Soybean and Bees

Robacker et al. (1983) studied the behavior of soybean plants grown at various day
and night air temperatures, soil temperatures and soil concentrations of N, P and K,
to investigate effects of environmental conditions on flower characteristics, including
flower production, color intensity, openness, size, nectar secretion and aroma emanation
and on attractiveness of the plants to honeybees. Most flower characteristics increased
as day air temperatures, at which plants were grown, increased from 20 °C to 24 °C and
reached maximum values at 28 °C, before plateauing or declining at 32 °C, although flower
size and nectar secretion continued to increase as growing temperature increased
to 32 °C. Out of the two flower aroma components, emanation of one component
increased while the other decreased, when growing temperatures raised. The hypothesis
suggested by Robacker et al. (1983) is that the two aroma chemicals may communicate
flower-readiness information to pollinators.
Flower production and flower openness responded linearly to night air temperature
at which plants were grown, attaining highest values at higher (22 °C, 26 °C) vs. lower
(14 °C, 18 °C) temperatures. Flower production also responded linearly to soil temperature,
attaining highest values at higher (28 °C, 32 °C) vs. lower (16 °C, 20 °C) temperatures.
Of two levels of N (75 ppm and 175 ppm) and P (15 ppm and 30 ppm) fertilization tested,
the higher level of N stimulated greater flower production, flower size and nectar secretion
while the higher level of P decreased the same three flower characteristics. Conversely,
lower N and higher P promoted higher fully flower openness.
Soybean plants attractiveness to honeybees varied positively with flower characteristics and
also with environmental conditions. For plants grown at a day air temperature of 28 °C, night
air temperatures of 22 °C or 26 °C, the higher level of N combined with the lower level of P
were the most attractive to honeybees.
Severson and Erickson Junior (1984) investigated the nectar characteristics of 17 soybean
cultivars in order to assess the potential for preferential foraging by honeybees. Nectar
secretion occurred between ca. 9:00 and 15:00 h and individual flowers bloomed only one
day. Mean nectar production per flower varied from 0.022 µl to 0.127 µl among cultivars,
while total nectar carbohydrate content varied from 301 µg/µl to 1,354 µg/µl. Fructose, glucose,
and sucrose content varied from 42 µg/µl to 314 µg/µl, 43 µg/µl to 262 µg/µl, and 97
µg/µl to 986 µg/µl, respectively. Total carbohydrate per flower varied from 16.0 µg to 134 µg.
The ratios of fructose:glucose:sucrose among cultivars were distributed along a broad continuum,
from those with low sucrose (ca. 1.2:1.0:1.4) to ones with high sucrose (ca. 1.2:1.0:6.7).
There were significant differences between purple and white flowered varieties for
fructose and glucose content, nectar per flower, and total carbohydrate per flower
46 SoybeAn and bees