Botrytis and Bees

/ Gerald Holmes

Wouldn’t it be wonderful if we could get bees to deliver an effective agent to strawberry flowers that would protect them from infection by Botrytis cinerea? Researchers in Sweden (Iqbal et al. 2022; https://doi.org/10.1094/PHYTO-05-21-0205-R) used bumblebees to pick up a biocontrol fungus (Aureobasidium pullulans) and deliver it to strawberry flowers, reducing Botrytis fruit rot by 45% and doubling the fruit’s shelf life.

That’s impressive, especially in light of our own results with the same biocontrol fungus where it showed no efficacy against Botrytis fruit rot under field conditions.

So, what’s going on?
Why such a big difference between their results and ours?
The biggest difference likely is related to the fact that they did their experiments in the greenhouse and we did ours in the field. The other difference is that we sprayed our biocontrol fungus onto the flowers at weekly intervals and they used bumblebees for application. One could argue that applying the material in a water suspension might work better than bumblebees, or that bees might deliver the biocontrol agent more intimately to the flower parts, but we don’t really know. 

Figure 1. Strawberry flower shows the proximity of male and female parts, making pollination by bees unnecessary. (photo by G. Holmes)
Figure 1. Strawberry flower shows the proximity of male and female parts, making pollination by bees unnecessary. (photo by G. Holmes)

Even if bees were effective at delivering a biocontrol agent that could reduce Botrytis fruit rot, it would be very hard to use this control measure in the field. Strawberries in the field are self-pollinated and bees are not necessary. At least that’s the way they’ve been grown in California all along. A little wind is all that’s needed to move the pollen the short distance from stamen to pistil (Fig. 1). In addition, in real-life outdoor conditions, honeybees would be preferred to bumblebees (Fig. 2) and it’s not clear if bumblebees would do the job as well as honeybees. Plus, bees don’t find strawberry flowers as attractive as many other flowers. Bees also run for cover during rain and that is precisely when Botrytis fruit rot flares up. Thus, the biocontrol fungus would have to persist during rainy periods or be transferred before and after a rain. Bees are also a nuisance to field workers who are in strawberry fields at least twice a week for harvest. Bee stings are at best painful and at worst life-threatening if the victim has an allergic reaction. In a tight labor market, anything that discourages the workforce could be more devastating to the growing operation than Botrytis fruit rot.

Figure 2. Honeybee and strawberry flower. Although bees are attracted to strawberry flowers, they are not necessary for pollination and California growers do not utilize beehives to pollinate their crop.
Figure 3. Bumblebee and strawberry flower. (photo credit: Yegor Aleyev)

The idea of using bees to vector a biocontrol agent to flowers is certainly clever and merits consideration, along with all the pros and cons that come with it.

Reference

Iqbal, M. et al. 2022. Bee-vectored Aureobasidium pullulans for biological control of gray mold in strawberry. Phytopathology 112:232-237. https://doi.org/10.1094/PHYTO-05-21-0205-R

2 thoughts on “Botrytis and Bees”

  1. While this is very very cool, I don’t think they characterized cull rates. Bumblebees are such effective pollinators that sometimes they cause deform due to physical damage. I’d be curious what marketable and cull rates are like in a control treatment and where the bumblebees are used for vectoring beneficials and for pollination.

    • Good point. In my brief exposure to indoor production in Europe, bumblebees were being used for strawberry pollination. I don’t think they’d be using them if they were causing a lot of deformed fruit. The authors didn’t mention the cull rate which leaves us to wonder what it was. I’m tempted to assume it was a non-issue since they didn’t mention it.

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