What is the ruling on using an insect zapper device for flies, mosquitoes, and the like?
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Question : Honorable Shaykh, what is the ruling on using an insect zapper device for flies, mosquitoes, and the like? They are widespread, especially in mosques. If the user is an elderly person who cannot drive these insects away from himself except with it, is it forbidden?
Answer : No, there is no harm in it. First, this does not burn them; rather, it shocks them, just as if a person touched an electric wire and died. But is that considered [killing]?
It is not considered [killing]; it is of this type. Second, if the only way to benefit from removing this harm is with this device, even if it were to burn them, there is no harm. Have you not seen the Prophet who was stung by an ant and burned the entire ant colony? Allah the Exalted said to him, "Why not just one ant?" and they did not reprimand Allah for the burning, because he wanted to do this to defend himself. But Allah the Exalted rebuked him for burning the entire colony. So long as it is to repel harm, there is no harm in it. We also all know that the Prophet (peace and blessings be upon him) burned the palm trees of Banu Nadir, and the palm trees usually do not lack young shoots or insects or other things, but they were not intended. Many of you know that the scholars (may Allah have mercy on them) said it is permissible to throw a trebuchet at a village of disbelievers, even though the trebuchet will kill those whose killing is not permissible, such as women and children, but that is incidental. Therefore, we see that there is no harm in these devices. This is one point.
Second, we see that there is no harm for a farmer to burn the land of the crop in order to remove the weeds that harm the crop, even if there are insects or the like in the land, because the person burns the land not in order to burn the insects, but in order to burn the harmful weeds. What is established incidentally is not established as a primary objective, and this is analogous to the burning of the palm trees of Banu Nadir by the Prophet (peace and blessings be upon him).
Answer : No, there is no harm in it. First, this does not burn them; rather, it shocks them, just as if a person touched an electric wire and died. But is that considered [killing]?
It is not considered [killing]; it is of this type. Second, if the only way to benefit from removing this harm is with this device, even if it were to burn them, there is no harm. Have you not seen the Prophet who was stung by an ant and burned the entire ant colony? Allah the Exalted said to him, "Why not just one ant?" and they did not reprimand Allah for the burning, because he wanted to do this to defend himself. But Allah the Exalted rebuked him for burning the entire colony. So long as it is to repel harm, there is no harm in it. We also all know that the Prophet (peace and blessings be upon him) burned the palm trees of Banu Nadir, and the palm trees usually do not lack young shoots or insects or other things, but they were not intended. Many of you know that the scholars (may Allah have mercy on them) said it is permissible to throw a trebuchet at a village of disbelievers, even though the trebuchet will kill those whose killing is not permissible, such as women and children, but that is incidental. Therefore, we see that there is no harm in these devices. This is one point.
Second, we see that there is no harm for a farmer to burn the land of the crop in order to remove the weeds that harm the crop, even if there are insects or the like in the land, because the person burns the land not in order to burn the insects, but in order to burn the harmful weeds. What is established incidentally is not established as a primary objective, and this is analogous to the burning of the palm trees of Banu Nadir by the Prophet (peace and blessings be upon him).
Source
لقاء الشيخ مع طلبة كلية الشريعة وأصول الدين بالقصيم — archive.org (1:19:20)
The Arabic text is a machine transcription of the audio recording.