In Northern California, butterflies have been mysteriously disappearing for decades.

According to new research published in Biology Letters, there's even more proof that the butterflies aren't just disappearing because land-use and the climate changing - they're disappearing because humans are killing them with neonicotinoid pesticide use.

The research team - lead by Professor Matthew Forister from the University of Nevada - looked at 40 years of data to separate the different factors that correlate with the declining butterfly population.

The authors found that the butterfly population in Northern California made it a biodiversity hotspot - but that populations have been in decline since the late 1990s without any correlation to recent temperature or land-use changes.

But the decline lines up neatly with the increase use of neonicotinoids - which were first used in 1995 in Northern California.