This is a jukugo word that uses the on'yomi of the kanji. You learned this reading for 昆 when you learned the kanji, but you haven't learned the on'yomi for 虫 yet, so here's a mnemonic to help you:

Did you know that insects are Chewbacca's (ちゅう) favorite thing? He's fascinated by all types of insects, and keeps them in boxes as pets. There are a good few of them crawling around in his fur, too.