The fireflies are under the protection of the spider webs. You need to remove that protection in as few moves as possible.
It doesn't really matter if you remove the spiders until later in the game, its the webs that we are more concerned with at the moment. The less webs, the more room you have for combinations etc.
Try clear the ones on the logs and the bottom row so that once the logs are clear, you can start making matches to get the fireflies.
You can use shovels on them if you are running out of moves.
Combinations and getting extra points will help you get all the crops you need. You will need to do this a lot because quite a few of your moves will be wasted on breaking webs.