Tomato Pollination Graphics
Images copyright 2000-2009, David L. Green

Tomato blossom drop

Blossom Drop

Only one of these five blossoms still has a chance to produce a fruit. Four have not been pollinated and are now useless. One on the lower right is still clinging to its stem; three have already dropped. Why did so many not pollinate?  This problem causes a serious loss of yield. If the growing season is short, one may not get tomatoes ripe in time before frost, as the early blossoms don't get set. If one is growing for market, one usually wants to have them for the early market, when prices are at their best.

Structure of the Tomato Blossom
(Shown upright for easier visualization; more often it extends out or down.)

  Big Boy hybrid tomato, with front parts removed to show the structure of the blossom. The ovary is at the base of the flower, the pistil, made up of style and stigma is in the center. The style is the long stalk reaching up to the bumpy and sticky stigma, which extends out well beyond the surrounding stamens.

Tomato blossom parts

     The wild progenitor of our domestic tomato, in its native Peru, was pollinated by a solitary bee that was specifically adapted to it. As tomatoes were carried to other areas, its pollinator did not go with it, and pollination was often lacking. Humans, sometimes intentionally, sometimes not, have been breeding tomatoes for the ability to self pollinate. That this does not always happen, is shown in the frequent problem of blossom drop.  Blossom drop is almost always explained in terms of temperature or other growing conditions and many tomato growers dismiss any possibility of bee involvement out of hand.  This prejudice may cost production, if even a small portion of the blossom drop is actually preventable by bee activity.

    The structure of the blossom indicates a possibility, perhaps even a preference, for cross pollination, as the stigma extends well beyond the anthers, and it grows into that position a couple days before the anthers begin pollen shed. A fuzzy, electrostatically charged, and well-dusted bee, that has just visited another flower (from another plant, called a pollenizer), can brush against the stigma and coat it with pollen. Unfortunately, cross pollination does not often occur. The native pollinator is absent, and other bees that might possibly serve, are in short supply.

   Tomatoes have another possible route to go. The blossoms are self fertile, an attribute which is a defect in nature, but which mankind has selectively bred for more pollination. A certain amount of self pollination of tomato flowers does occur. But pollen does not move well by itself from anther to stigma, as evidenced by the really poor pollination seen in greenhouses when no pollination aid is given. But shaking by wind or mechanical means can cause the release of the pollen, which drops down (the blossoms normally hanging downward) through the stamen tube to the stigma.  The best possible motion to release this pollen is from a bee that "sonicates" Sonication is the vibration of the wing muscles without flight, causing the whole flower to vibrate, and a cloud of pollen to be released onto the bee's body and at the same time, onto the stigma. Not all bees sonicate.  Today, bumblebees, if they are around, will also sonicate and serve as a substitue for the native pollinator bee.

Bumblebee pollinating tomato 

Bumblebee pollinating a tomato blossom,  Image copyright 2009, David L. Green

    Since tomatoes must have help from bees, especially sonicating bees, or from wind, or other mechanical shaking to reliably and consistantly produce a crop, they cannot truly be said to be self pollinating.  Greenhouse growers for many years employed humans with electric vibrators (one brand name: Electric Bee!) to accomplish pollination. Today these have been mostly replaced with cultured bumblebees who do it more efficiently and cheaply.

    Dr. Samuel McGregor, prior to the development of bumblebees as greenhouse tomato pollinators, noted their value in the field.  McGregor, the author of the USDA's pollination "bible," also suggested the possible importation of the native bee from Peru, as a  way to improve pollination. He commented...."the evidence indicates that if a heavy population of insect visitors could be established, the effects would be beneficial.

Baby tomato

    The culture of bumblebees has grown almost explosively and now is a multimillion dollar enterprise, proving McGregor a prophet. So far, the bees are almost exclusively used in greenhouses, but, as the price comes down, they should also be available and useful in field situations.

Halictid bee covered with pollen
Image copyright 2001, David L. Green

   Wild halictid bees, such as this (not sure of the species). are thought to be the best wild pollinators in field situations. Unfortunately they are very sensitive to pesticides.  Bumblebees are the second best, with honeybees a distant third, if they will work tomato blossoms.

Using a trimmer to mechanically pollinate tomatoes

I've got no worms in my 'maters!  Without poisons!  In South Carolina!

More on Sonication (buzz pollination)