Colony collapse disorder, a phenomenon that occurs when the majority of the worker bees disappear and leave the queen to die, has baffled scientists for years. Now, researchers have found a new cause for concern among honey bees – the cold.

Honey bee colonies are important pollinators, and in the U.S., millions of colonies are hauled across the country in semi-trailers to pollinate crops like California almonds. But some of these colonies don’t survive the trek as cold temperatures kill them, according to a study by Agricultural Research Services scientists.

Researchers found that weak colonies, those with ten or fewer frames of honey bees and larvae, are susceptible to temperature stress that may kill the colony. While colonies containing more frames are able to endure the cold, the weaker colonies die off days after arrival.

The results of the preliminary study raised questions about how to load semi-trailers to protect bees, since colonies near the front or back of the trailers experienced more temperature-related stress than colonies in the middle.

The researchers also discovered that during the recovery period from cold exposure, activity in the genes that support disease resistance and respond to cold stress decreased. But the genes involved in producing antibiotic peptides increased activity, possibly to help fight off new bacterial infections to which the weakened hive may be vulnerable.