What insects can do to mongrel and parrot at home upset (5-9)

'what insects can do' is the definition.
The definition suggests a singular noun but the answer is not.

'mongrel and parrot at home upset' is the wordplay.
'mongrel' becomes 'cross' (cross breed).
'and' is a charade indicator (letters next to each other).
'parrot' becomes 'poll' (poll is a kind of parrot).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'upset' becomes 'ate' (to eat up is worry or upset).
'cross'+'poll'+'in'+'ate'='CROSS-POLLINATE'

'to' is the link.

Can you help me to learn more?