By D.F. Mayer, Ph.D., Washington State University Cooperative Extension entomologist, Prosser; C.A. Johansen, Ph.D., Washington State University Cooperative Extension entomologist, retired and C.R. Baird, Extension entomologist, University of Idaho


Most bee poisoning occurs when insecticides are applied to crops during the blooming period. Other hazards are
  • Drift of toxic pesticides onto adjoining crops or weeds that are in bloom.
  • Contamination of flowering cover crops when orchards are sprayed.
  • Insecticidal dusts adhere to foraging bees and ultimately become packed with the pollen onto the hind legs. Penncap-M and Sevin are especially dangerous because they may be stored with pollen and kill newly emerged workers the following season.
  • Bees drinking or touching contaminated water on foliage or flowers.
  • Bees collecting contaminated pollen or nectar.

The most common symptom of bee poisoning is the appearance of excessive numbers of dead bees in front ot the hives. Another common symptom is lack of foraging bees. Aggressiveness in bees may be caused by most pesticides. Stupefaction, paralysis, and abnormal activities of bees are commonly caused by chlorinated hydrocarbons and organophosphorus insecticides. Regurgitation of the honey stomach contents is often caused by poisoning with organophosphorus insecticides. Bees may perform abnormal communication dances on the horizontal landing board at the hive entrance while under the influence of insecticide poisoning. Disorganized behavior patterns may lead to lack of recognition of affected field bees by guard bees.

Many bees poisoned with Sexin or dieldrin slow down and appear as though they had been chilled; such bees may take two to three days to die. Beekeepers familiar with Sevin poisoning quickly learn to recognize the "crawlers" that move about in front of the hive but are unable to fly. Dead brood in or in front of the hive is typical of Sevin, micro-encapsulated methyl parathion (Penncap-M), or arsenical poisoning. When not enough hive bees are left to cover the brood frames or care for the brood, desiccation or starvation kills the larvae. In severe cases, few bees in the hives survive, or the entire colony may be dead.

One forager returning to the hive with a load of contaminated pollen or nectar can cause extreme agitation and death of a number of bees. Several such foragers can seriously disrupt and damage the colony. Often, the queen is superseded because of the agitation of the workers, possibly aggravated by a reduction in the secretion of queen substance.

Queens may be affected, especially by slow-acting materials such as arsenicals, Sevin, and micro-encapsulated methyl parathion (Penncap-M), which may be taken into the hive with pollen. Queens may behave abnormally: for instance, lay eggs in a poor pattern. Severely weakened or queenless colonies will not live through the following winter. Queenlessness the following fall have been associated with the use of a wide variety of insecticides including arsenicals, Penncap-M, Sevin, and parathion. Typically, severe Sevin or Penncap-M poisoning makes at least half of the colonies queenless within 30 days.


A major consideration for the reduction of bee poisoning is beekeeper-grower cooperation. Many cases could be cited where a grower, simply through ignorance of the hazard to bees, has caused tremendous damage to a large number of colonies. The timing or materials of the pest control program could have been modified so that little or no poisoning occurred. In many cases this can be done without unduly increasing the control cost or inconveniencing the grower.

Beekeepers should get acquainted with the farmer on whose land they place hives. They should know about pest-control practices and other special problems that might occur.

When the grower rents colonies for crop pollination, definite verbal or written agreements can be made. One type of written contract emphasizes crop production and has the desirable effect of encouraging closer cooperation between the grower and the beekeeper. Such contracts should include details of the responsibility of the beekeeper in providing strong and effective colonies and of the farmer in safeguarding the bees from poisoning. In modern agriculture, the beekeeper often depends on the grower for bee forage and the grower depends on the beekeeper for pollination. Cooperation and understanding of each other's problems are essential.


Many states have regulations that attempt to reduce the hazard of insecticide applications to bees. These are based on the safest timing and bloom conditions for given chemicals on given crops. Note: Some of the listed pesticides have been discontinued and are no longer available or legal to use.


