Neonicotinoids : 9
and nectar from the plants treated with neonicotinoids. Various laboratory studies have shown that bees exposed to these sub-lethal levels can experience problems with flying and navigation and have slower learning abilities, all of which could impact foraging ability. When laboratory-based studies have seen an effect on the behaviour of bees it has included reduction of associate learning (at 12 ng/bee), decreased foraging activity, or reduced mobility (at levels between 2.5-20 ng/bee). By contrast, at colony level (using field-based studies), in most cases no observed effect was seen on survival, foraging, or colony development at 20 ppb (µg/l). Some effects (eg, decreased foraging activity) were noticed at levels >25 µg/l.
Studies using bees in their natural setting have found limited evidence that colonies near fields treated with neonicotinoids are suffering higher mortality than control colonies. It is clear from studying the research literature that conflicting results and conclusions are drawn regarding these effects. At this present time the exact effect of chronic exposure to these sub-lethal doses is unknown.
Experiment Design
The effect of neonicotinoids on honey bees has been studied by various groups in the past decade. These involved both laboratory- based studies, semi-field-based studies and field-based studies.
In the laboratory-based studies bees
were fed with a sugar solution containing a certain concentration of one of the neonicotinoids. The concentrations used were typically around 10 ng/g and should mimic bees’ access to flowers contaminated with low levels of neonicotinoids. However, in this set- up, bees have continuous access to this ‘contaminated’ source (ie, sugar syrup) which is not the same as when bees are foraging and may be feeding on nectar from other non-contaminated flowers. In the field-based studies, colonies were positioned near to a field treated with neonicotinoid and the conditions of those colonies were monitored over a certain period of time. In this set-up it is much more difficult to have full control over the experiment (unlike in the laboratory-based
May 2015 Vol 97 No 5
Figure 2. HPLC-MS system used to analyse levels of neonicotinoids
studies). Often the sample sizes used in the experiments were limited. Results from different studies are very variable. Laboratory-based studies show that bees are affected by these sub-lethal levels (10 ng/g) of neonicotinoids, although the field studies often show that bees are not affected by neonicotinoids. In one example there was higher worker loss when fed on pollen with imidacloprid at 40 ppb, but at colony level there was no increase in mortality if the colonies were exposed to up to 20 ppb in the field. In both cases the concentrations used are higher than those experimentally determined and this might be the reason for this discrepancy.
Current Knowledge on Levels in Pollen and Nectar
One of the difficulties in establishing the
real effect of any level of neonicotinoids on bees is the knowledge of how much exposure the bees will get. In recent years several studies have been conducted worldwide on a variety of agricultural crops such as maize, sunflower, oilseed rape (Canola), soybean and cucurbit. These studies were done in countries such as France, USA, Canada, Poland, Belgium, Greece and Austria. So far, no published studies exist in the UK for detecting the levels of neonicotinoids that will still be present in either pollen or nectar, after use as a seed treatment. The author of this article and his PhD student, Kris Wisniewski, recently collected pollen and nectar from several oilseed rape fields in the UK to determine the residue levels of these neonicotinoids.
Chemical Analysis
Owing to the complex matrix of samples, especially pollen, the chemical analysis is done in two stages: extraction of the pollen or nectar followed by chemical analysis of the extract, often with high performance liquid chromatography mass spectrometry (HPLC-MS).
A common extraction technique used for pesticide analysis is a method called QuECHeRS (Quick, Easy, Cheap, Rugged, and Safe). This is a well-established method that has been used extensively in the analysis of pesticides in fruit and vegetables. After certain steps, the extract is analysed by HPLC-MS. This process is liquid chromatography (separation of the various compounds in the extract) followed by mass spectrometry (detection and identification of each compound). To determine the levels present in these samples, the pollen is usually collected via pollen traps in front of the beehives. Some studies collect the pollen directly from the plant by shaking the flowers into a plastic bag. The advantage of the latter is that the levels are directly related to the plant material, which is more difficult to determine via pollen traps. However, collecting enough material is often difficult. Nectar can either be collected from the flowers or the bees as they return to the hives. Collecting from flowers is the more common approach and generally the flowers are collected and extracted as complete flowers. In cases where nectar is present in abundance (eg, cucurbit), the nectar can easily be collected from the flowers. This is not easy to do for smaller flowers such as oilseed rape.
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