Accurate monitoring of butterfly populations serves as a valuable indicator of ecosystem health, helps determine the impacts of environmental changes, and can contribute to the conservation of threatened species. However, several species are difficult to observe and researchers now turn to earlier stages in the lifecycle.
In Surrey & SW London Branch we have successfully trialled nocturnal surveys for caterpillars of one of our most elusive species - Black Hairstreak. This technique uses an interesting property of the caterpillars - they glow under UV torchlight. However, not all caterpillars glow under UV light and some, e.g. Purple Hairstreak, only glow via a 365nm UV torch.

A green leaf on a branchDescription automatically generated 

Black Hairstreak caterpillar under UV torchlight

A blue bug on a red leafDescription automatically generated

A close-up of a bug on a leafDescription automatically generated

Purple Hairstreak caterpillars under UV torchlight at 365nm (top) and 385-395nm

Larvae in our area tested so far

UV Photoluminescence
Species Wavelength
nanometers
Ovum Larva Pupa
Adonis
Blue
365   x  
385-395   x  
Black
Hairstreak
365 x Y x
385-395 x Y x
Brown
Hairstreak
365 x Y x
385-395 x Y x
Chalkhill
Blue
365 x x x
385-395 x x x
Clouded
Yellow
365   x x
385-395   x x
Green
Hairstreak
365 x Y x
385-395 x Y x
Holly
Blue
365 x Y x
385-395 x Y x
Orange-tip 365 x x x
385-395 x x x
Purple
Hairstreak
365 x Y x
385-395 x x x
Small
Copper 
365 x Y x
385-395 x x x
Small
Tortoiseshell
365   x x
385-395   x x
White-letter
Hairstreak
365 x Y x
385-395 x Y x

Dos & Don'ts

UV protective glasses must be worn at all times.
Never shine the UV torch into anyone’s eyes.
A standard torch is also essential and we advise against going out alone.
Ensure you have the landowner’s permission if required.
UV wavelength of 365nm and minimum power of 15W is recommended.

Further information
Gareth Tilley: [email protected]