I've watched recently a conference[1] by several renowned scientists. They affirm that neonicotinoids / neonics are responsible of at least 80% of the decline.
They also affirm that the "merchants of doubt" are working their best to muddle the evidence with things like "there are multiple factors", "it's complicated", "we need more research" etc.
But just like the overwhelming majority of lung cancer comes from smoking, the overwhelming factor in insects decline (and its consequences like birds decline) is PESTICIDES and neonicotinoids in particular.
[1]: sorry, in French : https://www.youtube.com/watch?v=6FQT7b2ExP4 Another one: https://www.youtube.com/watch?v=dLQKs1KBw9Y
Other articles:
https://www.nationalgeographic.com/animals/article/neonicoti...
https://journals.plos.org/plosone/article?id=10.1371/journal...