Protecting Honey Bees During Bloom

With almond bloom on the horizon, it is time to revisit best management practices for protecting pollinators during this critical time. Remember that communication is key during the bloom period. All parties should be kept informed so that beekeepers are aware of impending applications and applicators are aware of the requirements related to notification, materials, timing, location, and method of application. This includes growers, beekeepers, land owners-lessees, PCAs/CCAs, pesticide applicators, and county Agricultural Commissioners.

General guidelines:

  • Employ sound IPM practices:
    • Apply pesticides only when absolutely necessary based on monitoring and treatment thresholds.
    • Know all of the available materials and application timings. For insect pests, there are effective alternative timings for insecticide applications aside from the bloom period. If the weather remains dry and clear throughout bloom, there should be minimal need to apply bloom fungicides. One solid, every row application just ahead of full bloom should be adequate for good disease control under these conditions.
    • Be aware of the impacts of any treatments on pollinators and other non-target organisms.
  • Always provide adequate clean water for bees:
    • Cover or remove water sources prior to any application.
    • Keep water clean and fresh ensuring bees spend more time pollinating the crop than searching for water. Bees can forage up to 5 miles away seeking food and water if not available in the orchard, increasing their risk of contact with harmful pesticides.
  • Do not spray hives directly with any pesticide. Ensure the spray-rig driver turns off nozzles when near hives.
  • Do not spray flying bees with any applications. Aside from toxicity concerns, bees will not be able to fly because of the weight of spray droplets on their wings. Even water can impact their flight ability (and will also cause pollen grains to burst affecting pollination).
  • Avoid pesticide application or drift onto blooming weeds in or adjacent to the orchard.
  • Avoid applying systemic pesticides or those with extended residual toxicities pre-bloom.
  • Agree on proper hive removal timing:
    • Bees should be removed from the orchard when 90 percent of flowers on the latest blooming cultivar are at petal fall. Past this point, no successful pollination is taking place.
  • After removal of bees from an orchard, communication with neighbors remains important since other bees may still be foraging in the area.

Insecticide guidelines:

  • Do not apply insecticides during bloom. Much of the information and labeling related to honey bee toxicity is based on acute toxicity of foraging adults. In recent years, more research has indicated adverse effects of pesticides on developing brood, so even materials with “softer” reputations toward honey bees should be avoided.
    • One exception is Bacillus thuringiensis (Bt), which may be used at petal fall and shortly after for control of peach twig borer. For more information on monitoring and treatment of PTB using Bt during bloom, refer to the UC IPM Pest Management Guidelines for PTB in almonds.
  • Rely on other effective timing options (delayed dormant, post-bloom, in-season) for pest management. UC IPM Pest Management Guidelines for almonds provide monitoring information and insecticide and treatment timing options.

Fungicide guidelines:

  • Avoid tank mixes with insecticides, adjuvants, other fungicides. Increasing evidence shows that synergistic effects among materials can be more detrimental to both adult bees and the developing brood than applications of individual materials.
    • Addition of adjuvants for bloom fungicide applications are not necessary unless specified on the label, and may harm bees by increasing fungicide toxicity to the bee and/or impact their behavior directly. Limited canopy development should allow good spray coverage as long as the sprayer is well calibrated and properly set up, so addition of adjuvants should not be needed at bloom.
    • University of California trials are generally conducted without adjuvants, and excellent disease control is obtained with several fungicides in these trials.
  • Know the impacts of particular fungicides on honey bees and choose materials accordingly.
  • The University of California IPM Program has published a new online resource, “Bee Precaution Pesticide Ratings.”
    • Use this database to find precaution ratings for any material you are considering applying during bloom (searchable both by common name and trade name).
    • These precaution rankings (I, II, III) have been created based on all of the currently available scientific studies, but are still largely based on adult bee toxicity. The table does include effects on bee brood if that information is available. If the table does not indicate toxicity to honey bee brood, that does not suggest the material has no impact on the brood, only that such data is not available yet. Always proceed with caution and err on the side of bee safety.
    • The output table also lists known harmful synergistic mixtures based on IRAC and FRAC mode of action (in the column “Other Effects on Bees”). Again, absence of noted synergistic effects between materials only means that the data is not yet available (there are many possible combinations that still need to be investigated). Proceed with caution.
  • Apply fungicides in the late afternoon or evening when bees and pollen are not present. Each morning new flowers and anthers open to release pollen. Pollen-collecting bees often collect all of this pollen and leave the almond blossoms by mid-afternoon. Pollen that will be collected the next day is still protected inside closed flowers or anthers, which will not open until morning. It is important to ensure that fungicides have time to dry before new flowers open, anthers shed pollen, and bees begin foraging the following day.

If you suspect pesticide-related damage to honey bees, immediately report this to your county agricultural commissioner. Preserving some adult bees, brood, pollen, honey, nectar, and/or wax by immediately collecting and freezing in clean, labeled containers may be helpful for follow-up on the incident. Signs to look for:

  • Excessive numbers of dead or dying adult honey bees in front of hives
  • Dead newly-emerged workers or brood (developing larvae) at the hive entrance
  • Lack of foraging bees on a normally attractive blooming crop
  • Adult bees exhibiting stupefaction (dazed, unconscious, etc.); paralysis; jerky, wobbly, or rapid movements; spinning on the back
  • Disorientation and reduced efficiency of foraging bees
  • Immobile or lethargic bees unable to leave flowers
  • Bees unable to fly and crawling slowly as if chilled
  • Queenless hives

Additional resources:


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