I can currently attack up to level 24 bugs but I don't know what to improve to alow me to attack level..

Don't quote me on this, as my answer may be inaccurate, but to the best of my knowledge this actually depends a bit on which server you are a part of. In server 11 (which is the server I am a part of) you can only go after the next level of bug after successfully defeating the current bug level you are up against. (You said that you are currently able to attack level 24 bugs, so you should be able to attack level 25 bugs after you successfully complete an attack on the level 24 bugs.)