Bumblebees are disappearing from the planet at an alarming rate, and scientists think they know why: climate change.

And it’s a big problem, because bumblebees are responsible for the pollination of crops across Europe and North America, and therefore their disappearance could mean a major catastrophe for the world’s plants and for agriculture, according to a Christian Science Monitor report.

As the temperatures have risen across the globe, bumblebees have stopped occupying the warmest regions, and they aren’t spreading north to compensate, unlike most other species that have also had to adapt.

Honeybees are already a concern due to their declining numbers via a process called colony collapse disorder, which has been blamed on overuse of pesticides that disorient them to more bee parasites.

However, it’s the bumblebees that do the real hard work of pollinating, as they spend their time in the fields pollinating plants from the spring to the fall. Other species pollinate just a few plants.

Also, bumblebees are capable of pollinating in ways other bees can’t, such as “buzz pollinating” flowers like tomatoes. It’s estimated that the work they do is worth $3 billion per year to the economy.

In this latest study, the research team look at habitat ranges for 67 bumblebee populations in Europe and North American from 1901 to 1974 before the climate change effect, and from 1974 and 2010 as climate change began to take hold.

They found that in the southern areas, the range of the bumblebees shrank by 186 miles, or 5.6 miles per year.

But they haven’t compensated by going to northern climates, the study found. Instead, they are simply dying off — and that’s a big worry to scientists.