Bee colonies afflicted with the mysterious colony collapse disorder had more pathogens than healthy hives, according to a new study.

Penn State University entomologist Dennis vanEngelsdorp and entomologist Jeff Pettis, geneticist Jay Evans and virologist Yanping Chen with the AAgricultural Research Service in Beltsville, Md., studied more than 200 variables in 91 colonies from 13 apiaries in Florida and California.

They screened for bacteria, mites, the protozoan parasite Nosema, viruses, nutrition status and 171 pesticides.

They also sampled adult bees, wax comb, beebread and brood.

No single variable stood out as the single cause for CCD, which is marked by a sudden disappearance of adult bees from a colony, according to a news release.

But overall, CCD colonies had a greater number of total pathogens—55 percent of CCD colonies were infected with three or more viruses compared with 28 percent of non-CCD colonies.

The researchers also found detectable levels of residues from 50 different pesticides in all of the sampled colonies. There was no association between increased pesticide levels and CCD, according to the release.

To read their full report, click here.