A commonly used insecticide, and possibly an increasingly widely used form of that pesticide, could be a causal factor in bee colony collapse. It is not 100% certain that this pesticide’s effects can be counted as one of the causes this problem, but there is a very good chance that neonicotinoids can cause a drop in hive population, and thus, should be examined to see if there is a relationship in some cases. From the paper’s abstract:
Nonlethal exposure of honey bees to thiamethoxam (neonicotinoid systemic pesticide) causes high mortality due to homing failure at levels that could put a colony at risk of collapse. Simulated exposure events on free-ranging foragers labeled with an RFID tag suggest that homing is impaired by thiamethoxam intoxication. These experiments offer new insights into the consequences of common neonicotinoid pesticides used worldwide.
CCD (Colony Collapse Disorder) was identified as a phenomenon in which colonies of honey bees (Apis mellifera) would suddenly disappear from their own hive. Suggested causes have included pathogens, parasites, habitat loss, and even pesticides. So far studies have failed to make any clear links.
Pesticides have always been on the table as a possible cause because these days so many bees forage on industrial grown grasses (corn, sunflower, etc. etc.) which are periodically dusted with insect killers. In particular, Neonicotinoid pesticides, which are used to protect crops against sap-sucking insects (such as aphids) are associated with loss of functional behaviors such as navigational skills. Bees that are exposed to non-lethal doses of Neonicotinoid pesticides forage abnormally, have “olfactory memory” problems, are easily disoriented and become poor learners. Essentially, a dose of this sort of pesticide may make it impossible for a bee to return to the nest. If an entire colony of bees is infected by this chemical, by direct exposure as well as indirect exposure (among bees back in the hive after foraging bouts) the entire colony cold just fly off and not come back over a fairly short period of time.
In the present study, the research team “tested the hypothesis that a sublethal exposure to a neonicotinoid indirectly increases hive death rate through homing failure in foraging honey bees,” focusing on the recently marketed form “thiamethoxam.” They exposed bees to this chemical then measured their ability to find their way home, using tagged bees. Then, they modeled the effects of the combination this problem of getting lost with other problems bees have while foraging, like getting eaten and such on colony viability. For the later, they simply modeled honey bee population dynamics with the “I got lost because I ate thiamethoxam” factor added in.
The bees were tracked with a very tiny device called a RFID device, as in this photograph from the Science paper:
The researchers tagged 653 individual bees over four separate treatments, in an area of France.
To simulate intoxication events, foragers received a field-realistic, sublethal dose of thiamtethoxam… and were released away from their colony with a microchip glued on their thorax. RFID readers placed at the hive entrance were set to detect on a continual basis tagged honey bees going through the entrance. Mortality due to post-exposure homing failure, mhf, was then derived from the proportion of non-returning foragers.
Other bees, not dosed with the poison, were also monitored for comparison.
The bees dosed with the poison were less likely to return to the hive if released a kilometer away, than those not treated. In the case of bees released at a place they had been to before, from which they could use their internal bee-ish mapping abilities to fly home, the treated bees were very slightly less likely to return to the colony. Bees that were released at random locations that they were not likely to be familiar with were more discobobulated, and while both treated and untreated bees had a hard time making it back to the colony, the difference between the two was more dramatic.
A: Bees released at a location known to them a kilometer away from the hive; B: Bees released at a random location a kilometer away from the hive. The gap in the graph is overnight.
One thing that strikes me as especailly interesting here is that many bees don’t make it back over a fairly long period of time even under normal conditions, and that some bees stay out overnight!
Once these numbers were obtained from empirical fieldwork, they were plugged into models for bee population dynamics. The result is the following graph (see caption for explanation):
Comparison of honey bee population dynamics between simulated colonies exposed to thiamethoxam (red lines) or not exposed (blue lines), following six demographic scenarios. L: queens daily laying rate (nb. of eggs per day). Exp: proportion of foragers exposed to treated crops during the day. The non-exposed colony follows either (A and D) a normal development trajectory (at L=2000), (B and E) an equilibrium dynamic (L=1800), or (C and F) a slightly declining trajectory (L=1600). Shaded areas delineate the exposure period (e.g., oilseed rape). Pairs of trajectories in exposed colonies were obtains with the lower and upper bounds of homing failure mortality (0.102 and 0.316), in order to delineate the best and worse estimates for population dynamics, respectively. Dotted lines extend the declining trajectory expected for a sustained exposure.
As you can see, colonies with bees exposed to a disorienting chemical are more likely to reach a critical level of population size. In fact, the difference between non-affected and affected colonies can be quite dramatic, and for sustained exposure, heading towards the point of no return is well within the range of possibility over a period of just a few months.
The researchers conclude:
Our study clearly demonstrates that exposure of foragers to non-lethal but commonly encountered concentrations of thiamethoxam can impact forager survival, with potential contributions to collapse risk. Furthermore, the extent to which exposures affect forager survival appears dependant on the landscape context and the prior knowledge of foragers about this landscape. Higher risks are observed when the homing task is more challenging. As a consequence, impact studies are likely to severely underestimate sublethal pesticide effects when they are conducted on honey bee colonies placed in the immediate proximity of treated crops. Finally, this study raises important issues concerning exposed solitary bee species, whose population dynamics are probably less resilient to forager disappearance than honey bee colonies.
Henry, M., Beguin, M., Requier, F., Rollin, O., Odoux, J., Aupinel, P., Aptel, J., Tchamitchian, S., & Decourtye, A. (2012). A Common Pesticide Decreases Foraging Success and Survival in Honey Bees Science DOI: 10.1126/science.1215039