Protecting Honey Bees at Bloom

It’s time for your annual pollinator safety reminder! Everyone should know to avoid insecticides when honey bee colonies are in the orchard. The only insect pest that could potentially be considered for treatment when honey bees are in the orchard is peach twig borer (PTB). Applications of Bacillus thuringiensis (Bt) during this time have been shown to be non-toxic to honey bees. This material is the only viable option to manage PTB at this time, while protecting the pollinators you pay so dearly for. There are also alternative and equally effective timing(s) for managing PTB under different circumstances. The UC IPM guidelines have more detail on PTB management using Bt and alternative treatment timings.

Ongoing research continues to examine the impacts of fungicides and adjuvants on acute adult bee toxicity (how damaging sprays are to actively foraging adults), and also the effects on the developing brood that are fed pesticide-contaminated pollen. As you can imagine, there are a huge number of potential combinations of fungicides, adjuvants, and tank mixes that bees may be exposed to in the orchard during bloom.

Below are some take-home messages for bloom fungicide applications and links to resources:

Fungicides – how many sprays are needed?

  • Conditions will dictate the number of bloom sprays needed for disease management. If the weather is dry and clear throughout bloom, there will be minimal need to apply fungicides during this period. Under environmental conditions not conducive to disease development, UC researchers (Adaskaveg et al. 2017) suggest minimizing the total number of fungicide applications during bloom by making a single delayed bloom application at 20 to 30% bloom.
  • Under wet bloom conditions, multiple bloom fungicide applications may be warranted. Practice sound integrated pest management practices – treat only for those pathogens that are best controlled during bloom (see table below) and those you know are a potential threat in your particular orchard or block based on monitoring or history. The online UC IPM guidelines provide details on monitoring and treatment timings for key almond diseases.

Adjuvants – are they needed?

  • According to the authors of the annual Fungicides, Bactericides, And Biologicals for Deciduous Tree Fruit, Nut, Strawberry, And Vine Crops (Adaskavag, Gubler, and Michailides 2017), “most fungicides are formulated with adjuvants including wetting agents, spreaders, and stickers. Unless a material specifically indicates on the product label that an adjuvant should be added, the fungicide product does not need additional adjuvants mixed into the sprayer tank to improve performance. With few exceptions, adjuvants do not statistically improve the efficacy of fungicides for managing diseases of fruit and nut commodities.”
  • All University of California efficacy trial results (+++’s in the efficacy table) are based on this premise and materials are tested without addition of adjuvants unless expressly indicated on the product label.
  • Adjuvants may increase the potential toxicity of fungicides to honey bees. To save money and protect bees, only put what is absolutely necessary in the tank.

Choosing materials:

  • Know the impacts of particular fungicides on honey bees and choose materials accordingly.
  • Visit the University of California IPM Program’s “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, including adult bee toxicity and effects on bee brood. As there are many materials and tank mix combinations yet to be examined, use the information contained here conservatively and always proceed with caution (err on the side of bee safety).

“Bee-safe” applications:

  • Apply fungicides when available pollen is at the lowest possible levels (late afternoon through very early the following morning). Pollen is released in the mornings when temperatures reach 55°F, and is often removed by foraging honey bees by mid-afternoon. The “bee-safest” time to apply fungicides is in the evening or at night when temperatures are less than 55°F.
  • Never spray hives or bees directly with any material. Contaminated foraging worker bees will carry the fungicide back to the hive where other worker bees will clean them and contaminate the hive’s food supply. Aside from these toxicity concerns, bee flight ability can be impacted from the weight of any spray droplets (even water – which is why they don’t effectively pollinate during inclement weather) and any water, from sprays or rain, can cause pollen grains to burst affecting pollination.

The Almond Board of California pollinator resource pages provide additional information and links to best management practices for protecting honey bees during almond bloom.


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