It’s complicated

By Athayde Tonhasca

Himalayan balsam (Impatiens glandulifera) was brought to the British Isles in 1839 as an addition to Kew Gardens’ collection of ornamental plants. As usually happens with introduced species, Himalayan balsam escaped into the wild, causing consternation ever since. It has spread throughout damp woodlands and along rivers, flourishing in thick stands up to 2 metres high that overshadow the local vegetation. This plant does well in a variety of climatic conditions and soil types, and has a tremendous capacity to spread.

So nobody likes Himalayan balsam. Nobody but pollinators.

Himalayan balsam © MurielBendel, Wikipedia Creative Commons
Himalayan balsam © MurielBendel, Wikipedia Creative Commons

This invasive is a nectar factory. Each flower produces about 0.5 mg of sugar per hour, a rate far higher than any European plant; flowers of most species yield less than 0.1 mg/h. And because the plant flowers late in the season, nectar it available at a time when other sources start to become scarce. So naturally, bumble bees, honey bees and wasps go for it with gusto. And there is something in store for hoverflies as well; they feed on the copious amounts of pollen produced by these flowers. Predictably, the number of bumble bees and other insects increase in areas invaded by Himalayan balsam. 

This abundance of food could have undesirable side effects. Many bees get the proteins, carbohydrates, lipids and amino acids they need from a variety of pollen sources. But thanks to the plentiful and readily available pollen from Himalayan balsam, bees stick to this easy option: in some situations, up to 90% of the pollen collected by honey bees comes from this plant, with unknown consequences to bees’ development and health. The profusion of pollen and nectar could also indirectly harm other plants: if native species receive fewer visitors, their pollination could be compromised. But the evidence for such outcomes is contradictory. Some studies suggest that Himalayan balsam reduces flower visitation and seed production of native plants; others have demonstrated no differences, or a ‘magnet effect’: Himalayan balsam attracted pollinators to itself and to plants nearby. 

A marmalade hoverfly (Episyrphus balteatus) and a common carder bee (Bombus pascuorum), two Himalayan balsam beneficiaries © Charles James Sharp (L) and André Karwath, Wikipedia Creative Commons
A marmalade hoverfly (Episyrphus balteatus) and a common carder bee (Bombus pascuorum), two Himalayan balsam beneficiaries © Charles James Sharp (L) and André Karwath, Wikipedia Creative Commons

Alien species are a hot and controversial topic among conservationists. Some highlight the damage caused by introduced species to the native fauna and flora, habitats, the economy and even human health. But other conservationists point out that alien species may have neutral or positive impacts: that is, they are alien but not necessarily invasive. The invasiveness of Himalayan balsam has been well documented, but there are mitigating factors in its favour: in some situations, this plant had no effect on local species composition, or at worst it only replaced a few ruderal species (plants that colonise areas that have been disturbed). And its presence may check the spread of harmful alternatives such as the giant hogweed (Heracleum mantegazzianum).

Assessing the impact of alien species is important because a great deal of money and resources have been spent on controlling or eradicating them, quite often unsuccessfully. It is usually assumed that invasive plants are bad for pollinators, but there isn’t much evidence to support this assumption. Like many aspects of species’ ecology, data are scarce, results are often contradictory, and generalisations are risky. In summary: it’s complicated.

Drones, the derided and suffering warriors

By Athayde Tonhasca

As bad raps go, the one earned by male bees (drones) is hard to beat. Known as freeloading loafers whose single purpose in life is to mate, they have been called ‘flying sperm’, while the Germans have tagged them with the unflattering ‘lazy Willies’ nickname.

But drones deserve better. They do not help in stocking nectar or pollen, but are fundamental for the workings of a bee colony. In the case of honey bees, drones produce body heat that helps maintain the temperature of the hive. Honey bees do not overwinter, so thermogenesis (heat production) is a life or death matter for the colony. Because drones are bigger and chunkier than female workers, their contribution is disproportionately higher. As for bumble bees, their young drones appear to help care for the immature forms, including incubating pupae during their initial development. We don’t know much about the role of male bees because researchers have not given them the attention they deserve. Their reputation as secondary players in a bee colony certainly has not helped.

