Pesticide residues are known to appear frequently in wax combs. Since most pesticides are either lipophilic or dissolved in oil-based carriers, it is no surprise that we find them residing in beeswax. These chemicals can be brought into the hive by the bees themselves in the form or contaminated pollen and nectar, or they may be introduced by beekeepers in an effort to control Varroa mites. Earlier this year, a paper in the online journal PLoS ONE examined the negative effects of these chemical residues on colony health.
The authors designed an experiment in which areas of non-contaminated brood comb and pesticide-contaminated brood comb were affixed side-by-side within active brood nests. The contaminants, the brood, and the resulting adult bees were monitored over several generations in order to assess the differences between brood raised on clean vs. contaminated comb.
The researchers found several significant differences:
- Delayed adult emergence occurred in brood raised in contaminated cells.
- Increased brood mortality occurred in contaminated cells.
- Bees reared in contaminated comb had shorter lifespans (about four days) than those reared in clean comb.
- Over time, the amount of pesticide decreased in the contaminated comb and increased in the clean comb, indicating that pesticides migrate throughout the hive. This is probably caused by movement of the nurse bees during their daily activities.
These differences can have far-reaching ramifications for colony health.
- Delayed adult emergence means more mites can be raised to maturity. In this experiment, some foundress mites were able to produce an extra mite per brood cycle. This can have catastrophic effects on a colony over time.
- Increased mortality of brood puts a greater strain on both the queen and the workers. The queen must lay more eggs to make up for the losses, and a smaller number of workers are available to care for the brood. In addition, valuable resources are expended on brood that doesn’t survive.
- A shorter adult life span means that each forager has fewer total foraging days in which to build colony strength before winter. In some cases, “under-aged” worker bees may be forced to forage, which decreases the number of nurse bees available to raise young.
From a practical point of view, in-hive chemicals should be avoided whenever possible, and brood combs should be rotated out of the colony on a schedule commensurate with total pesticide exposure.
 Wu, J. Y., Anelli, C. M., and Sheppard, W. S. 2011. Sub-lethal effects of pesticide residues in brood comb on worker honey bee (Apis mellifera) development and longevity. PLOS ONE: 6(2): e14720. doi:10.1371/journal.pone.0014720.
More good reasons to change brood comb annually…I think the delay in production caused by the comb rebuilding needed is worth it to keep disease and contamination down.
How would this best be achieved? Removing one at a time throughout the year or many at a time?
If you remove the worst 25% of frames every year, then all frames get replaced every four years. Or, if there are not so many chemicals in your area, you could go 20% for a five-year rotation. On the other side, you could go 30% for a roughly three-year rotation.
Were the pesticides used in the experiment of a type that is typically brought to the hive by foragers, or were they of a type that is typically introduced into the hive by a beekeeper?