Are bumble bees causing my tomato flowers to fall off?

Early market hoop house tomato growers are reporting blossom drop soon after bumble bee delivery, but the flower abortions are probably not caused by bees.

Tomato flower

I get a number of calls from hoop house tomato growers in spring regarding flowers aborting at the elbow of their stems. There has been some speculation that the bumble bees have something to do with it since they notice a lot of blossom drop just after the bees were introduced. This has become an issue I hear about as frequently as symptoms related to ethylene from leaky heat sources.

Bumble bees can bruise the stamens and petals of tomato flowers because they grab on tight with their legs and mandibles as they pollinate. Usually, this is more obvious if there aren’t many flowers yet. The bees just keep hitting the same flowers over and over again. The thought that I’ve heard passed around is that maybe all those visits stress the stem joint and the flower falls off.

It is possible for bumble bees to damage the flowers to such a degree that they abort. Bumble bee visits to tomato flowers usually last between 10 and 115 seconds, according to Nunes-Silva et al. in 2013. A four-point scale can be used to measure visitation, where each level loosely represents one bumble bee visit. A level 4 indicates about four visits and was correlated with higher fruit weights and fruit set in at least one study by Morandin et al. in 2001. Other studies reported that one visit was enough.

So, bruising is not necessarily bad, but if flowers are bruised and misshapen higher than a level 4, then visitation is too heavy and blossom drop is a possibility. Koppert Biological Systems uses the pictorial scale below and adds a fifth level indicating an extremely high visitation rate.

Damage scale
Bumble bees bruise the stamens of tomato flowers as they visit. You can gauge how often they have visited a flower by the amount of bruising. A level 5 is more than enough, and will not lead to higher fruit set. It may actually cause blossom drop. Photo by Chris Anzell, Koppert Biological.

However, I do not think bumble bees are always the culprit, and I have another explanation for Michigan hoop house tomato growers in March through May. I think the main cause of blossom drop is temperature. Tomato flowers are unable to accept or transfer pollen if the temperatures are below 55 degrees Fahrenheit at night, or above 85 F in the daytime. Bumble bees do not like working in those temperatures either. If your tomato house fluctuates between these extremes, then that leaves fewer optimal pollination hours, and flowers will abort if they are not pollinated within 50 hours. Manually-operated vent and heat systems are prone to such fluctuations, especially on 40 F sunny days with spotty cloud cover and other farm tasks with demands on your attention.

New tomato flowers open every day, but are only receptive to pollination the first 50 hours after opening. Prior to the delivery of the bumble bees, the tomatoes that were already flowering were not getting pollinated very well. As the bees were introduced, more than 50 hours had passed for many of the open flowers without pollination and they aborted just as the bees started working.

Note, tomato flowers can be pollinated through vibration from wind or the grower as they prune, etc. So, some flowers will set fruit even before the bees come. In fact, in more than one tomato hoop house I visited, the first few plants along a well-traveled path had fruit with a 1-inch caliper, indicating that the frequent traffic had pollinated the very first flowers well before bumble bees were introduced.

If you see continued blossom drop, look for other reasons. The University of Florida bulletin #HS1195, “Blossom Drop, Reduced Fruit Set and Post-Pollination Disorders in Tomato,” has great explanations of primary and secondary reasons for blossom drop throughout the season. One way to diagnose blossom drop is to put bread bag twist ties around the stem of new flowers just as they open. Follow the development of those fruit. If they abort and do not have level 5 bruising, then there are other causes for the blossom drop. If you are seeing a lot of level 5 flowers, then it is likely you have too few flowers and lots of bees. This can be a problem when bees are introduced too early or if the colony you bought is larger than you need. You can close up the box for a few days or block one of their entrance holes to reduce their flights. Additionally, you plan to order a hive with fewer bees that will grow as your flower load increases.

Remember, growing tomatoes in these conditions is pushing the envelope for revenue. Doing it well requires considerable expense for your time or wallet. Every year, a number of growers will experience these hurdles, but when successful, it is a thing to be proud of.

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