UNDER THE MICROSCOPE:ONE THIRD of global agricultural production depends on pollination by the European honeybee, Apis mellifera. Unfortunately, a mysterious ailment called colony collapse disorder (CCD) has recently afflicted bees worldwide, putting nearly 100 crops that require pollination at risk. Research points to a complex disease in which a combination of factors make the bees vulnerable to viruses. The situation is described by Diana Cox-Foster and Dennis vanEngelsdorp in Scientific American(April 2009).
A flowering plant grows from a seed. Pollination must occur at the flower, producing a fruit with seeds. Dispersal of the seeds completes the cycle of plant reproduction. About 250,000 species of flowering plant require pollination. Pollination is the process whereby the male germ cell, borne in pollen, is transferred to the female sex organ. Many plants produce pollen that is heavy and sticky and doesn’t blow easily from flower to flower. These plants require agents such as insects to move the pollen. The honeybee is particularly well adapted for unconsciously picking up and moving pollen when feeding on flower nectar.
Some of the crops that require pollination by honeybees include alfalfa, clover, apple, avocado, strawberry, cherry, peach, pear, almond, macadamia, cotton, sunflower, broccoli, cauliflower, carrot, onion, squash and watermelon. Commercial beekeepers move beehives from field to field to pollinate crops. The annual value of bees’ work worldwide is $215 billion.
CCD came to prominence in 2006. Typically a honeybee colony suffering from CCD loses large numbers of its worker bees, leaving only the queen and the young workers. The dead bees are not found in the hives. Alternatively the colony may be wiped out completely. A US survey in spring 2007 revealed that more than 30 per cent of all colonies had died. The die-off resumed the following winter and also occurred in Europe, Australia, Brazil, China and Canada.
This is not the first time that the honeybee has suffered problems. Between 1987 and 2006 the number of managed bee colonies dropped by 45 per cent worldwide due to an onslaught by the varroa mite parasite. CCD colonies have been examined in detail and it is confirmed that the varroa mite is not present in sufficient numbers to explain the sudden die-off. Rather, the bees suffer from a variety of ailments, including many viral infections of known type, but no single pathogen explains the scale of the problem. The researchers hypothesised that something is compromising the bees’ immune systems, making them susceptible to many infections they would normally be able to resist.
Many suggestions were proposed as to what is weakening the bees. Radiation from mobile phones was suggested, but there is no evidence. Poisoning by pollen from GM crops was suggested, but this will not stand up because the particular insecticidal toxin produced by these crops is not activated in the honeybee gut.
Synthetic poisons used by beekeepers to control mites and new pesticides are more credible candidates for weakening the bees. A new class of pesticide, the neonicotinoids, were blamed by French beekeepers for harming insect pollinators. Research has shown that neonicotinoids interfere with the honeybee’s memory of how to return home to the hive.
The researchers also postulated that poor nutrition might be weakening bees. In a completely natural environment, bees will feed on a wide variety of flowers and are consequently very unlikely to miss out on any essential nutrient. However, bee colonies now feed on huge acreages of a single crop, which might be deficient in important nutrients. Nutritionally rich, weedy, flower-filled borders and hedgerows have also been eliminated. Beekeepers attempt to make up for these deficiencies by feeding protein supplements to the bees, but CCD still develops.
Intensive investigation of the CCD problem has failed to identify any particular chemical root cause of the problem. Initial tests failed to identify any new infectious disease that would explain the problem, but, eventually, one virus stood out – the Israeli acute paralysis virus (IAPV). IAPV was found in almost all colonies suffering from CCD. Cox-Foster exposed healthy honeybees, with no previous exposure to the virus, to IAPV. The infected bees began to die within two weeks. They didn’t die near the hives, which is characteristic of CCD.
However, not all colonies infected with IAPV develop CCD, so, either IAPV alone cannot cause the disease or some bees are IAPV-resistant. The growing consensus is that multiple factors, such as new pesticides and poor nutrition, combine to weaken colonies, making them susceptible to virus-induced collapse.
The long term solution to the problem is to identify and breed virus-resistant bees. In the meantime, beekeepers are improving colonies’ diets and keeping infections and parasites in check. And, simple changes in agriculture such as breaking up big expanses of single crops with hedgerows could restore balance to honeybees’ diets.
William Reville is associate professor of biochemistry and the public awareness of science officer at UCC –http:// understandingscience.ucc.ie
