Estimates suggest that 35% of the world's food crops - one out of every three bites of food - depend on animal pollinators. Honeybees in particular are responsible for billions of dollars in agricultural productivity in the U.S.
But in recent years, bee colonies have mysteriously collapsed. For no apparent reason, worker bees will abandon their hive, leaving the queen behind with her brood of immature bees.
The cause of colony collapse disorder is unknown, but it is theorized that pesticides may play a role. To examine how exactly these chemicals impact honeybees, researchers are tracing their movement and accumulation in colonies.
The new study was published in Current Biology by authors from the University of California (UC), Davis, the U.S. Department of Agriculture and Lawrence Livermore National Laboratory (LLNL). The researchers showed that a honeybee queen facing chronic exposure to pesticides will take up that contamination and pass it along to her eggs, a process called maternal offloading.
"The queen is the only member of the hive who can lay eggs that become the next generation of workers," said Sascha Nicklisch, senior author and an associate professor at UC Davis. "She keeps the colony alive, so understanding how pesticides can affect queen bees and also her offspring is important."
LLNL contributed to the work with the Laboratory's BioAMS, or biological accelerator mass spectrometry, capability - one in a suite of instruments that enable LLNL researchers to explore national security and emerging science challenges. The team labeled a model pesticide with a carbon-14 isotope "tag". Most carbon contains 12 neutrons, so when the heavier, 14-neutron version was observed in a sample from the colony, it indicated the presence and revealed the concentration of the pesticide.
"With BioAMS, we can trace very low levels of a pesticide. The pesticide concentrations we used were not lethal and were environmentally relevant to that seen in nature," said LLNL scientist and author Bruce Buchholz. "Worker bees have different jobs in the hive. We wanted to see if a specific worker bee niche or developmental stage was being overwhelmed by pesticide."
To answer that question, the researchers created "nanocolonies," which represent the inner workings and functions of a hive and colony using conical plastic containers fitted with netted bottoms. Each nanocolony contained one queen and 60 worker bees.
The authors tracked the pesticide over 10 days, watching it move from tainted pollen, water and food to the worker bees who harvested those sources to the queen and eventually her eggs. In addition to examining samples throughout the colony, they also analyzed the queens' ovarian tissue.
The results were striking. Worker bees were initially able to filter out the pesticides, reducing their concentration as they processed their food source to make honey. However, that ability became less efficient as more pesticide accumulated over time. In the first day, worker bees were able to filter out 95% of the pesticide and deposit it into the honeycomb, but that number fell to 86% by day 10.
"In our study, pesticides began to accumulate in queens over time, suggesting that worker filtration capacity can be overwhelmed," said Angela Encerrado-Manriquez, lead author and a recent Ph.D. graduate from UC Davis. "When this happens, queens have their own defense. Maternal offloading allows them to shunt the toxic burden to their eggs."
Queens accumulated much less pesticide than worker bees, perhaps because of that element of self-preservation.
"Queens seemed to off-load the pesticide in eggs," said Buchholz. "Some eggs may have a higher pesticide content and be less healthy and viable, but the queen survived and could keep laying eggs."
Honeybee queens can lay 1,500 to 2,000 eggs per day to support their hives and, subsequently, agricultural productivity and food security in the U.S.
Looking ahead, the researchers aim to examine how long queen bees can continue to pass along contamination, the long-term effects on colonies and the variation between different pesticides.
