Science - April 10, 2014

Imidacloprid or not?

Roelof Kleis

Are bee deaths due to neonicotinoids? There are many who say yes. They think the neonics are directly responsible for the high mortality among bees. But not according to Wageningen research. Meanwhile there are signs of an improvement in bee death rates.

Three weeks ago, a motion by the Party for the Animals calling upon the government to ban the use of all neonicotinoids got support from a majority of the Dutch Lower House.  That was on a Tuesday. Four days later, bee researcher Sjef van der Steen presented his study of the exposure of bees to imidacloprid, the best known of the insect-killing neonics. His conclusion after a two-year study is that imidacloprid has an effect on the number of bees, the brood and the urge to swarm but not on winter mortality.

The timing is remarkable but it was pure chance. Van der Steen presented his work at the annual symposium on bee health organized by Wageningen bee researchers in the Hof van Wageningen. Van der Steen’s study is part of BIJ 1, a programme set up by the Ministry of Economic Affairs to research bee deaths and what causes them. The official report, including press release, has yet to be published and he is still working on a scientific paper.


Even so, Van der Steen’s firm conclusions have not gone unnoticed. The Utrecht scientist Jeroen van der Sluijs, a specialist in new risks within the university’s Environmental Sciences group, was critical of the Wageningen study in an article on Kennislink. Van der Sluijs sees the neonics as the primary cause of bee deaths. His main criticism concerns the way in which the bees were exposed to imidacloprid. Van der Steen placed his poisonous sugared water in the wooden hive. In nature, bees fetch the nectar contaminated with imidacloprid elsewhere and then have to fly back to the hive, sometimes over long distances. Imidacloprid affects bees’ sense of direction. Van der Sluijs says that effect is much smaller in the Wageningen experimental design.

Bee researcher Romee van der Zee of the Dutch Centre for Bee Research (NCB), which monitors bee deaths every year, also has doubts about what exactly Van der Steen has demonstrated. ‘If you feed bees with sugared water, you may be further removed from the situation in practice than you intended.’ She also wonders how sure he can be that the control group did not collect any contaminated nectar or pollen. Van der Steen is aware of the criticisms but points out that this design was a deliberate choice. ‘This experiment is about the effect of imidacloprid on the vitality of bee colonies, not on bees’ sense of direction. It’s true that the foraging factor when bees fly out is not covered by this design. However, this is the only way to be sure of what comes into the hive and what the levels of exposure are. In fact, this gives maximum exposure. It was clear from the start that we wouldn’t be including the transport route in this experiment.’


Quite apart from Van der Steen’s experiment, the winter deaths remain a mysterious phenomenon. After years of alarming mortality rates of well over 20 percent, rates suddenly fell sharply in the winter of 2012-13 to a fairly normal level of 14 percent. The mortality in the winter that has just ended is not yet known. ‘We only start collecting data in April,’ says Van der Zee. ‘We don’t just ask whether colonies are alive or dead, we also want to know whether they are weak or strong and if they have problems with the queens, for instance. You can only answer those questions by taking a good look at the colony and you need good weather for that. We never publish the results before 1 July.’

Tackling the varroa mite effectively and in good time is crucial in keeping colonies strong
Sjef van der Steen

Even so, Van der Zee is prepared to make a prediction: she expects winter deaths to be low again this year. She says that is because of the weather. ‘The season was late starting last year. The winter was cold and long, so the varroa mites only started reproducing late, with a smaller population in the summer as a result.’ That made the efforts to tackle the mite, which Wageningen scientists see as a key factor in winter deaths, go smoothly. In addition, the winter just gone was mild without any cold periods. Van der Zee: ‘So the weak colonies were not put to a real survival test. Based on those two arguments, I expect winter losses of around 11 percent.’  Van de Zee says that the weather is the dominant factor in bee deaths anyway. ‘The likelihood of bees dying is closely related to the weather. If the weather is good so that there’s plenty of opportunity to fetch nectar and pollen and build up a strong winter population, the other factors that lead to bee deaths have less of an effect.’

Winter MOT

She is not the only person making this prediction. Spokesman Frank Moens of the Dutch Beekeepers’ Association (NBV) also thinks things are not so bad. He bases this conclusion on the results of the Winter MOT, a survey the NBV has carried out for the first time this winter. The survey (of 65 beekeepers with 530 bee colonies) looks at the relationship between winter deaths and the occupancy rate of hives just before the onset of winter. In fact, there is not really such a thing as winter deaths, explains Moens. ‘Winter deaths are actually autumn deaths. At the end of the summer, winter bees are created that survive until the spring. That transition takes place in September/October. The theory is that a poor population of winter bees will not survive the winter. And you will be able to see that already in the autumn by looking at the occupancy of the hive windows. If more than five of the ten windows in a hive are covered with bees, they should manage to survive the winter. If it’s less than five, the colony is too weak. Then the beekeeper has two options: merge weak colonies or let nature takes its course.’

Going by the results of the Winter MOT, the mortality rate is less than 7 percent.  Incidentally, the Winter MOT says nothing about why a bee colony is weak. Van der Steen and his Wageningen colleagues are convinced that tackling the varroa mite effectively and in good time is crucial in keeping colonies strong. Research by Van der Zee last year provided confirmation of this. Now Van der Steen’s fellow researcher Coby van Dooremalen is study­ing bees’ foraging behaviour using bees with a chip. She is zooming in on the effects of exposure to varroa mites, parasites and imidacloprid.

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  • Jeroen van der Sluijs

    Beste Roelof,

    Graag corrigeer ik jouw bewering dat "Van der Sluijs ziet in de neo’s de hoofdoorzaak van bijensterfte". Dat is een feitelijk onjuiste weergave van mijn positie in het wetenschappelijk debat over bijensterfte.

    Mijn positie is dat neonicotinoiden ook bij normaal gebruik (en ook bij gebruik op niet-bloeiende planten en in de bedekte teelt) onaanvaardbaar hoge verzwakkende effecten op hommels, bijen en wilde bestuivers hebben. Dat komt vooral door de schaal van het gebruik, de zeer hoge persistentie, de extreme potentie van het zenuwgif, en de translocatie naar stuifmeel van wilde vegetatie en onbehandelde gewassen. Zie onze studie:

    Voor mijn positie over de oorzaak van bijenstefte zie:

    Verder wijs ik er op dat het onderzoek van Van der Steen en collega's nog niet is gepubliceerd en nog niet is gepeerreviewd is en zelfs nog niet gesubmit, dus dat het wat voorbarig en eigenaardig is om de conclusies nu al in de media te brengen en andere wetenschappers een reactie te vragen op de conclusies van een nog niet gepubliceerde studie waarvan de (mij bekende) conceptrapportage nog vertrouwelijk is.

    Met vriendelijke groet,