[EvaH (OP)]

Paul asks:

Do flea collars work on cats and dogs and how do they work? Also, do drops on the neck or back of the head work?
There are some electrical gadgets which you can plug into a power supply and they are supposed to emit a high frequency which scares off rats, mice, cockroaches and other insects. Do they work?

What do you think?

[evan_au]

You could start with the references here: https://en.wikipedia.org/wiki/Collar_(animal)#Insect_collar/flea_collar

[alancalverd]

I've used flea collars with cats and dogs with some success, and the liquid antiparasitics are always applied on the nape of the neck. There are several possible explanations

1. There is nowhere else you can attach anything to a cat or dog without breaking the skin, and expect to find it in place an hour later

2. Human fleas (been there) tend to congregate in the darkest areas and where there is some constriction that brings blood close to the surface. Hence under a collar.

3. As for the liquid stuff, it's the one place that they can't lick it off.

So there may be no magic in the collar, just no other reasonable choice.

I've used ultrasound repellents to clear rats out of a loft, and cockroaches certainly make high-pitched noises but I don't know whether they find ultrasound attractive or repellent.