5-14-2001
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: http://www.health.state.ny.us/homens.html. You can contact the following DEC website to
locate phone numbers for your regional DEC office: http://www.dec.state.ny.us/.
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.
Sincerely,
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:
http://www.ag.ohio-state.edu/~ohioline/b641/index.html
http://www.ianr.unl.edu/ianr/entomol/beekpg/tidings/btid1998/btdmay98.htm#Item8
http://www.ianr.unl.edu/pubs/insects/g1347.htm
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 between 2.00 and 10.99 ug/bee are considered moderately toxic.
Those with an LD50 greater
than 11.00 ug/bee are considered relatively non-toxic.