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Introduction of a queen cage in a nucleus. © INRA, Claudia DUSSAUBAT

Bee declines: the combination of pesticides and parasites is also affecting the survival of queens

INRA scientists have now shown that chronic and indirect exposure to a very low dose of a neonicotinoid pesticide, and infection by a common bee parasite, can have a very important effect on the survival of queens under natural conditions, and modifies their physiology.  The interaction between imidaclopride and Nosema ceranae is even more harmful to the queens than each stress taken separately.  These findings are published on 31 August 2016 in Scientific Reports.

Updated on 09/12/2016
Published on 08/31/2016

In their natural surroundings, colonies of domestic honeybees (Apis mellifera) are constantly exposed to numerous pressures which include insecticides and pathogenic agents, whose combined action is suspected to be partly responsible for their decline.
Faced with greater mortality among worker bees, the queen's fertility is essential if she is to renew the population and ensure survival of the colony.  However, bee-keepers have been observing abnormal deaths among queens in recent years, to the point that in some cases they need to regularly change a large proportion of their queens so as to prevent the loss of their colonies, even though a queen can normally live for four to five years.

Insecticides in the neonicotinoid family are at the centre of this controversy because of their high toxicity to non-target organisms such as pollinators.  Until now, most research focused on the harmful effects of pesticides and other factors on worker bees.  The INRA team have now studied the behaviour of queens exposed indirectly to a neonicotinoid pesticide and/or to the pathogenic agent Nosema ceranae.

They thus reared four groups of ten queens, and pursued the experiment for two years.  In the laboratory, one group was fed by worker bees who themselves had been exposed to a neonicotinoid pesticide (imidaclopride), a second group was exposed to the parasite Nosema ceranae and a third to both stresses; the fourth group acted as controls. Thereafter, in line with the methods used by bee-keepers, the young queens were housed in small hives placed in fields (see photo) so that they could fly freely, mate and return home to lay their eggs.

Introduction of a queen cage in a nucleus. © INRA, Claudia DUSSAUBAT
Introduction of a queen cage in a nucleus © INRA, Claudia DUSSAUBAT
Labelled queen in laboratory cage. © INRA, Claudia DUSSAUBAT
Labelled queen in laboratory cage © INRA, Claudia DUSSAUBAT
   

The interaction between a pesticide and a pathogenic agent is even more harmful to queens than each stress taken separately

The results showed that chronic and indirect exposure to a very low dose of a neonicotinoid pesticide, imidaclopride (0.7 ppb, the dose that bees may encounter in nature), and infection by a common bee parasite, Nosema ceranae, had a very serious effect on the survival of queens under natural conditions, and modified their physiology. The interaction between the pesticide and pathogenic agent was even more harmful to the queens than each stress taken separately.  Indeed, during both of the experiments performed, between 90% and 100% of the queens died within a period of 45 to 90 days.

The INRA scientists observed a protective response to the action of imidaclopride, and particularly to the oxidative stress caused by the parasite-pesticide interaction; this consisted in an increase in the activity of specific enzymes in the heads and intestines of the queens.  However, these protective mechanisms were not sufficient to prevent their premature deaths.

These results may explain the loss of resilience of a colony when the death of the queen causes the discontinuation of laying, and hence the production of new worker bees.

Imidaclopride is an insecticide in the neonicotinoid family.  These so-called systemic insecticides are found in both the nectar and pollen of flowers, and subsequently in the products of hives.
Since the end of 2013, a European moratorium has banned their use on rapeseed, sunflower and cotton crops.
They are permitted in France for the spraying of fruit trees and forest conifers, and for the coating of seeds for winter wheat, winter barley, rye, sugar beet and triticale crops.  
Contact(s)
Scientific contact(s):

Associated Division(s):
Plant Health and Environment
Associated Centre(s):
Provence-Alpes-Côte d'Azur

Reference

Dussaubat, C. et al. Combined neonicotinoid pesticide and parasite stress alter honeybee queens’ physiology and survival. Sci. Rep. 6, 31430; DOI: 10.1038/srep31430 (2016)