Macro Over-Exposure of Flower Petals: Any Suggestions?

Hi Everyone:

I am wondering how to handle a situation where the central parts of flower petals appear to be over-exposed (see below). These are Asters taken at the local botanical gardens. I don't like the effect. I am using an SD15 105mm macro lens at f18, 1/60th sec, and built-in flash with a diffuser in sunlight, shooting raw. I would like for the rendition to be more subtle in the sense that the petals should not have a burned out central part. Should I stop using the flash?

Thanks,

-Charles

(By the way, why do these images look so crappy? They look much better when I look at the originals. I uploaded them to my site folder and sited their URLs in this message. Is there a better way to show images? I tried to grab them from my gallery last year but dpreview was having problems, and I got too frustrated.)