Re‐evaluation of a method used to study nutritional effects on bumble bees

Abstract

Bee declines are an increasingly recognised problem globally. Nutritional stress due to agricultural intensification is one of the drivers of bee declines. Therefore, understanding the nutritional requirements of bees is crucial to mitigate the effects of food scarcity on bee populations. Laboratory studies evaluating nutritional effects on bumble bees often use microcolonies as a model system for effects on queenright colonies. Microcolonies consist of workers, whereby one worker will lay male‐destined unfertilized eggs. Consequently, microcolonies exclusively produce males, while queenright colonies produce (female) workers and reproductives (males and queens). A comparison between Bombus terrestris (L.) microcolonies and queenright colonies was made by feeding three diets of varying nutritional quality. The experimental diets affected most fitness parameters of microcolonies differently than those of queenright colonies. Low‐protein, largely monofloral, pollen affected queenright colony fitness by reducing colony size and increasing larval mortality, while these fitness parameters were not affected in microcolonies. High‐protein polyfloral pollen reduced larval mortality in queenright colonies but did not affect larval mortality in microcolonies. For both colony types, a similar result was obtained when assessing offspring body mass, which was reduced by the low‐quality diet and increased by the high‐quality diet. In conclusion, our results demonstrate that microcolonies are inaccurate model systems and their use can lead to erroneous conclusions in terms of effects of nutritional stress or pollen quality on bumble bees. Our results further highlight the importance of high‐quality (here high‐protein polyfloral) pollen for sustaining colony development of bumble bees.

Higher Essential Amino Acid and Crude Protein Contents in Pollen Accelerate the Oviposition and Colony Foundation of Bombus breviceps (Hymenoptera: Apidae)

“…The workers with higher protein had higher productivity, and the drones were heavier [ 34 ]. In another study, a queenright B. terrestris colony was fed pollens containing 22%, 18%, and 14% crude protein; the results showed that the pollen group with 22% protein content had lower larval mortality and heavier weighted workers than the pollen group with 14% protein content [ 35 ]. In our study, combined with the reproduction performance and pollen nutrition, the results show that the higher crude protein and essential amino acid contents in the camellia pollen mix can contribute to the strong performance of B. breviceps queens and colonies.…”

“…Similar to some research results, our results suggest that it took a shorter amount of time for the colony to reach ten workers under the camellia pollen mix and camellia–oilseed rape–apricot pollen mix, with higher protein contents. Wynants et al reported that protein contents of 21.8%, 21.4%, 18.4%, and 11.9% were fed to a queenright B. terrestris colony, and they found that the number of offspring with a higher protein content increased faster [ 35 ]. This may be because when the first worker emerges successfully, it can continue to feed on pollen with high protein and essential amino acid contents, which allows the worker’s body size to develop faster, and helps the queen to brood, thus reducing the time required for the colony to reach ten workers.…”

“…In addition, in our study, the performance of the queens and queenright colonies was worse than that found in other studies on B. breviceps and B. terrestris . More specifically, the mortality rate of the queens was higher compared with the female mortality of B. terrestris when raised with or without a queen [ 32 , 35 ]. Some studies deem that the high death rate is caused by pollen with a low protein or amino acid content [ 40 , 69 ] or the preservation time of pollen [ 70 ].…”

“…Further, the number of drones in a microcolony fed sinapis pollen with a 21.8% protein content increased faster [ 33 ]. Moreover, an oilseed rape pollen and commercial pollen mix with a 22% protein content increased B. terrestris productivity and the size and longevity of male offspring [ 34 ] and reduced female mortality in the microcolony [ 35 ]. Pollen with a high ratio of protein and lipids can improve the collecting motivation of B. impatiens workers [ 36 ].…”

“…There are some advantages to using microcolonies over queenright colonies, such as the ease of creating and maintaining colonies, the possibility for larger sample sizes, and the easier standardization among microcolonies [ 44 , 45 ]; however, the nutritional conclusions based on laboratory-raised queenless microcolonies do not apply to queenright colonies of bumblebees in the wild, on account of the fact that a single queen collects pollen and nectar to supplement its nutrients after spring recovery. Therefore, it is more reliable to use a queen or queenright colony assessment than queenless microcolony assessments when trying to understand bumblebee queen nutritional requirements [ 35 , 46 , 47 ].…”