Plant-insect relationships, a double-edged sword

Flowering plants have had an intimate relationship with insects for millions of years. Indeed, this is often cited as an example of co-evolution , particularly with regards to the flowers and their pollination by the insects.

Accordingly, one is used to there being a high degree of mutualism in that relationship – apart from such famous examples as female mimicry and the deception that leads to flower-biased pseudocopulation in the orchids. However, when one delves deeper into that apparently cosy world of mutually-beneficial flower-insect relationships one can find surprises that upset that otherwise neatly symmetrical and supportive symbiotic synergy.

Take for example Peter Graystock et al. who’ve shown that flowers can aid dispersal and transmission of parasites between bee species, which is a bad thing. Investigating Campanula cochleariifolia (fairies’ thimbles) and Viola tricolor (pansy) they show that certain honeybee and bumblebee parasites – deposited into the flowers when infected bees visit – can be passed therefrom directly to visiting non-infected bees or transferred to other flowers by non-susceptible insects, and thence infect appropriate bee species who visit those flowers.

Whilst this work is relevant to pollination ecology and highlights an infection risks to the bees that humans rely upon to pollinate a great many of our essential crop plants, it also underlines some of the dangers inherent in having multiple pollinators (but which strategy otherwise increases the chances of pollination and hence reproductive success…).

Unfortunately, there are risks in having but a single pollinator, dramatically demonstrated in the case of Caladenia huegelii (grand spider orchid) by John Platt who reports on the work of Ryan Phillips et al. The orchid’s habitat is increasingly fragmented – due to human activity – and the chances of the males of its single pollinating wasp species (yes, it’s good old sexually-deceptive pollination and pseudocopulation here!) and the orchid being present in the same small patch are diminishing. Hence pollination success is low, so much so that the fact the orchid survives at all is more a testament to its longevity and capacity for vegetative propagation rather than success in seed-based sexual reproduction.

[Ed. – That decidedly negative pollinating parasite association is neatly ‘balanced’ by Leif Richardson et al.’s positive publication showing that certain plant-produced secondary metabolites in floral nectar reduce parasite infections in bumblebees…]