[tzs]

I'd reach for the insect repellant before the tinfoil hat. Anyone know offhand an insect with a two year life cycle that has some significant stage that peaks around September?

[nemonemo]

Even if such insect exists, they would have alternating two groups that would appear like annual, like cicadas.

[dmckeon]

Or appear every 13 or 17 years, like some cicadas. https://en.wikipedia.org/wiki/Periodical_cicadas#Lifecycle

[nemonemo]

This is a great natural wonder. Thank you.

It also invalidates my logic. Maybe there could be some vector with biannual periodicity.