterpillars

This image illustrates the variability in Emperor Moth caterpillars - all are very colourful! (A mixture from L3-5 feeding on Birch and Oak)

Saturnia pavonia Eggs

Caring for Pupae (Cocoons)

Allow the caterpillar to complete constructing the cocoon and pupate before attempting to move them. You can leave the cocoons in a container at room temperature for the rest of the summer, then refrigerate them from the autumn until early spring - this process of cold storage breaks dormancy and development begins once the temperature warms.

A mesh cage is probably fine to incubate cocoons of this species after removal from winter storage, as they are resilient to a wide degree of environmental conditions. Ensure you mist cocoons daily.

If you struggle to maintain temperature and humidity at suitable levels, a polystyrene box is ideal for keeping cocoons for emergence. Most of the cocoons will have emerged within 2-3 weeks kept at room temperature, though it is normal for some to emerge a bit more randomly.

Caring for Adult Moths

Being a member of the Giant Silk-Moth family, Saturniidae, the Emperor Moth does not eat during adulthood - it lives on fat stored during the larval stage - meaning its adult lifespan is only around one week.

This is a diurnal moth (active during the daytime), with most flight activity taking place in late morning through early afternoon. Males can fly for miles in pursuit of a female, detecting them by the pheromones they release. After mating, females deposit around 200-250 eggs. As a native species, these can be released, supporting populations local to you.