Information on Mosquito Control Program and Honey Bees
May 14, 2001

Dear Beekeeper,

As you probably know, the West Nile virus was discovered in New York in 1999.  The New York State Department of Health provides the following information on this virus:

West Nile virus is a mosquito-borne virus that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). West Nile virus is spread to humans by the bite of an infected mosquito. A mosquito becomes infected by biting a bird that carries the virus. You or your child cannot get West Nile virus from a person who has the disease. West Nile virus is not spread by person-to-person contact such as touching, kissing, or caring for someone who is infected. Persons 50 years of age or older have the highest risk of severe disease. 

As of 12/31/2000, West Nile Virus has been found in all counties in New York, except Chenango. In response to this public health threat, various county and city health departments throughout New York State have implemented spray programs to control the mosquitoes that carry this virus. Initially, an organophosphate pesticide - malathion - was used for control; but malathion is toxic to people, so most municipalities now use other chemicals. One of the products being used is Anvil®. The active ingredient (ai) in Anvil is Sumithrin, a member of the pyrethroid family. It is highly toxic to honey bees, as indicated by its LD50 value. The LD50  is the dose at which 50% of an exposed population is killed. Sumithrin has an LD50 of 0.06743 ug/bee (one ug = 1 millionth of a gram).  A second product being used is Scourge®. The active ingredient in Scourge is Resmethrin. It is also a pyrethroid and is highly toxic to bees, with an LD50 of 0.063 ug/bee. Both compounds have very short half-lives in the field.

The manufacturer's internal study of Sumithrin concluded that an application rate of 0.0674 lb ai/acre would be damaging to honey bees. Fortunately, the actual application rates for Anvil and Scourge will be no more than 0.0036 lbs ai/acre, considerably below the level that the manufacturer's study indicates would be damaging to bees. Unfortunately, some of the calculations involved in determining if a specific application rate will be damaging to bees are theoretical, and empirical data on which to base a recommendation are not available. Therefore, I cannot make any guarantees about the safety of the spray program at this time as it pertains to honey bees. However, since the actual application rates for these compounds will be about 1/20th of the rate that the manufacturer's study indicates would be damaging to honey bees, I do not anticipate serious pesticide damage to honey bees if spraying is conducted between one hour after sunset and one hour before sunrise.

Bear in mind that health department officials have a responsibility to balance public health concerns with environmental ones and may need to authorize sprays during early morning and evening hours, when mosquitoes are active. In addition, other compounds such as Malathion (Fyfanon®), fenthion (Baytex®), naled (Dibrom®), carbaryl (Sevin®) and chlorpyrifos (Dursban®) may be used. All of these are listed as highly toxic to bees and have longer residual half-lives than the aforementioned pyrethroids. I cannot make a general statement regarding the extent of damage that might occur with all compounds under all spray regimes because many factors affect the extent of the damage a pesticide will have on bees. The key factors to bear in mind when making your decision to protect your bees are: the pesticide's LD50  (the lower the LD50, the greater the risk to your bees), its residual activity (a longer residual period increases risk), the time of application (daytime applications pose the greatest risk), and the current status of the nectar flow (a strong flow increases risk). The actual risk is determined from a combination of these factors. For example, a daytime application of Malathion to flowering plants during a nectar flow poses a very high risk to bees. Nighttime applications of Anvil or Scourge at 0.0036 lbs ai/acre pose considerably less risk.  Contact your local health department and ask for a copy of their spray policy. Be sure to obtain the information mentioned above. You will have to make the final decision about protecting your bees based on local conditions.

Where the risk factors combine to pose a serious risk to bees, you will want to consider one of two options. Beekeepers with one or two colonies can confine their bees during and immediately after a spray. If you choose this method, you will have to confine your bees the night before the spray takes place, and leave them shut in for 24 hours. Before confining your bees, make sure they have sufficient space to prevent overheating - that may mean adding an extra super of empty combs. Remove the entrance reducer, if present, and screen off the entrance with 1/8" hardware cloth. Plug or tape all other holes in your equipment that the bees can use as entrances, and replace the inner and outer covers with a piece of 1/8" hardware cloth stapled over the top of the hive. Cover the hive with two layers of wet burlap, and keep the burlap wet while the bees are confined. Place a sheet of plastic loosely over the burlap during the spray to minimize direct contact with the pesticide, but remove it immediately after the spray. If your bees are in the sun, you must provide shade. A day of confinement is all that a colony can take without suffering damage, especially if it is hot. Beekeepers with more than a couple of colonies will want to move their bees out of the spray area. Be sure to contact the health department in the county where you plan to move your bees to be sure there is no spray program planned for that area.

If you leave your colonies unprotected in a spray zone, observe the entrances for several days after the spray takes place. If you note an unusual number of dead, crawling or dying bees in front of your hives, call your regional DEC office immediately and ask that a Pesticide Specialist sample your bees to determine if the kill is due to the pesticide that was sprayed in your area. Ask DEC for a laboratory assay to determine if the product used to control the mosquitoes is present in your bees. Also, report any confirmed pesticide damage to me, so that I can determine the statewide impact of the spray programs on honey bees.

I have contacted agencies in other states to learn about their experiences with these pesticides. The staff at the Florida Department of Agriculture Mosquito Control Program informed me that they have not had any bee-related problems with Anvil and Scourge when using nighttime, ground applications. Some minor damage to bees hanging outside their hives on hot nights has been noted, but that is all. In a similar vein, colleagues in Missouri have also informed me that they do not experience damage from pyrethroid sprays unless the spray contacts bees hanging out on hot nights. So, that is relatively good news.

You can contact the following New York State Department of Health website for more information on the West Nile Virus, control methods for mosquitoes, and the various pesticides being used as part of the control program:  You can contact the following DEC website to locate phone numbers for your regional DEC office:

I am contacting the state's county health departments and asking that they restrict any spraying to nighttime applications of Anvil or Scourge. Also, I am asking that they consider focusing on control of larval mosquitoes rather than the adults because larvicides are less toxic to bees. Compounds such as methoprene and Bt are effective against the immature stage of the mosquito, non-toxic to people, and relatively non-toxic to bees. Local community-based programs that focus on the elimination of breeding areas, such as old tires and cans with water, can also have a significant impact on mosquito populations.

Please share this information with all members of your organization.


Nicholas W. Calderone
Assistant Professor of Apiculture
Department of Entomology
Cornell University
Comstock Hall
Ithaca, NY 14853

Note: The following websites have good information on mosquito spraying:

Note: Generally, any pesticide with an LD50 between 0.001 and 1.99 ug/bee is considered highly toxic to bees. Compounds with an LD50 between2.00 and 10.99 ug/bee are considered moderately toxic. Those with an LD50 greater than 11.00 ug/bee are considered relatively non-toxic.

Master Beekeeper l Dyce Lab l Beekeepers Resources l General Information
B-Files l Bee Management l News l Events l CU Bee Courses l Home
© Copyright 2008, All rights reserved, Nicholas W. Calderone, Associate Professor,
Department of Entomology, Cornell University, Ithaca, NY 14853 


Updated July 2006
Web Site Design: Linda Fazzary