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4/2/2012 - 4/8/2012

Insecticidal Seed Treatments can Harm Honey Bees

Erin Hodgson, Department of Entomology (ISU) and Christian Krupke, Department of Entomology (Purdue)

Neonicotinoids are a relatively new class of chemistry to control insects. They are now widely adopted because they are persistent and systemic in plant tissues. Most field crops in Iowa have a neonicotinoid seed treatment. Common examples of neonicotinoids include: clothianidin (Poncho ®), thiamethoxam (Cruiser ®), and imidacloprid (Gaucho ®). Active ingredient rates range from 0.25-1.25 milligrams per kernel (sold as 250-1,250 rates).

Neonicotinoids are extremely toxic to bees. Lethal LD50 rates (the rate at which half of the exposed population dies) for clothianidin are 22-44 nanograms per bee for direct contact and 2.8-3.7 nanograms per bee for oral ingestion. In other words, a single corn kernel with a 1,250 rate of neonicotinoid seed treatment contains enough active ingredient to kill over 80,000 honey bees.

There has been an increased public awareness of pollinator health and the decline of bees in North America. Researchers have identified multiple contributing factors for honey bee decline, including: Varroa mites, disease-causing pathogens, habitat loss, malnutrition, the intensity of migratory pollination services and pesticides (Fig. 1). 


Figure 1. Bees exhibit neurotoxic symptoms when dosed with neonicotinoids. Dying bees have trouble flying, collecting food and getting back into the hive. Photo by John Obermeyer, Purdue Extension Entomology.

Bees are susceptible to many broad spectrum insecticides, but how are they getting exposed to a chemistry largely used for seed treatments? Christian Krupke, a field crops entomologist at Purdue University, and several others took a closer look at how honey bees might be interacting with neonicotinoids. They published a recent article reporting several potential exposure routes. Here is a summary of their findings:

  • Bees, pollen and nectar were collected from an apiary during the corn planting season in Indiana. All dead and dying bees had traces of clothiandin, and stored pollen had high neonicotinoid levels.
  • Soil samples collected from fields not planted with a seed treatment for two years still contained detectable levels of clothiandin.
  • Dandelions collected from around field edges before planting had detectable levels of neonicotinoids (Fig. 2).
  • Talc used as an additive for planting treated seed had extremely high levels of neonicotinoids. Planter exhaust expelling tainted talc could be coming in contact with bees or plants they forage.
  • Corn pollen collected by honey bees later in the season was screened; half of the corn pollen samples analyzed had neonicotinoids.

Figure 2. Dandelions are common pollen and nectar sources for insects
. Photo by John Obermeyer, Purdue Extension Entomology.

Their paper makes the following summary: neonicotinoid exposure is likely a combination of direct contact; indirect contact with dosed weeds/crops, talc or soil; and through ingestion from pollen in dosed plants. This year, approximately 200 million acres of crop land will be planted with crops that are treated with neonicotinoids, 94 million with corn alone. This means that some exposure is inevitable, but the following recommendations may help minimize the danger to honey bees during the planting season:

  • Farmers should communicate with nearby beekeepers or apiaries about your intentions to plant. Visit the Iowa Department of Agriculture and Land Stewardship Sensitive Crops website for more information.
  • Beekeepers should move hives away from production fields during the planting period if possible.
  • Always use the recommended amount of talc to allow proper planting, removing this lubricant is not recommended.
  • Do not clean planter equipment/hoppers near fields, especially around flowering plants.

Because of the importance of pollinators and the prevalence of these insecticides in our cropping systems, there is a great deal of research on this topic in independent labs all over the world. We will likely see more studies that explore the linkage between pollinator decline and pesticides in the near future, so stay tuned. For now, the best thing to do is minimize the high level exposures during planting as much as possible using the steps outlined above.


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847. Christian Krupke is an associate professor of entomology at Purdue University with extension research responsibilities.

