I can’t really answer for Bikkhus - but this is something that can be a challenge for Buddhists generally. Many Buddhists take a vow of not killing - it’s one of the five precepts you can take as a lay Buddhists. I’ve taken it myself.

So the way they work is that you can break them in major or in minor ways. Killing insects is not one of those major breaches.

MAJOR BREACHES

If you kill your parents, or an arhat, or a Buddha, or your guru if you do take on someone as your guru (most people don’t), that totally breaks the vow of not killing.

You would do a lot of purification practices if you did something like that, whether you’d taken the precept or not indeed. This is an example of one teacher’s advice to someone who had killed his parents in a delusional state Lama Yeshe Wisdom Archive - that sort of advice would be very dependent on who you go to in that situation so it’s just an example.

More generally the most important thing is to refrain from killing other people, if that’s something one is prone to do.

But it’s the intention also, so if you kill someone in an accident, you didn’t intend to do it. So you haven’t broken the vow, not if it was a complete accident.

To break the vow completely, you need the intention to do it, the action of killing that you try to do it, the completion - that you succeed, and finally you need to rejoice in completion of the action - if you immediately regret then that’s a much less heavy thing though you’ve still broken the vow.

MINOR BREACHES OF THE VOWS

In the same way if you kill insects while breathing, walking, drinking water, gardening etc there is no intention there either.

When you kill insects to stop spread of disease, or for medical reasons, or for instance, you have to kill ticks or fleas to protect a pet - then that’s a mixed thing. You still have the negativity of killing things, and you do have the full completion, but you have the positive side of it also that you are doing it to help them as well.

Your preceptor or your teachers or advisors would explain in detail how to deal with those situations and what to do to try to transform it as best you can. But yes, you would kill insects or such like in order to protect someone from disease or yourself, or an animal like a pet, or an animal you are caring for etc.

Myself I live in a Lyme disease district and have to remove ticks right away if they bite me, although that might kill them, to prevent myself from getting Lyme disease. Even though I do that, I actually got one of those circular rashes recently, just a few weeks ago - the doctor gave me a strong dose of antibiotics and it went away. So it’s a real risk here. Perhaps I left a tick for too long before I noticed it and removed it.

Also, you may do activities that obviously increase the number of insects and other creatures you’ll kill like gardening. It’s how your mind is that matters though. Again that’s something you’d discuss with your preceptors. Different traditions also may have different ways of handling it, but the main thing is you do your best to make it into a positive thing, but of course you have to protect humans and higher animals that you care for, and yourself, from insects and diseases that harm them. That’s recognized and though ideally you wouldn’t kill even insects, in our world that’s not practical and you do the best you can.

.And the main thing here is that it is a mind training thing. You do it not to stop insects from dying, which is obviously impossible. You do it to train your mind so that you are less involved in killing others. It’s hard to be compassionate and wise if you go around intentionally killing insects, and especially if you rejoice in doing that, it kind of blocks you off a bit from being able to appreciate that they also suffer.

In the same way also - it’s not like a crusade. You don’t have to go around freeing insects from spider’s webs for instance :). The spiders have to survive too. You can feel compassion for both. Nor do you have to go around freeing insects from insect eating plants either. The main focus is on training yourself, and then whatever compassion or wisdom comes out of this restraint, then that can give a bit more space, a bit more restraint, to follow the path.

CAN’T STOP KARMA

And - some people have the mistaken idea that the aim of Buddhists is to stop all karma. That’s impossible. You have countless seeds of karma from the past, even not doing anything is a form of action. It’s just not possible to not act, to cease acting at all in any way. It’s rather to create some space, to deal with the worst issues such as killing other people, and to open out to other beings generally, and within that then you have space for true boundless compassion and wisdom and loving kindness to arise.

Meanwhile, basic morality like this gives us some restraint to help us to avoid the worst excesses that can cause serious problems in your life and cause you to be a nuisance to others as well. So that’s how we see it, as Buddhists who have taken the vow of non killing.

BUDDHIST MONKS EATING MEAT

That also can help perhaps to understand why we also may eat meat. The Dalai Lama eats meat too, his doctor advised him that he needed to. Many Tibetan bikkhus eat meat though Therevadhan bikkhus maybe tend more towards vegetarianism.

That may be very hard to understand until you realize that it’s a mind training rather than a crusade to change the world so that animals don’t die any more. Again the animals will die anyway. Sadly, we can’t stop that. And for bikkhus, actually the guideline is that they should accept whatever food is given to them, so long as it is safe to eat.

So if you are a bikkhu and someone offers you a meal of meat, you are expected to accept it. The main exception there is if they offer to kill a creature for you, then if you are a bikkhu you would say that this goes against your vows and that you can’t eat an animal that’s been killed to provide you with food.