Following are some of the ways to help reduce bee poisoning:

What the Pesticide Applicator Can Do
  • Do not apply insecticides that are toxic to bees on crops in bloom, including cover crops in orchards and adjacent crops or interplants. With aerial application, do not turn the aircraft or transport materials back and forth across blossoming fields. Ground application is generally less hazardous than aerial application because less drift of the pesticides occurs, and smaller acreages are treated at one time.
  • Apply certain chemicals only in late evening, night, or early morning while bees are not actively foraging (generally between 6 p.m. and 7 a.m. in the north and 8:30 p.m. to 4 a.m. in the south). Evening applications are generally less hazardous to bees than early morning applications. When high temperatures cause bees to start foraging earlier or continue later than usual (5:30 a.m. to 8:00 p.m.) shift time accordingly.
  • Do not apply insecticides when temperatures are expected to be unusually low following treatment or on nights when dews occur. Residues will remain toxic to bees for a much longer time under such conditions.
  • Do not dump unused dusts or sprays where they might become a bee poisoning hazard. Sometimes bees collect any type of fine dust material when pollen is not readily available. Under such conditions, they may actually carry pesticide dusts back to the colony.
  • Use insecticides that are relatively nonhazardous to bees whenever such choices are consistent with other pest control considerations.
  • Choose the less hazardous insecticide formulations. Our tests have consistently indicated dusts are more hazardous than sprays of the same insecticide. Emulsifiable (liquid) formulations usually have a shorter residual toxicity to bees than do wettable powders. Granular formulations are low in hazard to bees.
  • Contact and ask the beekeeper to remove colonies from the area (or keep the bees confined during the application period) before applying hazardous pesticides when such measures are feasible and of value.
  • When roadside and other weed control operations involve 2,4-D and similar compounds on blooming plants, select the formulations or derivatives known to be least harmful to bees. Our tests have shown that at maximum dosage, alkanolamine salts and isopropyl esters are more toxic than other forms. Oily formulations seem to be more hazardous to bees. Spraying in late afternoon or evening will also lessen the hazard, since bees will not visit the blooms after they become curled. The only highly toxic herbicides are arsenicals and DNOSBP.
  • Observe State Department of Agriculture regulations aimed at reducing bee poisoning.
What the Grower Can Do
  • Mow or beat down orchard cover crops before applying sprays hazardous to bees. Treatment with 2,4-D is the best way to remove dandelion blooms. This is especially important in relation to the first cover spray on apples, applied during a critical foraging period when bees will fly several miles to obtain pollen and nectar from even a few blooms of dandelion, or mustard.
  • Blossom-thinning sprays have not been hazardous to bees in Washington orchards. However, Sevin used as a fruit thinner can be hazardous if cover crop blooms become contaminated.
  • Learn the pollination requirements of the crops you raise. Such information is not generally known for some insect-pollinated crops, such as lima beans. Application of insecticides hazardous to bees on these crops, or driving beekeepers out of your area by the use of insecticides on other blossoming crops will likely cause poor yields.
  • When insect pests have been damaging a crop every season, use a preventive program of early season application before pest population increases, foliage growth, and weather conditions reduce the effectiveness of insecticides. Such a program is usually less dangerous to pollinating bees and other beneficial insects as well.
  • Learn about the beekeeper's problems with chemical poisoning and enter into mutually advantageous agreements to best produce bee-pollinated crops.
What the Beekeeper Can Do
  • Do not leave unmarked colonies of bees next to orchards or fields. Post your name, address, and phone number in printing large enough to be read at some distance in all apiaries so you can be contacted readily to move the colonies when hazardous sprays are to be applied. Several regulations concerning such marking of apiaries are in effect in the Pacific Northwest.
  • Do not move hives back into fields treated with hazardous insecticides until at least 48 to 72 hours after the application. Our tests have shown that 50 to 90 percent of the killing of bees by insecticides occurs during the first 24 hours after application.
  • Choose apiary sites that are relatively isolated from intensive insecticide applications and not normally subjected to drift of chemicals. Establish holding yards of honey bee colonies at least 4 miles from orchards being treated with toxic materials.
  • Learn about pest control problems and programs so you can develop mutually beneficial agreements with growers concerning pollination service and prudent use of pesticides.
  • Be careful how you control insect pests around beekeeping storage facilities or apiaries. Vapona "No Pest Strips" will also contaminate beeswax and kill bees when the combs are put in colonies later. Use relatively low-hazard materials, such as Sevin bait granules for ant control and pyrethrum aerosols for fly control.
  • Cover honey bee colonies with wet burlap for two or three days to protect them from the initial hazards of an insecticide. Such covers should be put over the hives during the night before the crop is treated and should be kept wet during use. This method works; however, most beekeepers find it impractical.

