Oilseed rape blooms at the same time as apple trees and, like them, is mainly pollinated by honey bees. Researchers have now investigated whether nearby rapeseed fields compete with apple orchards or even attract pollinator insects. It turned out that while honey bees preferred rapeseed, more bumblebees and wild bees visited the apple blossoms – thus ensuring adequate pollination.
In spring, large fields with yellow-flowering rapeseed plants can be seen in large parts of Germany. Apple trees also bloom at the same time as the cultivated plant. Both plants are pollinated by insects – mostly honey bees. Theoretically, the flowers of both plants must therefore compete for pollinators if, for example, an apple orchard borders on rapeseed fields. The apple seems to have a disadvantage: “Apple blossoms produce a little less nectar than rapeseed blossoms,” says Julia Osterman from the Martin Luther University Halle-Wittenberg (MLU).
Honey bees fly on rapeseed
Osterman and her colleagues have investigated what influence the increased nectar production of the rapeseed plants has on honey and wild bees and whether this actually leads to a competitive advantage over apple trees. In advance they suspected that the mass flowering of the rapeseed plants either reduced the pollination of the apple trees because more insects came to the rapeseed, or increased it because the pollinators attracted by the rapeseed also visit the apple blossoms more. For their study, the researchers collected and counted the blossom visitors in apple orchards at twelve locations in Saxony-Anhalt, in each of which a different amount of rape was grown.
The result: the rape plants actually had an influence on the pollination of the apple blossoms. It was shown that the number of honey bees on the apple blossoms varied greatly from location to location. It was particularly high when the bee colonies were set up directly in the enclosures. But if there were many rapeseed fields in the vicinity of the trees, the honey bees were more attracted by these and pollinated less apple blossoms. In total, the honey bees flew up to ten kilometers a day to collect the nectar and pollen. In contrast to the honey bees, Osterman and her colleagues were able to detect the usual number of bumblebees and significantly more wild bees in the blossoms of apple trees that were close to rapeseed fields than in plants without rapeseed fields. The wild bees have a much smaller collection circle than the honey bees. “We saw most of the wild bees in the apple orchards that were directly surrounded by rapeseed,” says Osterman’s colleague Robert Paxton. “That could be because there is less competition because the honey bees are lured away,” speculates the researcher.
Wild bees compensate for pollination gaps
“We not only counted the bees, we also measured the pollination performance,” reports Osterman. For this, part of the apple blossoms was covered with a net so that honey bees, wild bees and bumblebees could not visit them to compare what happens when the natural pollinators are missing. “It is clear that apples hardly form without pollination by insects,” says Osterman. Interesting, however: Although there were fewer honey bees in the areas surrounded by rapeseed, just as many apples were formed as in areas without rapeseed nearby. Because, according to the researchers, the fruit and seed set on the apple trees persisted even without the usual number of honey bees. The stable occurrence of bumblebees and the increased existence of wild bees could have compensated for the loss of honeybees. “Local wild bee populations could compensate for the honeybees’ loss in providing pollination services in apples by providing particularly effective pollination,” the scientists sum up.
They conclude from their results that it could make sense for apple growers to specifically encourage the settlement of wild bees. “Commercially kept honey bees naturally continue to play a major role in the pollination of apple trees,” emphasizes Osterman. “Bumblebees and other wild bees could be more lucrative, however, because they are similarly effective pollinators and are less distracted by other food offers.” Possible measures for wild bee protection could be the creation of flower strips and, among other things, special nesting aids. It should be noted that some species nest underground and require open ground. Others, however, settle old barn walls and steep walls in the area. “Wild bees also need food options after the fruit has blossomed. That should be taken into account when growing apples, ”Paxton concludes.
Source: Martin Luther University Halle-Wittenberg, Article: Agriculture, Ecosystems and Environment, doi: 10.1016 / j.agee.2021.107383