Fig Trees and Wasps â€“ Competition at Close Quarters?
19 February 2008
by Dr Marié Warren
The Sycamore Fig (Ficus sycomorus) is a tall, distinctive tree with a yellowish bark and fluted stem that we see growing along the rivers in the Kruger National Park. This spectacular tree is home to a host of insects, reptiles, rodents and birds – many of which we will never see. The feast of figs it produces all year round is a source of food for all the savanna inhabitants from monkeys in the branches to impala and warthog eating the fallen fruit on the ground. But how many of us are aware of the intricate web of life surrounding these figs that hang so abundantly from the branches of the tree?
There is a fascinating relationship between the massive sycamore fig and the tiny specialized fig wasps that ensure its survival. Every fig tree species has a wasp species that only pollinates that tree species. When the figs become ripe, the wasps are attracted to the figs by the chemical ‘smell’ the figs release. The miniature wasps (Ceratosolen arabicus) crawl through the opening at the base of the fig. They lay their eggs into the flowers that are inside the fig. While they do this, they pollinate the remaining flowers. This relationship between pollinator fig wasps and fig trees is mutualistic, which means that both the fig and the wasp benefit and in fact both need each other to survive.
The delicate fig wasps have a lifecycle that is completed inside the fig. The wingless males emerge first from the flowers inside the fig and mate with the winged females. They then make a hole in the fig so that the females can escape. Once a ripe fig is opened, the oxygen in the air stimulates the females to emerge from the tiny flowers inside the fig. The females fly out and begin searching for the next fig to begin the lifecycle again. Some species of fig wasp have a very short life span, having only about 9 hours to escape the fig and find another fig to lay their eggs.
But, there are other wasp species that abuse this system. These non-pollinating wasps (Ceratosolen galili - known as cuckoo wasps) also crawl into the fig, or other wasps lay their eggs from the outside of the fig, and use the flowers for their developing larvae, but do not pollinate any of them. These imposter wasps trick the fig tree into providing food and shelter for the developing wasps but the fig will not produce any seeds.
Visitors to Skukuza camp will see gauze bags tied around certain bunches of figs. This is part of a long-term study lead by Prof. Jaco Greeff of the Department of Genetics at the University of Pretoria in conjunction with SANParks Scientific Services. With funding from the NRF (National Research Foundation) this research project, which was started in 2003, aims to understand what is happening between the different wasp species that use the Sycamore Fig because only the pollinator wasps benefit the tree.
To do this, the figs are covered with bags as they begin to ripen to make sure that no wasps can creep inside. Once the figs are ready, adult wasps are collected from these ripe figs on another tree nearby, often high up in the branches. The tiny wasps are put on the figs and they quickly wiggle their way inside. The experiment is designed to introduce fixed numbers of both the pollinator and non-pollinator wasps into these figs and to then see the effects of these two species and how they interact inside the fig. Some of the keys questions in this study include: Is the pollinator wasp’s egg-laying affected when the fig is shared with other, non-pollinating, wasps? Do they lay fewer eggs? Do they lay more male than female eggs when the fig is shared with many individuals?
The eggs inside the figs take about 5 weeks to develop into adult wasps, but this time could be shorter when the weather is warmer. Once the wasps start to emerge from the figs they will be kept inside the gauze bags and hopefully some answers to the mysteries of wasps and figs will be revealed.