Vegetables and fruits which rarely contain insects need not be checked. Vegetables and fruits which sometimes contain insects (even if a majority of the time they do not) must be checked. This is referred to in halacha as mee―oot hammasui. (Meaning: even though in the majority of instances, insects will not be found—they are present often enough to be of significance) 10.
In the case of a vegetable or fruit which rarely contains insects, which, under normal circumstances need not be checked: If three or more insects are found in a batch, the rest of it must then be checked 11.