How to say "a beneficial insect"? I want to be able to say to my son that an insect is beneficial.

I know I can use 害虫 for an insect that is harmful, but is there something I can use to say beneficial other than 好虫 which I assume is incorrect.
I would say 益虫．　This is related to the compound 益害 (literally - beneficial and harmful), which means effect (positive or negative). For example, you can say 喝咖啡的益害　（the effects of drinking coffee). 

Chinese has many similar antonymic compounds like this. Some examples include 高矮　(height of a person, or literally - tall and short), 长短 (length, or literally - long and short), 大小 (size, or literally - large and small), or 深浅 (depth, or literally - deep and shallow).