AS said kit lenses tend to have a pretty good close focusing aspect to them and for larger subjects like flowers and butterflies you can get some decent close up shots of them.
Manual focus can be a bit tricky with them though, since most are geared towards using AF and so have small focusing rings (at least mine is tiny) and they are not as finely controlled as a dedicated macro lens will be, but its certainly very usable.
If you want to get a bit closer you could look to getting some extension tubes which work by moving the lens away from the camera body and thus reducing the minimum focusing distance, letting you focus closer and thus get more insect into the shot. Note though that they will also remove the lenses infinity focusing as well, so you will be limited in how far off you can focus (a few cms to a few feet depending on the lens and the amount of tubes used). I would recomend getting a set of Kenko AF tubes over the ownbrand (Nikon) tubes since the Kenko are far better priced for what you get and since tubes have no optics in them at all you get identical performance no matter which you choose. However do make sure you get the tubes with contacts in them - the ultra cheap tubes (around 5$/£) might be very cheap, but they don't have any contacts in them at all, which means you won't have any contact from lens to camera, so not only is AF not an option, but you also cannot close the aperture blades on the lens either, which means you have to shoot wide open - not something that you want with macro work (typically done between around f8 and f13)