Christmas is coming! To celebrate this post is all about a group of wasps that pollinate figs, which are needed to make the traditional English Christmas dish Figgy Pudding.
Fig trees are unusual in that their flowers are not on display. Instead the flowers are contained within fleshy tissue, the fig. The only access point to the flowers within the fig is a narrow tube called the ostiole. Female fig wasps enter the figs via the ostiole to lay their eggs within some of the flowers. It’s a tight squeeze for the wasps and they get stripped of their wings on the way in. While they are laying their eggs they leave pollen (carried from the fig they were born in) on the flowers. Afterwards, the female wasps will die, usually still within the fig.
The eggs develop within their flowers and within a few weeks the offspring mature and the males and female mate. The males will never leave the fig but they make holes for the pollen-covered females to escape. Once they have emerged the adult females have a lifespan of only around 48 hours so they need to quickly find another fig to lay their eggs in.
The interaction between fig trees and fig wasps is beneficial to both parties. The fig tree is able to protect its flowers within the fig but still get them pollinated, at the cost of some of the flowers not producing seed because they contain wasp eggs. The wasps get to lay their eggs in a protected environment so their offspring are more likely to survive until adulthood. The fig wasps are specialised for entering figs, having flattened heads and short spiny legs (bit like running spikes). Fig and wasp interactions are highly specialised with each fig tree species having only a few species of wasp capable of entering their figs and pollenating their flowers.
In a recent paper published in PLOS ONE the authors studied the behaviour of fig wasps that pollenate Ficus hispida (1).
As the figs age they become less attractive and harder for the larger wasps to enter (1). In the figure above, the head widths of female wasps that successfully reached the fig cavity decreased as the figs aged (1). The ostiole appears to become narrower with wasps with smaller head widths getting stuck as the figs get older. The head widths of the wasps that remained outside the fig (not attempting to enter) increased with fig age, suggesting that the larger wasps were less attracted to the older figs leaving the smaller wasps to pollenate them.
The interaction between figs and pollinating fig wasps can involve another group of organisms. The pollenating fig wasps can be parasitized by other wasps, which inject their eggs into the fig from the outside. The presence of the parasitic wasps was originally thought to be a bad thing for the interaction as it leads to death of pollinating fig wasps. However, evidence now suggests that the parasitic wasps may actually stabilise it (2). The pollinating fig wasps avoid laying their eggs near the edges of the fig (where they are more vulnerable to the parasitic wasps) so those flowers are left to develop seeds (2). If the pollinating fig wasps didn’t avoid laying eggs in the flowers at the edges of the fig there would be fewer fig seeds produced, leading to a decline in fig tree numbers, which in the long run would be bad for the pollinating fig wasps too. Therefore, the parasitc wasps may help maintain the optimum balance of pollinating fig wasp egg laying and fig seed development that is required to sustain both populations.
1. Fig wasp (Ceratosolen sp.). Image by Anthony Bain (CC BY-SA 3.0) via Wikimedia Commons.
2. Fig. Image by Rainer Zenz released into the publish domain via Wikimedia Commons.
1. Liu et al (2013). Larger fig wasps are more careful about which figs to enter – with good reason. PLOS ONE.
2. Dunn et al (2008) A role for parasites in stabilising the fig-pollenator mutualism. PLOS Biology.