|Photo, Ben Smart
Feeds inside alder and Birch catkins, it may move from one catkin to the next through the side of the catkin.
Holes may therefore be apparent as in the photo.
Reddish frass may also be present.
The catkin itself will feel soft as it is being gradually hollowed out.
If you carefully open the catkin by pulling it at both ends, you will hopefully see the larva.
I am told they are quite easy to rear in a decent size pot (8x8cm). Just add a fresh catkin every 4-5 days, and remove the old one once vacated by the larva.
This is the first brood (April) only, the second brood (Sept) feeds in rolled leaves.