Moed Katan, Chapter 1, Mishnah 4. This mishnah deals with two topics: trapping moles and mice in a field on Chol HaMoed and during the Sabbatical year, and repairing a breach in a wall.
"Tzadin es ha'ishus ve'es ha'achbarim mi'sdeh ilan u'mi'sdeh lavan, shelo k'darko, ba'moed u'va'shevi'is" - one may trap moles and mice that damage a field, provided that it is done in an unusual manner:
"Mi'sdeh ilan" - a field of trees, that is, an orchard.
"Mi'sdeh lavan" - a field that grows grain.
"Shelo k'darko" - in an unusual manner: they would strike the ground with a hoe rather than set up a trap in the ordinary way.
"Ba'moed u'va'shevi'is" - this permission applies both on Chol HaMoed and during the Sabbatical year.
The reason for the leniency is that these pests are liable to cause damage to the field. Therefore trapping them was permitted, even though on Chol HaMoed it involves exertion, and even though during the Sabbatical year digging in a field looks like preparation for plowing.
The view of the Sages:
"Va'chachamim omrim: mi'sdeh ilan k'darko, u'mi'sdeh lavan shelo k'darko". In their view one must distinguish between the two types of fields:
From an orchard - in the ordinary manner: in an orchard one may trap in the usual way, and may even set up an actual trap on Chol HaMoed and during the Sabbatical year, because the damage there is extensive.
From a grain field - in an unusual manner: in a grain field one may trap only in an unusual manner.
Repairing a breach in a wall:
"U'megadrin es ha'pirtzah ba'moed" - a breach that opened in a garden wall may be closed up on Chol HaMoed only in a temporary fashion. This distinction stems from the nature of the place:
A garden: the purpose of the wall is to keep people out, and there are no items in it liable to be stolen. Therefore one may place stones one atop the other, but may not use mortar to join them, since that would make the closure permanent.
A courtyard: a courtyard wall that was breached may be repaired in the ordinary way on Chol HaMoed, because of the concern that thieves may enter.
"U'va'shevi'is - boneh k'darko": during the Sabbatical year one may build up the breach in the ordinary way. Although at first glance this appears to be guarding the produce, which is forbidden during the Sabbatical year - since the produce must be left ownerless and available to all - nevertheless, since the intent is only to protect the land and not to guard the produce, it was permitted in the ordinary manner.
In summary: in this mishnah we learned that one may trap moles and mice that damage a field on Chol HaMoed and during the Sabbatical year - according to the first opinion, in an unusual manner in any field, and according to the Sages, in the ordinary manner in an orchard and in an unusual manner in a grain field. We also examined the laws of repairing a breach: in a garden, only a temporary closure; in a courtyard, an ordinary repair because of thieves; and during the Sabbatical year, ordinary building, since the intent is to protect the land and not to guard the produce.