Early Planted Acres May Lose Replant Insurance

By William Edwards, Department of Economics

Unusually warm and dry weather has allowed crop producers to start spring field work earlier than can be remembered by most people. Getting an early start on tillage and planting reduces the risk of getting behind schedule later if an extended period of rainy weather occurs, but it also has its risks. The Risk Management Agency (USDA) has some specific rules about early planted crops with regard to crop insurance coverage. For each insurable crop RMA has set an “early planting date.” The earliest planting dates allowed for counties in the state of Iowa are April 11 for corn and April 21 for soybeans. Dates will vary in other states, and by county within a state.

Acres planted before these dates are no longer eligible for replant coverage payments should it be necessary to replant them. The maximum replant payments each year are equal to 8 bushels of corn and 3 bushels of soybeans, times the RMA projected price for that year, which is the price used to establish the value of the insurance guarantees that the producer  purchases. For 2012 the projected prices are $5.68 per bushel for corn and $12.55 for soybeans, so the maximum replant payments are $45.44 and $37.65 per acre, respectively.

Any acres that are planted before the earliest planting dates lose replant coverage, even if the entire farm or insurance unit has not been planted. However, early planting does not affect a producer’s normal yield or revenue insurance guarantee. That guarantee is still in effect, and any indemnity payments will depend on the final harvested yield. Normal good management practices must still be followed, included replanting of crops if the potential yield increase is enough to offset the costs of replanting.

Records should be kept of when all acres are planted. Check with your local crop insurance agent for questions.

William Edwards is an Iowa State University professor of economics with extension responsibilities in farm business management. Edwards can be contacted at (515) 294-6161 or by emailing

Prospective Corn Acres in 2012: Largest Ever for Iowa

By Roger Elmore, Department of Agronomy

The USDA-NASS prospective corn plantings reports released last week by the are just that, prospective. The report released on March 30 indicates greater planting intentions than ever before in Iowa. If realized, we’ll have 14.6 million acres. Prospective nation-wide corn plantings, 95.864 million acres, if realized is the most the U.S. has had since 1937. 

These numbers of course may change between now and planting. Historically, although the U.S. numbers shift by up to a 15 percent overestimate - as they did in the drought year of 1983, they typically range within 5 percent of the final estimates (see figure). Recently though the prospective plantings estimates have matched final numbers well. The ten year average is less than 1 percent range around the final acreage reported for each year.

Stay-tuned as corn growers adjust to planting conditions and markets as we progress through the planting season. See the ICM news for more information on the ‘Best corn planting dates for Iowa.’


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or (515) 294-6655.

Alfalfa Weevil Hatch is Early in 2012

By Erin Hodgson, Department of Entomology and Adam Sisson, Integrated Pest Management

Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults can feed on plants, but the larvae typically cause the majority of damage. Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Gradually maturing larvae (Fig. 1) move down the plant and begin feeding between leaf veins. Adults (Fig. 2) eat along the leaf margin, leaving irregular notches. A heavily infested field will look frosted or silver (Fig. 3).

Figure 1. Alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Fully grown larvae are about 5/16 inches long.
Photo by Clemson Cooperative Extension Slide Series,

Figure 2. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color.
Photo by Joseph Berger,

Figure 3. Heavily defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University,

Scouting and management

Alfalfa weevils develop based on temperature, or accumulating degree days. Scouting in fields should begin at approximately 200 degree days for areas south of Interstate 80, and 250 degree days north of Interstate 80. Based on accumulated temperatures since January, weevils are active now (Fig. 4). To follow accumulating degree days (base 48°F) throughout the year, visit the ISU Mesonet website.

Figure 4. Accumulated growing degree days (base 48°F) in Iowa from 1 January – 2 April 2012.
This base 48°F degree day map is updated daily. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

To initially detect alfalfa weevil larvae in the spring, use a sweep net to sample. After finding larvae, collect six alfalfa stems from five locations throughout the field. Take each stem and shake into a bucket to dislodge larvae from the plant. Average the number of larvae per stem and plant height to determine if a foliar insecticide is warranted (Table 1). Remember, cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days.


Table 1. Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847. Adam Sisson is an Integrated Pest Management program assistant. Sisson can be contacted by email at or by calling 515-294-5899.

This article was published originally on 4/9/2012 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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