The decline in the population of honeybees is likely due to lethal chemicals found in pesticides sprayed on crops, the Los Angeles Times reported.

In a study published Wednesday in the journal PLOS One, researchers said they discovered 35 pesticides, some lethal, in the pollen collected by bees. The bees had been servicing major food crops in five states, including California.

According to the report, pesticides were found in all the pollen samples, including the ones taken from nearby wildflowers. The chemicals discovered were reportedly strong enough to kill half the population of bees in a matter of days.

While most researchers have analyzed a bee's reaction to single chemicals, this study examined the complex mixture of agricultural chemicals and pesticides to which domesticated bees are exposed to on a regular basis. The results for the insects, crucial to the world's food supply, were a weakened immune system.

"Bees are getting exposed to a lot of different products, including fungicides," said Dennis van Engelsdorp, an entomologist at the University of Maryland and lead author of the report. "What's surprising is that it seems to weaken the bee's ability to fight off infection."

Along with climate change, habitat destruction and handling procedures, combined with pesticides, are believed to be responsible for massive collapses of domesticated bee colonies.

The study connected eight pesticides to the increase of Nosema infection. Neonicotinoids, a banned class of pesticides in Europe, was found on pollen from a single apple orchard.

"It certainly wasn't very common," van Engelsdorp said. "It suggests there are other products that should be getting attention from the regulatory agencies."

The research team could not determine how pesticides made their way to non-crop pollen. The study examined almond crops in California, pumpkins and apples in Pennsylvania, cranberries and cucumbers in New Jersey, blueberries in Maine and watermelons in Delaware.

"It could be drift from when they sprayed their crop, but it also could be that the bees are picking it up and contaminating the pollen on the forage trip," van Engelsdorp said.