Indiana experienced a massive honey bee die-off during 2010 resulting from poisoning by clothianidin, an insecticide highly toxic to honey bees, which is widely used on corn. Honey bees do not forage corn, a wind-pollinated grass, for nectar. However, they readily fly through corn tassels collecting poisoned pollen on their bodies when the plants have been treated with systemic insecticides. Clothianidin is in a class of insecticides called “neonicotinoids,” nicotine-based neurotoxins that are sprayed on foliage, sprayed on the soil, or coated onto seeds to kill gnawing or chewing insects that eat foliage or other plant parts. Systemic insecticides are carried throughout a plant and poison all plant parts, including nectar and pollen. Purdue University researchers studied the Indiana bee die-off to determine how neonicotinoids are transported from corn fields to honey bees and bee hives. The scientists identified several methods of insecticide contamination of bee hives near neonicotinoid-treated Indiana corn fields. Most corn is planted with seed coated with systemic insecticides. Talc is added to mechanical planters to prevent seeds from clumping. The scientists found clothianidin levels up to 700,000 times the lethal dose for honey bees in talc dust exhausted from planters. Also, significant levels of insecticide were found in the soil of corn fields as well as fields not currently planted in corn. Neonicotinoids are considered persistent; they remain toxic long after use. Outside the corn fields, dandelions, wildflowers attractive to honey bees, were also found to contain clothianidin.
Clothianidin was found in pollen stored in nearby bee hives. An exceptionally toxic effect occurs when honey bees gather clothianidin-contaminated corn pollen from fields treated with common fungicides, a widespread practice in North America. Dead bees found surrounding the hives contained clothianidin, either eaten by the bees or contacted with the bees’ bodies. The researchers caution that “sublethal doses of insecticides can weaken bees and increase susceptibility to key parasites or pathogens.” The study by Krupke et al. may be viewed at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0029268. Today’s photo: clothianidin-treated broom corn.--Richard