No, unfortunately, you can't do anything. The law lets you take action against "nuisances" created by neighbors, but a nuisance is an action, use, etc. which is not reasonable and appropriate for the location. Having your lawn chemically sprayed, however, is reasonable and appropriate; the fact that you may be unusually sensitive to the chemicals does not turn a common action into a nuisance--the law judges appropriateness by the average or "reasonable" standard, not by the standard of the most senstive person who may be affected--and therefore would not give you legal grounds to complain or take action.