Question: Is it permitted to kill flies or other insects which are causing discomfort on Yom Tov?
Discussion: The melachah of shocheit, or netilas neshamah, which refers to taking a life, is permitted on Yom Tov for the sake of ochel nefesh. For that reason, it is permitted to slaughter animals or fowl for consumption on Yom Tov. The principle of mitoch, which dictates that since a melachah is permitted for ochel nefesh, it may be performed even for other purposes, applies to this melachah.122 It could be argued that killing flies, mosquitoes, or other insects is permitted under this rule since it is being done for the sake of a legitimate Yom Tov need. Indeed, some Poskim123 suggest that it might be permitted to kill flies or bees if they are causing a disturbance. Nevertheless, it is actually prohibited for a few reasons:124
Firstly, although slaughtering is permitted on Yom Tov, trapping is not. Therefore, practically speaking, one may slaughter only animals that have already been captured. Since the fly is not yet trapped before killing it, killing it would be tantamount to trapping it125 (aside from the melachah of netilas neshamah) and, therefore, prohibited on Yom Tov.126
Secondly, as other Poskim explain:127 Slaughtering an animal in order to eat its meat provides a positive benefit, whereas killing mosquitoes does not provide a positive benefit; rather, it removes a disturbance. Therefore, although a melachah may be performed for purposes other than food preparation (based on the principle of mitoch), perhaps that is only when there is a positive benefit, not the removal of a disturbance.
Thirdly, some Poskim128 voice the concern that one might not discriminate between those pests which are actively disturbing and those which are not. Consequently, one may end up killing even those pests which are not permitted to be killed on Yom Tov.
122 Mishnah Berurah 518, note 1.
123 Melachim Umanayich (11, footnote 4) cites R’ Ya’akov Kamenetzky as permitting this. However, see Emes LeYa’akov 518, footnote 478, where R’ Ya’akov Kamenetzky is reported to have left the issue unresolved.
124 This is clearly implied by Mishnah Berurah 533, note 20, who permits killing flies that are disturbing someone on Chol Hamoed – indicating that on Yom Tov it would not be permitted.
125 See Ra’aviah 194, and Orchos Shabbos 14, footnote 3, citing R’ Yosef Shalom Elyashiv.
126 See Yom Tov Kehilchaso 3:33. Although trapping is sometimes allowed in order to prevent pain, it is not allowed in order to prevent minor discomfort; see 316:9.
127 Melachim Umanayich, ibid.; Chut Shani, page 114.
128 Minchas Shlomo Tinyana, 60:28.