Much of the research data on the effects of insecticides on species of wild bees has been done at WSU. Our work on the effects of chemicals on the alkali bee, Nomia melanderi, and the alfalfa leafcutting bee, Megachile rotundata, has been the most extensive to date.

The alfalfa leafcutting bee can be safeguarded by storing the nest units in a cool room or root cellar for a few days while the field is being treated. Nests with females in the ends of the tunnels can be moved at night. This bee is nearly inactive at 70ºF and completely inactive at 60ºF. Leafcutter nest shelters can be built to be covered or closed during insecticide applications to reduce the drift of dusts or sprays into the nest structures. When placing leafcutters on fields in a rotation plan, do not move nest shelters in until at least 1 week after Lorsban, Cygon, Supracide, Furadan, or malathion ULV treatments.

Do not allow insecticide dusts or sprays to drift onto alkali bee nest sites or blooming crops on which these bees are foraging.

Do not spray chemicals on or burn adjacent wild land or fence rows around red clover, cranberry, or other berry crops. Such areas provide nest sites for bumble bees that aid materially in pollinating these crops.

A classification of the relative hazard of insecticides to wild bees is presented in Table 4.

  1. There is a special tendency for Penncap-M to adhere to bees foraging on contaminated flowers. Ultimately, this material is combed from the bee hairs and deposited with the pollen on the pollen baskets. It can be a long-term hazard when stored in pollen in beehives from one season to the next.
  2. Do not use Thimet G, Di-Syston G, Phosdrin, TEPP, and methyl parathion where there is a possible fumigation hazard to alfalfa leafcutting bee shelters, alkali bee nest sites, or honey bee apiaries.
  3. Undiluted or ultralow volume technical malathion spray treatments can retain a high residual toxic hazard to honey bees for at least 5 days, and to alfalfa leafcutting bees for at least 7 days.
  4. Bees are temporarily inactivated by direct contact with oil sprays and some loss may occur.
  5. Acidified spray mixtures with Dylox are more hazardous to bees than nonacidified sprays of this material. Do not use more than recommended rates of acidifiers.
  6. Alfalfa leafcutting bees are much more sensitive to all chemicals after they have been in the field for 3 weeks or more. Time late applications to occur 6 to 7 weeks after the start of activity in the field to coincide with the natural lull between peaks of bee emergence.
  7. Specific miticides such as Kelthane and Comite should not be applied in mixtures with insecticides because this increases the hazard to bees.
  8. Do not treat during warm evenings when honey bees are clustered on the outside of the hives.
Note: Some of the listed pesticides have been discontinued and are no longer available or legal to use.

TABLE 1. TOXICITY OF INSECTICIDES AND ACARICIDES TO HONEY BEES (length of residual toxic effect in hours or days)

CAUTION: Timing of insecticide applications in respect to bee poisoning hazard can be drastically modified by abnormal weather conditions. If temperatures are unusually low following treatment, residues on the crop typically remain toxic to bees about twice as long as during reasonably warm weather. Conversely, if abnormally high temperatures occur during late evening or early morning, bees may actively forage on the treated crop during these times.

!Warning. Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

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Copyright 1999 Washington State University