The decline in bees: Are pesticide surfactants to blame?

Bees and pesticides

The current sharp decline in honey bee populations around the world has been attributed to a number of factors like parasites, pesticides and pathogens but, at present, they remain suspicions. There is no hard evidence to suggest that any of these is to blame.

However, that has not prevented the controversial banning of the neonicotinoid group of pesticides in the EU for two years from December 2013, specifically clothianidin, imidacloprod and thiamethoxam. The ban is temporary because eight EU member states voted against the proposal and four more abstained.

The ban has been met with dismay by agrichemical companies and Syngenta has just announced that it is taking the matter to the European Court of Justice. Its chief operating officer announced: "The proposal is based on poor science and ignores a wealth of evidence from the field that these pesticides do not damage the health of bees. Instead of banning these products, the Commission should now take the opportunity to address the real reasons for bee health decline: disease, viruses and loss of habitat and nutrition."

Given the contentious debate over the effects of pesticides on bees, one research group in the US decided to take a look at other components in pesticide formulations. In 2012, Christopher Mullin and colleagues from Pennsylvania State University reported that certain surfactants present in pesticides restricted the learning ability of honey bees by weakening the proboscis extension reflex for sucrose. This will have a direct effect on their feeding ability.

In their follow-on study, the group has now studied the beehive itself to see if these surfactants accumulate in the pollen, honey or beeswax. The compounds in question are the silicon-containing trisiloxanes, which have fully methylated silicon atoms alternating with oxygen atoms in a chain, completed by various end groups.

Siloxane solutions

There are a lot of different trisiloxanes, so the first point was to decide which ones to target in the beehive. Mullin did this by analysing the trisiloxanes present in the most popular pesticide formulations sold in California, using an LC/MS method. They were removed and concentrated by solid-phase extraction.

The siloxanes were injected onto a high-speed C₁₈ column and eluted into a high-speed quadrupole mass spectrometer operating in positive electrospray ionisation mode. The ammonium ion adducts produced from the presence of ammonium formate in the LC mobile phase were monitored in order to distinguish between the various siloxane compounds. The three principal trisiloxanes in the commercial preparations were those with methyl, acetyl or hydroxyl end groups attached to the ends of the (polyethyleneoxy)propyl chains. These chains were of varying lengths, with up to 13 ethylenoxy groups in the polymer sections. During the LC separation, the trisiloxanes with hydroxy, methyl and acetyl end groups eluted over 12.8-13.0, 14.8 and 16.7 and 15.1-16.6 minutes, respectively. Within these three groups, the polymers eluted in order of their chain lengths, with the longer chains eluting first.

Contaminated beeswax

Having established that the siloxanes can be distinguished by mass spectrometry, the researchers set out to see if they were finding their way from the pesticide formulations in the field into the beehives. They collected honey, pollen and beeswax samples from hives across seven US states from the east to the west coast. Following an extraction procedure based on the original QuEChERS method and the addition of an internal standard, the presence and contents of the target trisiloxanes were determined in the hive samples by the same LC/MS method.

Beeswax was the most contaminated of the three hive materials tested, with at least one trisiloxane found in every sample. Some beeswaxes contained all three siloxanes. The total concentrations ranged from 12-390 ng/g, with an average total of 116 ng/g. These figures contrasted with pollen, which contained only the methylated or hydroxylated trisiloxanes at levels up to 39 ng/g in six out of ten samples, with no trisiloxanes being detected in four pollens. In contrast, honey was found to be free of trisiloxanes, possibly due to the fact that the acidic environment of the honey matrix would hydrolyse any of the three siloxanes that might enter the hive.

The most common of the trisiloxanes in the beeswax and pollen was the methylated compound but the apparent distribution of the three types might reflect their initial concentrations in the pesticide formulations. The only way that these compounds could become part of the beeswax and pollen in the hives is through pesticide application to nearby fields, from where they are picked up by the honey bees. The beeswax comb is where bees are reared and their pollen and food are stored but this is the most contaminated section of the hive, so would be the best section in which to carry out further investigations

At this stage, no-one is blaming trisiloxanes for the fall in bee populations, but their effects on the health of bees should be examined. There has been little research to date, a gap which needs to be addressed bearing in mind the high trisiloxane contents found in this study.