A close up of a scale parasite

David uses his own garden as well as the Museum’s Wildlife Garden to monitor arrivals of new species

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Australian flies found in London

Cryptochetidae are a family of flies new to Britain, and its scale parasite fly (Cryptochetum iceryae) has been found in the Museum's Wildlife Garden.

David Notton, Senior Curator of Hymenoptera at the Museum, discovered two scale parasite flies in the Wildlife Garden meadow.

Scale parasite flies are native to Australia and belong to the family Cryptochetidae, representing about 30 species.

They are about two millimetres long and have a shiny, metallic body, a broad head and clear wings. No species from this family had been seen before in Britain.

David says, 'I'm very excited by the discovery of the scale parasite fly family in Britain, with this particular species being new to Europe as well.'

Cottony cushion scale

Cottony cushion scales (Icerya purchasi) are small pests also native to Australia, but are now widespread, having been moved about with the plant trade.

They eat more than 65 families of woody plants, many of which are common garden and greenhouse plants.

Cottony cushion scales infest twigs and branches. When mature, the females lay a large white egg sac, made of cottony, waxy secretions. The eggs hatch into nymphs which feed on the midrib veins of the leaves, causing the bulk of the damage.

Scale parasite flies are natural enemies of cottony cushion scales. The former are often introduced to specific infested regions in an attempt to control the latter, considered pests.

A cottony cushion scale

The cottony cushion scale lives by sucking the sap of its host, decreasing its vitality. It particularly likes citrus plants such as oranges and lemons. © Katja Schulz/Flickr.

Scale parasite flies as natural pest control

A scale parasite fly kills cottony cushion scales by inserting eggs inside its body. The larva feeds on the scale, killing it, then pupates inside the remains and bursts out of the body when it is ready to leave.

Despite their gruesome feeding habits, which is also how they get their name, scale parasite flies do not harm other insects or people.

Scale parasite flies have a short life cycle, lasting about a month in summer and two in winter. They therefore have between six and eight generations a year, twice as many as the cottony cushion scale.

Scale parasite flies have been deliberately introduced to California, South America and Israel as natural pest control. This has proven to be one of the most successful examples of biological control on record.

Their introduction to the UK is probably accidental, as they most likely arrived with ornamental potted plants. David hopes they can be used as a form of natural pest control here, too.

A scale parasite fly

Scale parasite flies are natural enemies of the cottony cushion scale - they locate and destroy even the smallest, most isolated host colonies © Protasor AN/Shutterstock

'Cottony cushion scales are increasing in Britain,' says David. 'They can now survive outdoors in southern England, probably due to increased winter temperatures.'

David also monitors country-wide patterns and changes in the UK's biodiversity.

'It's important to monitor insects because of the climate crisis and global trade,' he says. 'Many insect species are arriving in Britain from elsewhere, or changing their distributions.'