Large flowering fields attract bees and provide them with plenty of food for a short period of the year. But how do agriculturally grown flowering plants affect the health of bees? A new study comes to the conclusion that blooming monocultures do indeed increase the number of wild bees, but also contribute to the spread of parasites. According to the researchers, a variety of non-agricultural flowering plants in the area can help.
Monocultures in agriculture are increasing worldwide. In addition to non-flowering plants such as cereals, numerous flowering plants are also grown, including sunflowers, rape and fruit trees. For a short period of the year, all plants in a monoculture bloom at the same time and thus offer a rich source of food for insects – often in otherwise barren habitats. Professional beekeepers bring their honey bees to the flowering fields at the right time. But wild bees are also attracted by the large number of flowers. Previous studies have already shown that blooming monocultures can have a positive influence on the population size of wild bees. However, little attention has been paid to the health aspects of insects so far.
How monocultures affect bees
A team led by Hamutahl Cohen from the University of Oregon has now analyzed how sunflower monocultures in Northern California affect the occurrence and parasite load of wild bees. The researchers also included which plants were blooming in the vicinity of the field. To do this, they collected wild bees at about weekly intervals for one summer in six fields with sunflowers and six fields with non-flowering plants, half of which had specially created flowering margins or hedges on the edge of the field, the other half with uncultivated weeds on the edge of the field.
In total, Cohen and her colleagues caught 3,376 wild bees of 35 different species. In line with previous studies, they found that the mass flowering of monocultures had a positive impact on the number of wild bees: during the main flowering time of the sunflowers in July, they found most of the animals in the fields. The effect was also evident across the years: In areas where sunflowers had been grown in the previous year, more wild bees were found at the beginning of the season – probably because the high food availability in the previous year resulted in more offspring among wild bees.
More bees, more parasites
The further analyzes showed, however, that the blooming monocultures also have disadvantages for the wild bees. Cohen and her colleagues examined 1509 of the captured bees for parasites. More than four out of five bees had at least one type of parasite. “The presence of parasites in wild bees had a significantly positive correlation with the number of bees,” the researchers report. If a particularly large number of bees came into contact with one another through a sunflower monoculture, the risk of parasites spreading also increased.
But Cohen and her colleagues also identified a potentially protective factor: “Fortunately, the non-agricultural flower resources mitigated the parasite prevalence rates. With increasing abundance of flowers at a location, the positive effect of the number of bees on parasitism decreased, so that in locations with the highest abundance of flowers the relationship between the number of bees and the occurrence of parasites was even negative, ”they write. The reason: “If the abundance of flowers is large, the bees can spread out over the resources, and the likelihood that a single bee will meet an infected individual is lower.”
Restore diverse food sources
According to the authors, the diversity of the food supply could also play a role: “Bees in flower-rich environments can collect more types of pollen, and it has been found that the diversity of pollen improves nutrition and reduces susceptibility to parasites.” They therefore recommend next to flowering monocultures if possible create many alternative food sources for wild bees. “We assume that diversification measures such as the creation of flowering hedges can promote healthy wild animal populations in agriculture, especially if they are used in a high ratio to intensively farmed areas across the landscape,” the researchers say.
“Some studies have concluded that abundant flowering plants increase the density of bees – but we have found that this appears to increase the presence of parasites in wild bees. We therefore caution against the conclusion that mass flowering cultures can promote healthy bee populations, ”summarize Cohen and her colleagues. “While the restoration and diversification of agricultural habitats are challenging, this study underscores the importance of these measures in containing the spread of disease.”
Source: Hamutahl Cohen (University of Oregon, USA) et al., Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098 / rspb.2021.1369