Has anyone tried Audacity to find FF of insects sounds?

More specifically wing beat? I have audio and have used the pitch detect plugin, but the value varies vastly depending which part of the spectrum I select. I think this means that I have a lot of noise. Can I restrict the pitch detect to only 100 to 300 Hz?

You can restrict the range of frequencies displayed on the spectrogram
insect buzz.png

Nice soundwave! Yes, I tried this and it is great for visualizing it, but the pitch detect still defaults back…

Which plug-in is that?

Your plugin: pitch-detect.ny

Or should I export the plot spectrum data and compute the f0?

What does Plot Spectrum show? (Perhaps you can post a screenshot)
A short audio sample (just a few seconds, in WAV format) would be useful (See: https://forum.audacityteam.org/t/how-to-post-an-audio-sample/29851/1)

What sort of insect is it, and what frequency do you expect?

This is a fly and I expect around 200 Hz.
I am using the spectrum algorithm, have set it at the highest size at 65536 then export the freq and db into excel. There I can select only frequencies between 100 and 300 and then sort by the highest db which should be the fundamental frequency. Does this sound correct?

BTW if you want a spectrum plot with frequency-zoom try SPAN plugin (free).

Not necessarily: the frequency-response of the mic (& electronics) may not be flat.
This could result in some harmonic being louder than the fundamental on the spectrum.