By Athayde Tonhasca
Bright lemon-yellow globeflowers (Trollius europaeus) bring a bit of gaiety and colour to damp and shady areas such as woodlands, river banks, upland pastures and meadows across Scotland, northern England and northern Wales. These globe-shaped flowers are unusual: they have tightly-closed sepals that encase the true petals, which have nectaries at their base. They look like big yellow buds at the top of long stems, and rarely open fully.
This type of arrangement does not encourage most pollinators because they have hard times getting to the pollen and nectar. But one group of insects doesn’t mind: the root-maggot flies of the genus Chiastocheta, of which five species have been recorded in the UK. These are anthomyids – from the family Anthomyiidae, a name derived from the Greek anthos (flower) and myia (fly). The less flattering ‘root-maggot flies’ comes from the fact that their larvae grow inside stems or roots of various plants.
Male and female flies wiggle their way towards the flower’s nectaries. Once inside, they feed and mate, pollinating the flower in the process. Male and female are hairy, so pollen gets attached all over their bodies. But females need more than food: as true root-maggot flies, they deposit their egg on the carpels (the flower’s seed-bearing structures), and the resulting larvae develop and feed on the seeds. So Chiastocheta spp. are simultaneously pollinators and seed parasites.
Insect pollination is a mutualistic relationship, where each partner benefits from the other: the pollinator gets food or some other reward, while the plant gets fertilized. Another way of seeing it is as reciprocal exploitation: flowers produce as little pollen and nectar as necessary to attract pollinators, and insects would take away as much of those resources as flowers allow. So flowers must maintain a fine equilibrium between attractiveness and the metabolic costs of producing pollen and nectar. In the case of globeflowers, there is the added burden of seeds lost to root-maggot flies.
To avoid over-exploitation by Chiastocheta spp., globeflowers resort to chemical defence. Like other species of the buttercup family (Ranunculaceae), globeflowers are slightly poisonous: they produce compounds that discourage plant feeders. One of these chemicals, adonivernith, plays a regulatory role in the pollination/seed parasitism balance. Adonivernith is spread all over the plant, but concentrates in the flowers (it contributes to their bright yellow coloration). If the number of larvae growing inside the flower increases, the amount of adonivernith also increases, eventually inhibiting larval growth and feeding. It’s the globeflower’s way to regulate the loss of valuable seeds for the sake of pollination: when flies become a liability, they are curtailed by intoxication.
Male flies are also important for maintaining the right balance between pollination and seed predation. They are smaller and transport less pollen than females, but make about twice as many flower visits during the same time period. Male pollination incurs no seed losses, so they help reduce the pressure on their host.
In nature, it’s often a matter of harmonizing antagonistic interests.