Moed Katan, Chapter One, Mishnah Four
The first section of the mishnah deals with trapping pests in a field and the second half deals with repairing breaches in a fence surrounding a field. Both of these may need to be done in order to protect the crops and therefore they may be permitted.
1) They may trap moles and mice in a tree-field or a white field in an unusual way during the festival and in the sabbatical year.
a) But the sages say: in the tree-field in the usual way and in the white field in an unusual way.
2) And they may block up a breach in a wall during the festival, and in the sabbatical year they may build it in the usual way.
Section one: According to the first opinion in the mishnah (this is probably Rabbi Eliezer b. Yaakov, the sage from yesterdays mishnah), one can trap moles and mice in the normal way of trapping them, from both a field of trees and a field of produce (called a white field). The normal way of trapping them seems to have involved digging a hole so that they would fall in. We might have thought that this was prohibited on the sabbatical year because it looks like plowing. On the festival it might have prohibited because it is a lot of work.
The sages are stricter. In a tree-field, where the moles and mice can do more damage, one can trap them in the usual way. However in a white field, where the loss that they cause is more minor, they can only trap them in an unusual way. According to the Talmud this means that they dig the hole in an unusual way so that everyone will know that the person is cognizant of the fact that it is either the Sabbatical year or the festival. This is another general principle we will see frequently in Moed Katan. Work which is forbidden may sometimes be done with a change, even though this change may cause the work to be more laborious. This seems to me to be a way that the rabbis could allow people to prevent a financial loss, while still making sure that they knew that it was a festival.
Section two: If a wall has opened up on the festival and the crops are left exposed to wild animals, it is permitted to make a temporary wall to close it up during the festival. It is forbidden, however, to build a more permanent wall because this is too much work. This is with regard to the festival. During the Sabbatical year it is permitted to build even a new wall because this is not similar to plowing. Only plowing and activities similar to it were prohibited during the sabbatical year, not all work involved in maintaining a field.