Hornet Moth

Sesia apiormis

SPECIES OF THE MONTH

August 2022

The Hornet Moth (Sesia apiformis) is a large moth native to the UK.

Their appearance is an excellent example of Batesian mimicry, where a harmless species evolves to look like one that is harmful to avoid predation. Looking like a hornet makes them unappealing to many predators based on a previous unpleasant experience with a hornet or similar species.

They are not common but can be found across Suffolk, with numbers varying from year to year.

Hornet Moths have clear wings that span 34–50 mm. They have yellow and black striped abdomens, but the number of stripes varies; females have two stripes, whereas males have three. Females are, on average, larger than males. They are the size of a Hornet but can be identified by their feathery antennae, smaller eyes, yellower colouring and chunkier body, lacking a waist between the abdomen and thorax.

Arthropod, Plant, Insect, Organism, Pollinator

Hornet

Hornet Moth

Lifecycle: Eggs

Eggs are brown and ovular, around 0.43–0.85 mm in diameter, and are usually laid around the base of an isolated tree or on the surrounding vegetation. Females fly around a tree continuously depositing eggs, laying hundreds to thousands at a time. Comparing the number of eggs to the number of adults shows significant mortality rates. CLICK DOTS BELOW FOR MORE...

Lifecycle: Larvae

Larvae hatch from September to May and spend two or three years living in and around the roots of host trees. Once ready to pupate, they bore a tunnel up to 10 cm long into the host tree trunk. They disguise their tunnel with a ‘door’ over the entrance made from a thin layer of bark. Once inside, they build a cocoon from silk and excavated tree material. CLICK DOTS BELOW FOR MORE...

Lifecycle: Pupae

Pupae are lined with rings of rigid spines (adminicula) that allow them to manoeuvre. Before adult moths can emerge, the pupae must make their way to the tunnel entrance. They bend and straighten, causing their spines to catch on the tunnel’s walls, helping them to wriggle their way back out of the tree, where they stay until the adult moth emerges. CLICK DOTS BELOW FOR MORE...

Lifecycle: Adults

Adults emerge between mid-June and July and expel liquid waste of up to 70% of their body volume within seconds of emerging. Observers have noted that males tend to emerge several days before females. Once they emerge, females spend several hours on the tree and typically do not begin to fly until after mating. Conversely, males begin to fly almost immediately in search of a mate.

Reproduction

Hornet Moths only have a lifespan of only a few weeks to reproduce. Therefore, females are ready to mate soon after emerging and release pheromones almost immediately to attract a male. They don’t appear to show courtship behaviour, with pairs mating as soon as they come into contact. Females mate several times with different males before laying eggs.

Distribution in Suffolk

Habitat

Hornet Moths prefer open habitats such as parks, hedgerows, golf courses, quarries, fens, pond edges and pits, often where trees are in open habitats.

Plant, Wood, Organism, Trunk, Font, Grass

Look for larvae exit holes at the base of trunks

Insect, Arthropod, Snail, Wood, Organism

Or empty cocoon casings

Terrestrial plant, Insect, Arthropod, Wood

Diet

The larvae feed on several species of trees, including Aspen, Black Poplar, Lombardi Poplar and Goat Willow. Adult moths prefer to feed around trees surrounded by heavy vegetation. It was found that trees near heavy vegetation had more significant numbers of larvae than those without vegetation.

Conservation

The UK population has been declining over the past couple of decades. While the adults are challenging to observe, the empty cocoons left partially protruding provide a proxy for the number of moths in an ecosystem. For example, in several sites around southern England, no new exit holes were found in trees with existing exit holes, suggesting local population extinction. This, as well as under-reporting, has led it to be classified as nationally scarce in the UK.

A Pest?

Hornet Moths have been considered a primary cause of the dieback of Poplar trees. However, recent evidence suggests that the moth is a secondary threat, with their tunnel-boring larvae magnifying the effects of drought and human influence.

Spotting Hornet Moths

As Hornet Moths only fly for a few weeks a year they can be hard to spot. Look around the base of Poplar or Goat willow trees to spot their larvae's telltale exit holes and empty cocoons.

If you are lucky enough to spot Hornet Moths don't forget to record your sighting!

Find out more

at Suffolk Moths

Created by:

Suffolk Biodiversity Information Service