Readers’ wildlife photos

Today we have another text-plus-photo story of pollination by our resident expert, Athayde Tonhasca Júnior. Athayde’s captions are indented and you can enlarge the photos by clicking on them.

Oil-prospecting and private messages

The yellow loosestrife (Lysimachia vulgaris) is a familiar sight along riverbanks, pond edges, marshes, swamps and other wetland habitats in Europe and Asia. This plant is a favourite of gardeners and landscapers, but when conditions are right it forms dense stands that take over riparian habitats and displace other species. Because of its vigorous growth and the difficulties in controlling it, the yellow loosestrife is considered an undesirable introduced species in the US and Canada.

A yellow loosestrife inflorescence © AnRo0002, Wikimedia Commons:

During the summer months, boggy, wet landscapes can be blanketed by masses of yellow loosestrife blooms. Each one of those flowers looks like a run-of-the-mill source of nectar or pollen for insects. But the yellow loosestrife belongs to a special group of plants: its flowers have specialised oil-secreting glands called elaiophores – from the Ancient Greek élaio (oil) and phóros (bearing).

The five stamens of a yellow loosestrife flower are fused together and covered by thousands of glandular hairs, the elaiophores © Stefan Vogel, IWF (Göttingen):

Detail of the tip of a stamen’s filament secreting fatty oils © Stefan Vogel, IWF (Göttingen):

The lipids, saturated fatty acids and free fatty acids secreted by elaiophores are of no interest to most insects, but one ground-nesting solitary bee depends on those oils. The bee, unsurprisingly known as the yellow-loosestrife bee (Macropis europaea), is mostly active from June to July—about the same time that yellow loosestrife blooms. Male and female bees feed on nectar from different plants, but females get pollen and floral lipids only from yellow loosestrife.

A female yellow-loosestrife bee © Bee Inventory and Monitoring Lab:

Most bee species feed their young pollen dabbed in nectar, but all 16 known loosestrife bees (Macropis spp.) swapped nectar for floral oils from Lysimachia spp. loosestrifes (Lythrum spp. are closely related and also known as loosestrifes, but play no part in our story). This substitution makes bees’ baby formulas about eight times richer in calories per unit of weight than nectar-pollen mixtures. And a female bee needs that boost. She visits an average of 460 flowers to collect enough pollen and oil to provision a single nest cell; on each trip, she carries a 10.6 mg load (25% of her body weight), and 20,000 flowers are needed to maintain a population of 50 individuals (Schäffler & Dötterl, 2011).

A very busy day ahead: a yellow-loosestrife bee at work © Stephen Boulton, Cambridgeshire Bees, Wasps and Ants:

And there’s more to oils. Like most ground-nesting bees, loosestrife bees line their nests to prevent waterlogging and fungal infections. Nearly all solitary bees rely on glandular secretions or materials such as leaves or pebbles to waterproof their homes, but loosestrife bees use oils gathered from their host plants (Cane et al., 1983). Oily waterproofing must be quite reliable, considering that these bees tend to nest near their host plants, that is, in soggy, soft and crumbling ground.

The oil-collecting business is not for ordinary bees, which are mostly equipped to pick up and transport pollen grains and nectar. Gathering fatty oils require specialised morphological tools such as comb-like setae (‘hairs’) that mop up and retain greasy substances. In the case of Macropis spp., their fore- and mid-legs have short hairs that apparently suck up oils by capillarity and long, stiff hairs that support the shorter hairs when laden with oil or pollen. This film, shot on the banks of the river Rhine, shows all phases of loosestrife bees’ complex relationship with their host plants.

An oil bee’s tarsal segment from the front or middle leg. The bee collects oil and pollen in the same foraging trip, sometimes simultaneously © Stefan Vogel, IWF (Göttingen):

Some loosestrifes don’t depend entirely on loosestrife bees: a variety of insects help pollinate their flowers, and reproduction is easily achieved vegetatively via rhizomes. But loosestrife bees could not do without their hosts. As far as we know, all loosestrife bees are monolectic, that is, they collect pollen from a single host plant. Monolectic bees are perfectly fine-tuned to their narrow menu, but they first have to find their hosts.

Pollinating insects locate flowers by relying on signals such as colour, shape, electric fields and texture, but by far the most important sensory attractant is scent. Polylectic species (those that collect pollen from many plants) have an aromatic smorgasbord of single chemicals or blends to choose from, but monolectic species must follow olfactory clues that unmistakably take them to a particular plant. And one chemical acts as a reliable beacon for loosestrife bees: diacetin. Remarkably, this diglyceride compound works for other oil-secreting plants and oil-collecting bees, all from different taxonomic lineages and from all over the world, which indicates that diacetin-bee associations evolved independently multiple times. Even more remarkably, diacetin appears to have no effect on insects that do not harvest oils, so this compound acts as ‘a private communication channel between oil plants and oil bees’ (Schäffler et al., 2015).

Diacetin (C₇H₁₂O₅) synthetised in the laboratory is used as a food additive (one of the E-numbers) and as a flavouring in e-cigarettes. When secreted by loosestrifes, it is a potent bee-attractant © Emeldir, Wikimedia Commons:

There are only four Macropis species in North America, and the yellow-loosestrife bee is their sole British member. These relatively rare bees are representatives of plant-pollinator associations that require various morphological, behavioural and chemical adaptations. The complexities involved would explain why only 360 to 370 of the ~20,000 known species of bees collect floral oil. But since its discovery by Stefan Vogel in 1969, pollination by oil-collecting bees has been increasingly reported among the at least 1,500 plant species from several families that secrete floral oils. Certainly many other cases will come to light, adding evidence to one of the most striking and specialized types of pollination mutualism.

Oil is my business: the shining oil-digger bee (Centris nitida), a native to South and Central America and introduced to USA, has spatula-shaped front and middle legs with hairs specialized to gather floral oils © The Packer Lab, Wikimedia Commons: