How to analyse a experiment on repellence

Dear all,
I have a fairly simple experiment with a petri dish, some worms and two treatments (water, anti-worm treatment). I segmented the petri dish into 8 sectors and repeated the experiment with 6 dishes in two independed trials. For each experiment some worms will be released in the middle of the dish and the influence of the worm treatment (repellency) will be observed after an hour. So I though of a simple SAS / R model like this:
Y(worms) = treatment + trial + sector + treatment*trial*sector

So I did a typical variance analysis, however, I feel that there is a better way to include the spacial factor into the model. Does anyone have an idea?
Thank you for thinking!