Species Neodiprion nanulus
Keys to the insects of the European part of the USSR. Vol. 3. Hymenoptera. Part 6. By Zhelokhovtsev A.N., Tobias V.I., Kozlov M.A.
Keys to the Fauna of the USSR 158: 1-432, 1993
English translation from the 1988 Russian edition [which i use].
Covers the 'Symphyta' (close to 900 spp.) and a number of minor taxa of parasitic wasps (Trigonaloidea, Stephanoidea, Evanioidea, and Paxylommatidae). Somewhat outdated yet thorough treatment with lots of useful info.
The Hymenoptera of Costa RicaBy Paul E. Hanson and Ian D. Gauld, editors
Oxford University Press, 1995
A comprehensive treatment of the biology, systematics, and diversity of Hymenoptera in Costa Rica. Keys to superfamilies, families, subfamilies and in many cases, genera are included. Given the high level of biodiversity of Hymenoptera in Costa Rica in comparison to most areas of the world, this is a valuable reference book for students of Hymenoptera in any part of the world.
Order Hymenoptera. In: Zhang Z-Q (ed) Animal biodiversity: An outline of higher-level classif. and survey of taxonomic richnessBy Aguiar AP, Deans AR, Engel MS, Forshage M, Huber JT, Jennings JT, Johnson NF, Lelej AS, Longino JT, Lohrmann V, Mikó I, Ohl M.
Zootaxa 3703: 51–62, 2013