Bees are a crucial member of our world, found on every continent save Antarctica. They are responsible for the majority of insect plant pollination — for agricultural as well as wild plant species. Crops such as apples, broccoli, carrots, cucumbers, and coffee all require bee pollination and would fail to reproduce without them. Without pollinating bees, dozens of food crops and other flowering plants would go extinct without human intervention, creating a major crisis for the human species.
Asa result of this crucial role bees play and because dramatic declines in bee populations are already being observed worldwide, it is no surprise that the scientific community is studying bee populations voraciously. One such study performed by Gill et. al. of the University of London, studied the impact of pesticides on bumblebee populations. The study deliberately chose the bumblebee instead of the more studied honeybee because bumblebees are also crucial pollinators and their small colonies (less than 50 bees) allow for a more detailed study of pesticide effects on individuals as well as colonies. This study also attempted to account for the combinatorial effects of more than one pesticide on bumblebee colonies, which no previous study had monitored.
In total, 40 colonies were studied — 10 control colonies without pesticides, 10 colonies treated with a pesticide known as imidacloprid (which I will refer to as “I”), 10 treated with a pesticide called λ-cyhalothrin (which I will refer to as “LC”), and 10 treated with both pesticides (which I will refer to as “M” for mixed). Both pesticides are frequently used on flowering crops and are commonly found in bumblebee habitats.
The pesticides were applied as they are commercially, not exceeding the current “sublethal” limit. “LC” was administered as a spray onto flowering plants and “I” was dissolved in a sugary solution and absorbed by plant tissues. The pesticide exposure lasted 4 weeks.
The colonies’ current population survival rate is not the only factor in overall colony success — the efficiency of worker bees is also crucial to the colony’s survival. This is because worker bees care for young bees and provide food for the whole colony. Changes in something as simple as worker bee behavior can lead to the death of a colony.
Approximately 36% of all workers died in “LC” colonies, and about 40% died in “M” colonies. This is four times the normal rate of death as seen in the control colonies. In fact, of the 40 colonies studied, two failed entirely within 8 days, both within the M subset, suggesting colony death and damage increased when more than one pesticide was in play.
It was also found that most newly born worker bees in “LC” and “M” colonies died within four days — presumably before they were mature enough to have contributed any type of work to the colony. This demonstrates a deadly amount of colony resource waste, contributing to colony decline.
There was a delay in pesticide impact in the 40 colonies. “I” colony worker production rates did not deviate from control colonies’ until the end of week 2. “LC” colony worker mortality did not become significant until the end of week 3, but “M” colony worker mortality rates became significant as early as the end of week 1. Once again, we can see the dramatic effect of multiple pesticides on bumblebees.
Additionally, the total number of bee larvae and pupae was significantly lower in “I” and “M” colonies compared to controls.
Among things that did not seem to be effected were queen bee loss rates and the mass of the wax nest structure. The latter suggests that “I” and “M” colonies tried to raise as many worker bees as control groups, even though their success rates were much lower.
The study of worker bee foraging performance was carried out using RFID technology, or radio-frequency identification, which uses microchips to track individuals. Two-hundred fifty nine foragers were used to collect data, making about 23 foraging bouts each. Workers in “M” colonies on the whole performed fewer foraging bouts than control colonies. And unexpectedly, both “I” and “M” colonies had significantly more foragers than control colonies. This suggests that “I”-exposed foragers were significantly less efficient at collecting pollen than their control colony counterparts. The increase in foragers also results in more foragers getting lost on foraging bouts and never returning to the colony — the percentage of lost workers was 50% and 55% higher in “I” and “M” colonies respectively than in control colonies.
The resultant decrease in pollen collection in “I”-exposed colonies seemed to stunt the survival rates of larvae and pupae. “LC”-exposed colonies, on the other hand, caused additional mature worker mortality. The combinatorial effects were therefore increasingly severe. “M” colonies were consistently observed to be negatively affected in terms of worker behavior, worker mortality, and forager losses.
Conservation efforts so far have ignored the the risk of multiple pesticides. There are also no guidelines for testing the effects of pesticides on bees for more than 96 hours. This study is therefore precedential in determining multiple pesticide effects and in screening pesticide effects in bee species other than just honeybees. Protocols can now be adjusted to incorporate more complex and realistic scenarios.
Gill, R. J., Ramos-Rodriguez, O., & Raine, N. E. (2012, October 21). Combined pesticide exposure severely affects individual- and colony-level traits in bees.Nature.com. Retrieved October 31, 2012, from http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11637.html
Bauer, C. (2004, June 30). File:A bumble-bee on a flower.jpeg. Wikimedia Commons. Retrieved October 31, 2012, from commons.wikimedia.org/wiki/File:A_bumble-bee_on_a_flower.jpg
List of crop plants pollinated by bees – Wikipedia, the free encyclopedia. (n.d.).Wikipedia, the free encyclopedia. Retrieved November 2, 2012, from http://en.wikipedia.org/wiki/List_of_crop_plants_pollinated_by_bees