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Honey bees pollinate $15 billion worth of crops each year and 75 percent of flowering plants require pollinators. It is common for one third of a hive’s population to die each winter, and from 2007 to 2010 a problem termed “colony collapse disorder” resulted in larger losses of honeybees. The cause of these large losses appeared to have multiple explanations, including varroa mites, virus infections, disease, starvation, or other causes including insecticides. Recent scientific information has been found linking honeybee death to chemicals on seed corn. Purdue apiarist, Edward Dunn, and field crops entomologist, Christian Krupke presented information at a recent web seminar about their recent scientific findings.
Neonicotinoids are a class of insecticide now being used in virtually all seed corn treatments, to protect the seed and seedling plants. Clothianidin, in this class of chemistry is the chemical in Poncho, which dominates the seed treatment market and is extremely toxic to bees. The oral dose to kill bees (LD50) is 3 nanograms per bee. Thiamethoxam, a chemical very similar to clothianidin, is used in Cruiser, for seed treatments, but is metabolized into clothianidin by the insects. A single kernel of corn treated with Poncho 1250, treated at 1.25 mg/kernel, contains enough insecticide to kill 80 thousand bees or more. This chemical is systemic in the plant, and can be found in the corn pollen.
Studies done in 2010 and 2011 at Purdue by Dunn and Krupke looked at dead bees around the bee hives. Bees were analyzed by liquid chromatography and mass spectrometry, and they all contained traces of neonicotinoids, such as clothianidin or thiamethoxam. Seed treatments of field crops (primarily corn) are the only major source of these compounds in the area tested. Bees and frames all had traces of seed treatment insecticides. Dead bees contained clothianidin at 7.9 parts per billion (ppb). Stored pollen from hives with dead bees also had very high levels of pesticides. The bee-bread inside the colony, which contains both pollen and honey, ranged from 9 to 12 ppb of the chemical. None was detected in the healthy bees, but the healthy hives had trace levels of the chemical at around 3 ppb.
How do the bees get access to the insecticide in seed treatments? We know that soil from crop fields blows around during planting, and that dandelions are in bloom at planting time. Tests found that bees were feeding in the blooming dandelions in the field border. The insecticide clothianidin was found in the soil of the corn fields. It was also found in the soil of soybean fields where two years of soybeans were grown, and where no seed treatment was used in the previous two years. The neonicotinoid chemical was found on all the dandelions collected near the fields.
The question still remains, “how does the chemical get there?” Modern corn planters use vacuum or air to move the seed to the row. Treated seed is sticky, and requires talc in the planter to ensure uniform planting. Used talc is exhausted with the air during and after planting. Test indicated the talc contained clothianidin at levels 15,030,000 ppb, which is 700,000 times the LD50 dose needed to kill bees. It is presumed that the talc along with the insecticide becomes part of the cloud of dust moving away from the planter.
Neonicotinoid exposure is likely a combination of direct contact, indirect contact with contaminated weeds or crops, talc or soil, and through ingestion of pollen from plants treated with the insecticide. Researchers found a strong reduction of a honey bee worker’s ability to find their way home after feeding with sublethal doses of thiamethoxam. This adds to the speculation that seed treatment insecticides may be another possible cause of losses of bees from hives.
With more than 95 million acres of corn to be planted this year in the nation, some exposure is inevitable, but Krupke has the following recommendations to help minimize the danger to honey bees during planting. Farmers should communicate with bee keepers about their planting operations. Bee keepers should move bees away from corn production fields during planting if possible. Planter operators should always use the recommended amount of talc to allow proper planting; removing this lubricant is not recommended. Do not clean planter equipment or blow out seed delivery tubes near fields, or around flowering plants.
Expect to hear more about this issue, and don’t be surprised at possible changes in seed treatment or planter design in the future, reflecting the risk of bee losses. To view the entire presentation by the Purdue entomologists about honey bee deaths, see the following link: https://gomeet.itap.purdue.edu/p32228058/
By GENE McCLUER
OSU Extension Educator