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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Neonicotinoid Insecticides and Honey Bees

This article was published originally on 5/10/2013

Pollinators and particularly honey bees have been in the news a great deal lately because of concerns about declining health and reduced numbers.  The reasons for these losses are complicated, and according to a recent USDA and EPA comprehensive report there are multiple factors that include habitat loss, poor diet, diseases, parasites and pesticide exposure.   
 
No one factor is believed to be the cause of bee losses, rather it is a combination of problems.  Unfortunately, pollinator protection is complicated and there is no quick solution to their recent decline. Some factors affect both managed pollinators like honey bees and native wild pollinators (habitat loss, pesticides) and some are more particular to the honey bee (parasites, disease, stress).
 
One reoccurring piece of this puzzle is the role of neonicotinoid insecticides.  Neonicotinoids have been singled out as a possible toxic source, and because of concerns about bee health, the European Union has taken steps to reduce or eliminate the use of neonicotinoids in Europe.
 
What are neonicotinoids?
 
Neonicotinoids are a class of synthetic insecticides that are chemically similar to nicotine, the naturally-occurring toxin that is found in plants of the nightshade family.  Neonicotinoid insecticides are designed to be less harmful to humans than pure nicotine, but this chemistry is poisonous to all animals.  Neonicotinoids include several different chemicals, but the one most widely available to homeowners is imidacloprid.  Others are acetamiprid, clothianidin, dinotefuran, thiacloprid and thiamethoxam.
 
Why are neonicotinoids a particular concern?
 
There are other insecticides and all insecticides, by definition, harm insects, including bees.  When using insecticides, common sense and following the pesticide label directions will reduce exposure of pollinating insects to the insecticide.  For example, most insecticide labels will say not to treat when plants are blooming.
 
Neonicotinoids are being examined for a couple of reasons: 1) they are a relatively new type of insecticide that has quickly been widely adapted world-wide; 2) they are systemic, meaning that they move throughout the plant and are present in all the plant tissues for a period of time, so pollinators can be exposed even if the chemical is applied before the plant is blooming; and 3) they can be present in the ecosystem for a long time.  For instance, imidacloprid can remain present in soil for up over 5 years.
 
The neonicotinoids are also a concern because they are highly toxic to bees and much more so than some other commonly-used insecticides.  To measure this we use something called the LD50 rate.  The LD50 refers to the amount of chemical it takes to kill 50% of a test population.  LD50 values are available for a wide variety of compounds including table salt, medications, and pesticides.  Toxicity is measured for a variety of organisms, including humans although compounds are not directly tested in us.  So when you look at an LD50, the smaller the number the more poisonous a compound is because it takes less to kill 50% of the population. 
 
For a honey bee, the amount of imidacloprid that must be ingested to kill 50% of the test subjects is 0.0037 micrograms.  Compare this to carbaryl (brand name Sevin) which requires 0.14 micrograms to kill 50%, and bifenthrin that takes 0.1 micrograms.  This means that imidacloprid is 27 to 38 times more poisonous to bees than these other commonly used insecticides.
 
What is imidacloprid used for in the home landscape?
 
Imidacloprid is sold under a variety of brand names (Merit; Bayer Advanced 12 Month Tree and Shrub Insect Control; Bayer Advanced 3-in-1 Insect, Disease and Mite Control; Bonide Annual Grub Beater; Ortho Max Tree & Shrub Insect Control; Premise and others).  It is used to control a variety of insect pests including cockroaches and bed bugs in homes, white grubs in the lawn, and tree-feeding pests like Japanese beetle. 
 
There are presently 370 active insecticide product registrations containing imidacloprid in Iowa (registered with the Pesticide Bureau of the Iowa Department of Agriculture & Land Stewardship).
 
If I use imidacloprid to control grubs lawn will it hurt bees?
 
When we consider if using imidacloprid on a particular plant will hurt pollinators, the first questions is, does this plant produce flowers that pollinators visit?  In the case of grass, it does not, so exposure of bees to the imidacloprid applied to the lawn is probably minimal.  However, if there is clover or other plants in the grass that bloom, bees will visit them and be exposed to the imidacloprid.
 
If I use imidacloprid on my roses to control Japanese beetles will it harm pollinators?
 
Maybe.  Some types of roses produce abundant pollen and are well visited by bees and other pollinating insects, and they could be exposed to the imidacloprid.  Similarly, honey bees would be exposed to other insecticides that were applied to open flowers close to the time of bee visit.
 
What about using it on my trees for Japanese beetle or emerald ash borer?
 
Again, first ask if the tree produces a flower visited by bees.  In the case of ash trees being treated for emerald ash borer (and in Iowa unless you live in Allamakee County don't treat your ash!) the exposure to bees is probably minimal because ash trees flower very early in the spring and they produce a flower that is less attractive to pollinators. 
 
On the other hand, linden and basswood trees are a favorite of Japanese beetles and also produce a flower that is highly visited by bees and other insects.  In this case the risk of exposure is high and we recommend not using imidacloprid.  An alternative is to use a faster acting neonicotinoid such as dinotefuron (Safari) later in the season, after flowering.
 
How much imidacloprid is actually in the pollen and nectar? Is it enough to harm bees?
 
There is little information on this.  A recent report from the Xerces Society states that ornamental plants treated with a soil drench of imidacloprid have concentrations of imidacloprid high enough to kill bees in the blossoms for months to years following treatment.  Imidacloprid lasts longer than a year and using it annually on plants may increase the amounts found in pollen and nectar.  
 
An insecticide does not have to kill in order to have an effect.  Organisms may survive exposure to low doses but suffer physiological or behavioral changes.  There is limited information on such sublethal effects on bees, but research has indicated that exposure to neonicotinoids can affect bees ' ability to fly and navigate, learn, and reproduce.
 
What can I do as a homeowner to help beneficial insects?
 
Before you use any insecticide, including neonicotinoids, evaluate if it is necessary.  Is the damage already done, will treating improve the health of the plant, is the damage cosmetic?  Are there other things you could do to reduce damage by the insect pests.  Follow Integrated Pest Management practices to reduce insecticide use. 
 
If you have determined that an insecticide is necessary, you should consider using least toxic options first like soap or oil based insecticides.  Then consider if the plant is a flowering plant and if the blooms are attractive to bees.  If it is we do not recommend using imidacloprid.  You can use spray insecticides that cover the outside of the plant after bloom or you use a neonicotinoid if you use it after bloom.  For instance dinotefuran applied as a trunk spray moves into the plant quickly and can be applied after bloom.