Shouldn't I kill even mosquitoes that bite me by first precept?

The precepts are training rules for training your mind. The first rule is to learn to control your hatred, and impulsive behavior based on that, like killing or violence.

Sometimes killing is necessary, for example killing of pests to get things to balance, or killing for food. But that is killing in clarity, not in hatred - done with wisdom and understanding. We are all connected here you know, and our lives will feed into others' in many different ways.

When I eat meat, I appreciate life of a sentient being who gave me their flesh for nutrition and promise to make good use of it. When I kill a mosquito I do it in full awareness, as a conscious choice, not an act of hatred. There's no regret in my killing of a mosquito and no guilt. For a moment I pause and think to make sure there is no other way to solve the problem, and if there's none - then with an attitude of "thank you, forgive me, I love you, but I have to do this" I kill it.

From Bhikkhu Bodhi's discussion on the first precept, we find some comments on the varying gravity of the kamma of killing:

The first distinction given is that between killing beings with moral qualities (guna) and killing beings without moral qualities. For all practical purposes the former are human beings, the latter animals, and it is held that to kill a fellow human being is a more serious matter ethically than to kill an animal. Then within each category further distinctions are drawn. In the case of animals the degree of moral gravity is said to be proportional to the animal, to kill a larger animal being more blameworthy than to kill a smaller one. Other factors relevant to determining moral weight are whether the animal has an owner or is ownerless, whether it is domestic or wild, and whether it has a gentle or a vicious temperament. The moral gravity would be greater in the former three alternatives, less in the latter three. In the killing of human beings the degree of moral blame depends on the personal qualities of the victim, to kill a person of superior spiritual stature or one's personal benefactors being more blameworthy than to kill a less developed person or one unrelated to oneself. The three cases of killing selected as the most culpable are matricide, parricide, and the murder of an arahant, a fully purified saint.

The five precepts are training rules and not strong prohibitions.

Furthermore, even for monks, based on the Patimokkha rules, intentionally bringing about the death of a human being is a parajika offense, while taking the life of an animal is a pacittiya offense. A parajika offense is the gravest offense causing immediate expulsion, but a pacittiya offense is milder, requiring only a confession.

Parajika Rule 3. Should any bhikkhu intentionally deprive a human being of life, or search for an assassin for him, or praise the advantages of death, or incite him to die (saying,): "My good man, what use is this evil, miserable life to you? Death would be better for you than life," or with such an idea in mind, such a purpose in mind, should in various ways praise the advantages of death or incite him to die, he also is defeated and no longer in affiliation.

Pacittiya Rule 61. Should any bhikkhu intentionally deprive an animal of life, it is to be confessed.

And also on the motivation for killing (from here):

Another factor determinative of moral weight is the motivation of the act. This leads to a distinction between premeditated murder and impulsive killing. The former is murder in cold blood, intended and planned in advance, driven either by strong greed or strong hatred. The latter is killing which is not planned in advance, as when one person kills another in a fit of rage or in self-defense. Generally, premeditated murder is regarded as a graver transgression than impulsive killing, and the motivation of hatred as more blameworthy than the motivation of greed. The presence of cruelty and the obtaining of sadistic pleasure from the act further increase its moral weight.

In my opinion, if rulers, government officials, neighbourhood resident associations and even individuals decide to kill mosquitoes and mosquito larvae due to a proven rising dengue or malaria epidemic, it would be a reasonably justifiable motivation for killing. After all, isn't it wrong to endanger fellow humans knowingly?

From the Lonaphala Sutta:

"Now, a trifling evil deed done by what sort of individual takes him to hell? There is the case where a certain individual is undeveloped in [contemplating] the body, undeveloped in virtue, undeveloped in mind, undeveloped in discernment: restricted, small-hearted, dwelling with suffering. A trifling evil deed done by this sort of individual takes him to hell.

"Now, a trifling evil deed done by what sort of individual is experienced in the here & now, and for the most part barely appears for a moment? There is the case where a certain individual is developed in [contemplating] the body, developed in virtue, developed in mind, developed in discernment: unrestricted, large-hearted, dwelling with the immeasurable. A trifling evil deed done by this sort of individual is experienced in the here & now, and for the most part barely appears for a moment.

I take the killing of mosquitoes and mosquito larvae with the motivation of dengue or malaria eradication, to be a trifling evil deed, and if we do it with the correct motivation and the correct state of mind, then it is acceptable although not ideal. The same applies to rats and diseases spread by rats like leptospirosis and hantavirus.

