You have to mechanically check under loose bark, in dead wood or to search green plants for the larvae. It depends on which groups of the Buprestidae are you speaking about.

There is no a single method to search for buprestid larvae. Every plant and buprestid life-form as well as the species and systematic group has its own peculiarities (see Bily, 2003). Julodine larvae live in the soil near the roots of different plants (for example, Alhagi) and pupate within soil cases, however there is no way to spot them in the soil without digging. Leaf-mining larvae (only Trachys and ?Habroloma in Iraq) can be found within the leaf mines on different plants. Xylophagous larvae can be found under the bark or within a sapwood of usually sick or dying (but sometimes apparently healthy) trees and bushes; sometimes infested area marked with exit holes on the surface or gall-shaped swelling (usually on the live branches or roots, see Korotyaev et al., 2005). Rhizophagous larvae can be found within the roots based on the same symptoms but many of them also live in the roots of herbaceous plants (some Acmaeoderella, Sphenoptera). Larvae of  stem-boring Aphanisticini develop within the stems of herbaceous plants, their presence can be indicated by partly drying stems or usually black eggs on the stem surface. At last, Cyphosoma larvae develop inside root tubers of Bolboschoenus maritimus (L.) Palla (Cyperaceae) (see https://www.zin.ru/animalia/coleoptera/rus/cypeupbi.htm or https://www.researchgate.net/publication/275823293_Volkovitsh_BA_Korotyaev_2007_Biology_of_larva_of_Cyphosoma_euphraticum_Buprestidae_in_Russian on this site).

Good luck!!! 

Literature

Bílý S. 2002: Summary of the bionomy of the Buprestid beetles of Central Europe (Coleoptera: Buprestidae). Acta Entomologica Musei Nationalis Pragae, Supplementum 10: 1-104.

Korotyaev B.A., Konstantinov A.S., Lingafelter S.W., Mandelshtam M.Y., Volkovitsh M.G. 2005. Gall-inducing Coleoptera. Pp. 239-271. In: Raman A., Schaefer C.W., Withers T.M. (Eds.). Biology, ecology, and evolution of gall-inducing arthropods. Vol. 1. Science Publishers, Inc., Enfield (NH), USA, Plymouth, UK. 817 pp.

Data Volkovitsh, B.A. Korotyaev. 2007. Biology of larva of Cyphos...

Dear Wand Khalis Ali

Usually it's depend on the spp. , host plant…

Pls. find the attached files

http://link.springer.com/article/10.1007/BF00379911

http://academic.evergreen.edu/projects/ants/tescbiota/kingdom/animalia/phylum/arthropoda/class/insecta/order/coleoptera/family/buprestidae/home.html

http://eol.org/pages/7473/details

https://www.ncbi.nlm.nih.gov/pubmed/20021763

https://www.cambridge.org/core/journals/canadian-entomologist/article/estimates-of-emerald-ash-borer-coleoptera-buprestidae-larval-galleries-in-branch-samples-from-asymptomatic-urban-ash-trees-oleaceae/880636A2E9D2971EF00C7D09017B721A

https://www.cambridge.org/core/journals/bulletin-of-entomological-research/article/physiological-and-ecological-studies-of-the-species-of-capnodis-in-palestine-col-buprestidae-iistudies-on-the-larvae/BD69EBA1094D2E0ED05B72499136DA44

https://www.cambridge.org/core/journals/canadian-entomologist/article/detection-and-sampling-of-emerald-ash-borer-coleoptera-buprestidae-infestations/671D5F7160E19CDA09A4159D4B903A1B

http://www.bioone.org/doi/abs/10.1603/EN12334

page 26 in:    Darrell Addison Posey, Kristina Plenderleith&Kristina Plenderleith ed. 2004.  Indigenous Knowledge and Ethics: A Darrell Posey Reader Psychology Press,Pp.Pp.274. ISBN0415323630, 9780415323635

http://bup.xoom.it/where.htm

http://www.nrs.fs.fed.us/pubs/jrnl/2008/nrs_2008_bauer_002.pdf

http://borneobeetles.myspecies.info/content/buprestidae-leach-1815-0

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5320268.pdf

Hoping this will be helpful

Regards

Prof. Houda Kawas

Dear friends and colleagues, I'm really very thankful to all of you what you provide were very useful  thanks again 

You most welcome

Houda

Most Buprestidae species are host specific, so it depends on which species you want to collect, for instance for the large Euchroma gigantea larvae you must search on Ceiba pentandra trees, which is the main host plant in the Neotropical region.

Buprestidae will tend to infest certain species of trees. The species vary from one region to another. Look for freshly infested trees. this you can check for exit holes on the stems or branches.  I have managed to recover a number of  Buprestid species from Acacia species which have been cut or dying. They rarely attack fresh trees.  They tend to be just below the bark of the tree. Many don't survive once exposed or removed from the stems.