Thank you for the question.

We are reasonably confident that these are hazelnut aphids (Corylobium avellanae). Due to their appearance, sometimes they are called “wooly hazelnut aphids. When feeding on the plant juices of the hazelnut, they produce a sticky liquid called honeydew. This liquid attracts ants and a variety of flying insects including wasps and hornets.

If this aphid infestation is limited to a few shoots, it is unlikely that the plant will die. If many shoots are covered, or if leaves become sticky/blackened, then additional control measures should be employed:

1). Using a strong stream of water from a hose, try to “blast off” as much of this material as is feasible.

2). Prune out and discard the worst-covered shoots.

3). If there appears to be a heavy infestation, spray the tree with insecticidal soap or horticultural (e.g., Neem) oil. Either of these products can be purchased at a gardening center or greenhouse. Follow the application instructions on the product label. Repeat this application at least one additional time after a week from the initial spraying.

See the following for further information on these and related points:

https://pnwhandbooks.org/insect/nut/hazelnut/hazelnut-aphids

https://www.gardeningknowhow.com/plant-problems/pests/insects/woolly-aphid-control.htm

https://www.youtube.com/watch?v=IhcE2TiERHI

https://extension.umn.edu/yard-and-garden-insects/aphids

https://extension.oregonstate.edu/news/keep-aphids-under-control-low-risk-natural-strategies

https://bohart.ucdavis.edu/sites/g/files/dgvnsk4616/files/media/documents/note_42_woolly_aphids.pdf

Good luck. Please get back to us if you are additional, related questions.