Protecting flowers, pollen and bees at bloom.

Can fungicides harm bees, pollen, flowers, and /nut set?  I got an e-mail from a local PCA yesterday asking that question.  That prompted some research followed by a phone call to Eric Mussen, University of California Entomology Specialist at UC Davis.  Here’s what I learned…

  • Certain fungicides can harm immature bees — larvae.  In a controlled lab experiment, conducted by Dr. Mussen and others, artificial bee diet containing one of a range of fungicides was fed to bee larvae.  Larvae fed captan, iprodione (Rovral®, etc.) or ziram did not develop into adults.  Larvae fed Abound® (axoxystrobin), Elevate® (fenhexamid), Flint® (trifloxystrobin), Rally® (myclobutanil), or Vangard® (cyrodinil) reached maturity – became adult bees – at the same rate as the bee fed fungicide-free diet.
  • Under certain conditions, fungicides can interfere with pollen germination.  In a separate study with the same fungicides, Dr. Mussen found that when almond pollen from Ne Plus Ultra, Peeless, Norpareil, Carmel, Butter, or Mission was placed on a drop of germination fluid (water + sugar, boric acid, and calcium nitrate) containing field dosages of certain fungicides, the result was complete inhibition of pollen germination.  Abound® (axoxystrobin), Elevate® (fenhexamid), Flint® (trifloxystrobin), Rally® (myclobutanil), captan, and Vangard® (cyrodinil) all inhibited almond pollen germination.  Pollen germination occurred in solutions containing ziram and ipodione (Rovral®, etc.), but at levels less than in germination fluid that didn’t contain fungicide.  Interestingly, Nonpareil and Butte pollen failed to germinate in distilled water.  Pollen from the other varieties tested did germinate, but grew less than pollen placed in germination fluid (without fungicides).
  • Dried fungicides do not appear to interfere with pollen germination and pollen tube growth.  Dr. Mussen found that if flower parts were treated with fungicides – the same eight listed above – and allowed to dry for 24 hours, pollen grains placed on the female flower parts (stigmas) germinated and grew.  There is at least one published study suggesting that fungicides damage the surface of stigmas.  Dr. Mussen didn’t examine the treated stigmas for damage, but did document the germination and growth of pollen tubes through stigmas treated with fungicides 24 hours before pollination.

All almond growers, but especially those in the generally wetter Sacramento Valley, need options that allow flower protection (fungicide sprays) to coexist with essential pollinators and flower function (bees and functional flowers) as much as possible.  What can be done to minimize any potential damage to both bees and nut set?  Dr. Mussen suggested a general goal and a range of practices to meet that goal.

GOAL:  Spray orchards when pollen is gone for the day.  Anthers containing pollen open in the early morning, after the flowers have dried and relative humidity decreases with increasing temperature.  [Research by Drs. Gradziel and Weinbaum, UC Davis, showed that almond anthers don’t open and release pollen under really high RH (97%).]  Fresh pollen is then available when bees begin to forage.  In orchards with strong bee activity, the flowers are often stripped of pollen by the early afternoon.  In a perfect world, that’s the time to start spraying fungicides.  When exactly is that?  Hard to tell without actually checking, so it is best to have someone with some skin in the game check the bees in the afternoon on a day you are planning to spray fungicides. If the pollen collecting bees – the ones with the pollen packed on their legs – are moving quickly from flower to flower in a frustrated search for pollen that isn’t there, it is time to start spraying.  The nectar collecting bees work the flowers all day, digging down into the base of the flower where the nectar is found and avoiding the ends of the anthers and stigma in the middle of the flower.  Spray late but knock off spraying so the fungicide will have time to dry on the flowers before the pollen is released.

Waiting for the pollen collection to be finished does several things:

  • Pollen is not directly treated with pesticides.  This protects bees and pollen.
  • Flower surfaces are not wet when pollen is placed on them, improving pollen germination.

Spray practices and timings that take care of bees AND pollen increases the potential for a good, healthy, profitable crop in the orchard.

Print Friendly, PDF & Email

4 thoughts on “Protecting flowers, pollen and bees at bloom.

  1. The comment that timing the spray to avoid bees collecting sprayed pollen implies that we know when bees will collect pollen, and that their collection of pollen is limited to a specific period of time. If so, that should be specified in the article to be of use to farmers: do you mean during the winter and spring, until fruit has set? If so that might cover it. Also some of the pesticides that harm bees are systemic such as neonicotinoids. I would like to know if you have information, and share it regarding the impact of neonicotinoids on bees and your advice to almond farmers regarding these pesticides.

  2. Theresa,
    Valid concerns about spray timing in regards to bee activity As you can read in the paragraph starting with GOAL, observations of bee activity and bee load can provide an understanding of progress of pollen foraging and should dictate spray timing. Although many farmers try to avoid spraying fungicides during bloom, the weather may have different plans.

    Neonicotinoids (neonics) have been implicated in bee health issues. The good thing is that neonic use is very limited in almonds. Only one product is registered, and if used (which is rare), timings are typically in May when there are no bees placed in the orchard.

    If you need more information, please check pesticides labeled for use on almond at

    Hope that helps,

Leave a Reply

Your email address will not be published. Required fields are marked *