Whatever input honey bee drones may have in the functioning of a colony, all are dramatically and fatally surpassed when they reach sexual maturity and the moment of copulation arrives. On a warm and sunny day during the mating season, drones fly out of their nest and join other drones 10 to 40 m above ground. These clouds of bachelor bees can be thousands strong, and attract drones from hundreds of nests. After all nearby drones have been gathered, virgin queens leave their hives and join in. Love is in the air.

A honey bee drone © Waugsberg, Wikipedia Creative Commons.

Thanks to its big eyes, a drone spots a queen in the melee. It zeroes in and grabs her. Mating is completed in less than 5 seconds, during which the drone’s endophallus (a penis equivalent) is turned inside out into the queen and inflated by haemolymph (‘blood’) under high pressure. As haemolymph rushes into his endophallus, the drone loses control of his body and falls back, unable to move. Ejaculation happens at such speed and force that it can be heard by people as a ‘popping’ sound. 

Alas, these amorous moves will not end well for the lucky suitor. His endophallus breaks off, leaving its extremity inside the queen. The drone dies shortly after. However, his anatomical sacrifice does not guarantee sole paternity. Another drone may remove the piece of endophallus from the queen and mate with her, and the process may be repeated with up to 20 successive drones. 

A honey bee’s everted endophallus © Michael L. Smith, Wikipedia Creative Commons.

We know much less about the mating adventures of bumble bees, but in general they seem to be less melodramatic. Copulation may take up to 45 min, and for many species drones mate more than once before fading away and dying. Sexually mature bumble bee drones leave the nest for a vagabond life. If you have seen bumble bees clinging to flowers at night, sleeping, they are probably drones, who have no home to go to. 

Life has more misfortune in store for our drones. They have no father or sons, although they have a grandfather and grandsons. Such surreal family settings come about because of haplodiploidy. This tongue-twister refers to a reproductive system where females have two sets of chromosomes (just like us), but males have only one. After mating, a queen bee stores sperm in an internal sac called spermatheca. If she releases sperm as an egg passes down her oviduct, the egg is fertilized and generates a female with two sets of chromosomes. If no sperm is released from the spermatheca, the egg will produce a male with a single set of chromosomes inherited from the queen: there is no genetic input from daddy. Sex is determined in a similar way for all Hymenoptera (bees, ants, and wasps), Thysanoptera (thrips) and a few other insects.

The consequences of haplodiploidy are profound. For us humans, offspring share 50% of their genes with their mothers and 50% with their fathers, so siblings share on average 50% of their genetic material: (50% + 50%)/2. Female bees however share 50% of their mother’s genes, but all of their father’s. So they are related by (50%+100%)/2, which is 75%. You can find a detailed explanation of these calculations here

This 75% helps solving Darwin’s ‘one special difficulty’, stumbled upon when writing On the Origin of Species.Almost all bees, ants and wasps in a colony are sterile workers whose function is to gather food, protect the nest and help the queen lay eggs; they do not reproduce therefore are not subject to natural selection. How could these social insects evolve?

The answer to Darwin’s conundrum is kin selection: because sister bees are more related to each other than they are to the queen or any daughters they could have, it makes evolutionary sense to pass on their genes by helping their mother produce more sisters, rather than by reproducing themselves. The theory of kin selection involves self-sacrificial behaviour and altruism, including in Homo sapiens. So naturally, it has been controversial and debated for years.

It’s time to get back to our humble drones, who seem always to be lurking in the background, even though the survival and evolution of bees depend on them. In Henry V, Shakespeare vilified The lazy yawning drone. The bard, like many writers since, was too quick to judge.