Hitting a mosquito that bit you is also considered impulsive killing, rather than premeditated killing, and according to Bhikkhu Bodhi's description above, impulsive killing is less grave than premeditated killing.

Despite this, it is clear that not killing any sentient being for any reason, would be the higher dhammic decision.

Killing a mosquito should not be the first and initiative step. If you are honest and really struggle to maintain first precept better to make preventive measures. Just adhere to precepts will not create effective results. you have to adhere to precepts with wisdom in your capacity. As preventive measures 1.you can use mosquito repellent. 2.Use mosquito nets 3.Close windows and doors early in evening before enter mosquitoes to your house 4.close small spaces and ventilation holes with fine nets in your houses 5.remove mosquito breeding places.(keep clean your surrounding) above will protect your family effectively diseases spread by mosquitoes.Simply kill mosquitoes one by one is not effective and useless way.Above protective measures are more effective than killing them.In other hand you can protect first precept. "The protector of Dhamma will protect by Dhamma. May all being in peace and harmony. Thanks.

No, you should not even kill mosquitoes that bite you, according to the First Noble Precept.

I think the formula is actually I will restrain from the killing of sentient beeings. Keep in mind that the 5 precepts are not obligations, but rather rules you take up voluntarily. Nobody forces you to keep them, but they are an absolute minimum of Buddhist morality. The mind that has the intention of killing, stealing... is not able to have a clear perception of reality, at all.

The 1st precept is a training rule to refrain from killing 'breathing' things.

The Pali is: "Panatipata veramani sikkhapadam samadiyami", where the word "pana" means "life" or 'breath", such as in the term "ana-pana-sati".

The 1st precept does not apply to bacteria because they do not breathe.

Strictly speaking, bacteria do not “breathe” as humans and other animals do, but they must still engage in the act of cellular respiration. Cellular respiration is the method by which cells convert chemicals into energy that the cell needs to stay alive.

Bacteria

As for mosquitoes, the probability of contracting malaria or dengue fever is very low. I lived in Thailand for 9 years and had dengue fever only once. I can contract dengue fever where I have lived for the last 10 years & have never got it (although one friend did). When I lived in Thailand, there was a period of a few years when hundreds of mosquitoes would bite my feet when I was meditating at sunset. I never got dengue fever or malaria during this period. I felt very free when my mind became accustomed to not worrying about & fearing mosquitoes.

Not killing mosquitoes is beneficial for cultivating metta (loving-kindness), fearlessness & freedom. However, that said, killing mosquitoes is not particularly bad karma. When I first visited Thailand & learned meditation, I did not care about killing mosquitoes & killing a mosquito never affected my samatha (calmness; concentration) meditation. This is because killing a mosquito is kammically a natural act. The mosquito wants to steal our blood & can (possibly) kill us. This is why the kammic results (vipaka) of killing a mosquito are trifling (insignificant).

Most people have killed many mosquitoes & remain mentally normal. Their minds do not become deranged (crazy). Where as the majority of soldiers that kill in war are often mentally affected for life in a negative way. Some ex-soldiers become mentally deranged & even commit suicide. (Here, an ex-soldier must learn to forgive themselves!) This shows the difference between killing many mosquitoes & killing just one or even many human being/s. Kammic consequences (vipaka) follow the law of nature.

The Buddha taught "kamma is intention" (AN 6.63). A mosquito killed with the intention of avoiding malaria or dengue fever is not an unwholesome intention. However, it is still an intention rooted in fear, attachment & a lack of liberation.

For example, to kill a mosquito so it does not bite a small baby or child is probably good karma. If it was known you let mosquitoes bite your small baby, many people would criticize you. You would be blamed & censured for being a bad parent. The government may take your child away from you.

Personally, I would not worry too much about it since not killing mosquitoes is a higher level of dhamma practise. The Buddha did not make strict rules for monks about killing mosquitoes or even about killing animals (for food) for laypeople.

And which is intolerant practice? There is the case where a certain individual doesn't tolerate cold, heat, hunger, & thirst; the touch of flies, mosquitoes, wind, sun & reptiles; ill-spoken, unwelcome words; & bodily feelings that, when they arise, are painful, racking, sharp, piercing, disagreeable, displeasing & menacing to life. This is called intolerant practice.

And which is tolerant practice? There is the case where a certain individual tolerates cold, heat, hunger, & thirst; the touch of flies, mosquitoes, wind, sun & reptiles; ill-spoken, unwelcome words; & bodily feelings that, when they arise, are painful, racking, sharp, piercing, disagreeable, displeasing & menacing to life. This is called tolerant practice.

Khama Sutta