Oriental bumblebee: queens can survive underwater

The bumblebees are generalist pollinators that, just like bees, are crucial for pollination. The Bombus impatiens, known as the Common Oriental Bumblebee, is a species of bumblebee that is widespread in North America and is essential for pollination of numerous wild and cultivated plants. Currently, there are many causes for the decline of this species. One of these relates to the use of insecticides, but mention must also be made of the loss of habitat and, above all, the climate change, which, as is now well known, represents one of the greatest threats to the planet and biodiversity. Understanding the habits, ecological needs and threats that affect this species is crucial to promoting its conservation and preserving the balance of the ecosystems in which it lives.

In temperate regions, numerous organisms must cope with adverse conditions during inhospitable periods, such as winter or dry seasons. For this reason, each has developed its own survival strategies, such as synchronising its life cycle with seasonal changes. In insects, for example, one of the most common strategies to avoid the harsh winter temperatures is the diapause. During this period, the animal minimises its respiratory metabolism, ceases food intake and stops developing. Each species has its own way of dealing with diapause. For example, in the bombs (Bombus spp.), unpaired queens are produced by the colony in late summer and mate before wintering underground for 6-9 months. These represent the only individuals that survive the winter, while the female workers and males die before the winter months arrive.

With the advent of climate change, however, more attention needs to be paid to how inhospitable climatic conditions affect living beings. This includes the need to study potential adaptations and resilience mechanisms in response to expected temperature changes and escalation of extreme weather events, such as droughts and floods. Inwild bees, some deleterious effects of warmer winter temperatures on diapause individuals, including bumblebee queens, have already been reported. However, there is a lack of knowledge regarding the potential impacts of flooding on subterranean 'diapausing' species. Given the importance of bumblebees as pollinators and given the global decline in their populations, it becomes crucial to undertake research actions that feature this taxon.

Against this backdrop of growing concern about climate change, there emerges a new study that offers a glimmer of hope. It would appear that the queens of thecommon eastern bumblebee (Bombus impatiens) possess the amazing ability to survive even underwater. This discovery is not only fascinating, but also extremely relevant at the scientific and adaptive level. It means that these insects may be able to resist flooding caused by climate change, which would otherwise threaten their survival during the winter diapause. Their burrows may indeed be subject to flash flooding, but the resilience of the queens of the bumblebee offers an encouraging prospect for the future.

The curious thing about this discovery is that it happened by chance. The lead author of the study, Sabrina Rondeau, recounts that she was studying the effect of pesticide residues in the soil on bumblebee queens that burrow underground during the winter near agricultural landscapes. At one point, however, he noticed the unintentional accumulation of water in certain containers that housed the diapause queens. She immediately thought the worst, but to her surprise, she realised that, against all odds, they had survived.

At this point, the researcher wanted to get to the bottom of it. So she conducted a new study to better understand how these insects managed to remain undamaged. Specifically, 143 unpaired and diapause queens were selected. Each queen was placed inside a plastic tube with a perforated cover to allow breathing and containing moist soil. Afterwards, the tubes were stored in a refrigerated unit in the dark for one week. Subsequently, 126 test tubes were subjected to two different conditions. In one half the queen was allowed to float above water, while in the other half it was pushed underwater with a plunger. The remaining 17 tubes, however, were kept as a control group. For each condition, one third of the queens were left submerged in water for eight hours, another third for 24 hours and the remainder for seven days, simulating various flooding situations.

The conclusions of the study met expectations and confirmed the researchers' hypotheses. 81% of the hibernating bumblebee queens, which had been submerged in water, and 88% belonging to the control group not only survived for seven days, but also remained alive eight weeks after the end of the experiment, once they returned to dry conditions. Furthermore, the tests showed that queens with a higher weight had a higher probability of survival. Such a finding is more unique than rare and has probably never been highlighted before precisely because we are talking about insects that shelter underground, so it is difficult to study their adaptations.

The answer may lie in the way they breathe during diapause. During this period, these insects practise an intermittent breathing pattern, known as discontinuous gas exchange. This involves the closing of the spiracles for long periods of time alternating with short, intermittent gaseous exchanges. This strategy not only reduces the risk of dehydration and exposure to environmental challenges such as hypoxia, but also limits the entry of water through the spiracles during immersion. In addition, it should be considered that the diameter of the spiracles is extremely small and the cuticle is highly hydrophobic, both of which contribute to further reducing water ingress. It is plausible, moreover, that during the opening of the spiracles, cuticular gas exchange and the presence of trapped air bubbles act as physical crushes, satisfying the low oxygen demand associated with diapause.

Actually, 'submersion tolerance' is nothing new to the scientific world. It is known, in fact, that in many insects inhabiting floodplains this adaptation is widespread. In this context, however, it is the eggs that go into diapause. There are studies showing that this phenomenon occurs in collemboles, grasshoppers and cicadas. In contrast, few examples of flood tolerance are available among adult diapause species, making bumblebee queens a more unique than rare case among documented flood-tolerant species. Indeed, previous studies have shown that most insects that hibernate as adults (e.g. many terrestrial beetles) are unable to tolerate winter submergence in floodplains and migrate instead to sheltered areas to escape unfavourable conditions.

These results highlight the unequivocal ability of diapause queens of B. impatiens to be able to survive any flooding for at least a week. This brings a sigh of relief to all those who study or care about this species and who were concerned about the damage that the effects of climate change might cause it. However, we cannot consider the debate concluded. Further research is needed to confirm whether this ability is common to other species of bumblebees, assessing how widespread this trait is. This insight could provide valuable information for the conservation and management of bumblebee populations, offering a better understanding of the adaptation mechanisms of these insects to environmental challenges.