Do Soybeans Require Insects for Pollination?

Soybeans are considered to be a self-pollinating legume. This means that pollen produced within a flower fertilizes the ovary of the same flower on the same plant. Therefore insects are not required to pollinate a soybean crop. Since soybean flowers do not readily attract insects like the flowers of other legumes, crosses in nature between two soybean plants are rare. Field experiments have shown that cross pollination is usually less than 1% in soybeans.

Soybean flowers are often fertilized by the time the flower opens and may occur a full day before the flower fully opens.1 Some cultivars are entirely "cleistogamous", which means that the flower buds do not open at all and fertilization takes place with self-pollen. With some cultivars, flowers only open under the right environmental conditions. In a study of 12 soybean cultivars where both honey bees and indigenous insect populations were present, cross-pollination varied from as low as 0.09% to as high as 1.63% based on a two year average.2 Therefore insects including honey bees are not required to pollinate soybeans and the presence of insects would not be expected to significantly improve yields.

Does A Lack of Fertilization Cause Flower Abscission?

The soybean plant produces many more flowers than will actually develop into pods. Over 80% of the flowers may abscise (fall away) producing no yield. There has been speculation that soybean flowers abscise because they have not been fertilized. However, failure of fertilization is not the cause for floral abscission since almost all abscised flowers are already fertilized and contain proembryos that have undergone two or three cell divisions.3

Do Insecticide Seed Treatments Result In Contaminated Soybean Pollen?

If pollinators do happen to feed on soybean flowers does the insecticide placed on the seed contaminate the pollen? Research conducted to evaluate the potential exposure of pollinators to neonicotinoid insecticides used to treat seed analyzed 560 samples from various crops for concentrations of clothianidin, imidacloprid, thiamethoxam, and their metabolites. They concluded that "there was no detection of neonicotinoid insecticides in flowers collected from four soybean tests where neonicotinoid seed treatments were being evaluated.4

Is There Research That Shows Bees Can Increase Soybean Yields?

Anecdotal evidence has been reported that the presence of honey bees may increase soybean yields under certain conditions. These reports are difficult to verify. There have been a few trials that showed increased yields under specific circumstances.5 A recent study from Brazil purported yield increases of 18% from the introduction of honeybee colonies to a soybean field.6 However, this trial was only conducted one year with limited replication, a small experimental area with a tropical soybean variety. The benefits of either cross- or self-pollination appear to be highly depend on cultivar, temperature, moisture, and the number of insects present. Studies that have shown yield increases from bees have often employed "caged" bees, forcing them to forage on soybeans or excluding them from the crop. These limited trials with unique research circumstances should not be considered representative of what may happen in an Ontario soybean field. No link between soybean yields and the presence of bees has been demonstrated under Ontario growing conditions. If bee hives are placed next to soybean fields it is important to communicate with the soybean grower. A foliar insecticide application to control soybean aphids or spider mites will cause harm to bees.

1Dzikowski, B. 1936. Studia nad soja Glycine hispida (Moench) Maxim. Cz. 1. Morfologia. Mem. Inst. Natl. Pol. Econ. Rurale 254: 69-100.

2Ahrent, D.K. & Caviness, C.E. 1994. Natural cross-pollination of 12 soybean cultivars in Arkansas. Crop Science 34:376-378

3Abernathy, R.H.,R.G. Palmer, R. Shibles, and J.C. Anderson. 1977. Histological observations on abscising and retained soybean flower. Can. J Plant Sci. 57:713-716.

4Environ Sci Technol. 2014 Aug 19;48(16):9762-9. doi: 10.1021/es501657w. Epub 2014 Jul 23.

5Erickson, E.H. Berger, G.A., Shannon, J.G. and Robin, J.M. 1978. 'Honey Bee Pollination Increases Soybean Yields in the Mississippi Delta Region of Arkansas and Missouri'. Economic Entomology, 71: 601-603.

6Marcelo de O. Milfont, Epifania Emanuela M. Rocha, Afonso Ode'rio N. Lima, Breno M. Freitas. Higher soybean production using honeybee and wild pollinators, a sustainable alternative to pesticides and autopollination. Environ Chem Lett (2013) 11:335-341

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