Plants produce large quantities of nutrient-rich pollen in their flowers – a real powerhouse for many insects, including honey bees: they mix pollen that is rich in protein and fat into the food they use to nourish their larvae. Younger bees also consume the pollen themselves to build up their strength.

These plant-based energy sources are extremely important for honey bees. A lack of pollen can delay their development, make them more vulnerable to pathogens and more sensitive to pesticides. Scientific studies also show that a more diverse pollen mix improves the health and winter survival of bee colonies.

Possible Double Burden: Food Shortage and Pesticides

Late summer should therefore be a particularly difficult time for honey bees. At this time of the year, few plants are in bloom and the food supply is dwindling. In addition, the pollen that is still available may contain residues of pesticides used in agriculture.

This double burden could be particularly hard on the bees, particularly during a season that is already critical for them: in late summer, the long-lived winter bees develop in the hives. Their vitality determines whether the colony will survive the cold season unscathed.

Beehives Placed in Lower Franconia

So what is the nutritional situation of honey bees in late summer – particularly in agricultural regions? Researchers from the Chairs of "Animal Ecology and Tropical Biology" and "Behavioral Physiology and Sociobiology" at the University of Würzburg's Biocenter investigated this question. They placed 36 honey bee colonies at nine different locations in Lower Franconia. The proportion of annual cropland varied significantly around each beehive, ranging from 43 to 97 percent within a radius of two kilometres.

From the beginning of July to mid-August, the researchers took samples from the pollen traps in front of the beehives every third day. They analysed which plant species were visited by the bees and studied pollen diversity and pesticide residues. The results have been published in the renowned journal Agriculture, Ecosystems and Environment.

Same Pollen Diversity and Low Pesticide Levels

The results surprised the team led by Professors Ingolf Steffan-Dewenter and Ricarda Scheiner: the diversity of the pollen collected was always similar, regardless of whether there was a large or small amount of annual cropland surrounding the beehives. Equally unexpected: the pollen contained only small amounts of pesticides.

So could honey bees also thrive in intensively farmed agricultural landscapes in late summer?

Doctoral student Sarah Manzer, the lead author of the publication, offers a caveat: "Our results should not be overgeneralised. In our study, landscapes with a high proportion of annual cropland still contained many different fields. These can provide flowering resources along field edges as well as a range of pollen-producing weeds within the various crops." In other countries with significantly larger areas of arable land, such as the USA, the landscape may offer significantly less food.

Furthermore, different countries have different regulations on pesticide use, which can lead to varying levels of pesticide contamination. In order to obtain a comprehensive picture of the impact of agricultural land on honey bees, further studies in other areas and at other times of the year are necessary, especially in spring, when more pesticides are used.

Clover at the Top of the Pollen Suppliers

"DNA metabarcoding revealed 140 different pollen species in our 540 pollen samples," says doctoral student Sarah Manzer. The bees most frequently collected pollen from clover species, followed by cornflower pollen and sunflower pollen.
On average, bees carried pollen from ten different plants into the hive over a period of three days. This is neither a large nor a small amount. Similar values have been reported in other studies. The bees may compensate for the scarcity of flowers in late summer by flying greater distances in search of food.

Around 60 Per Cent of Samples Pesticide-free

The researchers found pesticide residues in 39 per cent of the pollen samples. Among the 16 pesticides detected, fungicides (agents used to combat fungi that damage plants), were the most common.

"In terms of concentration, the pesticide doses found are moderate to low; direct lethal effects on honey bees are not to be expected," says Sarah Manzer, based on the results of various scientific studies. However, "we cannot rule out the possibility of gradual damage to bees through negative long-term effects. Furthermore, there is limited knowledge about potential interactions with other pesticides. We also do not know the impact on significantly smaller wild bees, for example."
Further studies are needed to shed more light on this issue.

Partner Universities

In addition to the Würzburg researchers, teams from the Technical University of Kaiserslautern-Landau (Carolina Honert, Carsten Brühl / pesticide analysis, pesticide evaluation) and Ludwig-Maximilians-Universität München (Alexander Keller / evaluation of DNA metabarcoding data to determine pollen diversity) were involved in the study.