Worker termites industriously carry dry plant material to a small hole in the ground. Others climb onto the few blades of grass and cut them into pieces of two to three centimetres long, ready to be taken into the subterranean nest of the colony. Two species of harvester termites occur in Namibia: a type of Microhodotermes is found only in the south between Ariamsvlei and Lüderitzbucht, while Hodotermes mossambicus is spread across the rest of the country and even thrives on the fringe of dunes. In contrast to almost all other termite species, harvester termites can ‘harvest’ in broad daylight because pigmentation protects them against dehydration and UV exposure. All other species in Namibia – there are some 60 species of 33 genera – have to get to their food sources either under cover of darkness or through the protective tunnels and the galleries which they construct on the surface. These self-made structures also serve as shelter from enemies. Without those structures harvester termites are at the mercy of predators. Especially in times of drought, when no protecting dry vegetation is left, they are at great risk.
Hornbills, shrikes, rollers, starlings, weavers and many other bird species make sure that they do not miss out on this high-protein food. They eat as many workers as possible before the termites manage to scurry into the hole to their nest. The larger soldier termites are armed with big pincers but they can do little to defend the workers.
Farmers believe that termites are destroying grazing areas during a drought, harvesting the very last blades of grass for their colony. The fact is that it is easier to see these small insects when hardly any grazing is left and the ground is practically bare. Termites fulfil a very important task in dry areas: they do the work of earthworms. The countless tunnels of their subterranean nest loosen the soil. They carry dry plant matter underground but they do not take more than they need, as researcher Dr Sabine Grube found already twenty years ago when she studied termites in Etosha National Park. She offered the usual quantity of dry vegetation to four termite colonies and deposited a lot more of the biomass elsewhere. Nevertheless all the termites collected more or less the usual amount of dry plant mater.
Harvester termites are one of the lower termite species, i.e. the more ancient type. Higher termite species cultivate mushrooms and they are able to digest cellulose with their own enzymes. Lower termite species decompose cellulose in the fermentation chamber in their hindgut, where unicellular organisms help with the process. There are many other species which also collect grass but only harvester termites cut the blades into short pieces. When outside on the ground Hodotermes mossambicus usually does not move more than a metre away from the hole from which it exited and usually does not use it a second time. The hole is sealed and a new one is dug a few metres further.
In times of good rains the harvesting activity of Hodotermes mossambicus drops substantially, Dr Grube points out. Available plant mass and its water content are the deciding factors for the scope of the harvester termites’ activity. Good plant cover (not just grasses) with high water content causes harvesting activity to approach zero, the researcher found. When no grass is available on the plains (i.e. in November) the termites will also harvest litterfall, such as from amaranths (Amaranthaceae), mimosas (Mimosaceae) and daisies (Asteraceae). The harvester termite Hodotermes mossambicus eats a wide range of food. When the dry season starts, harvest activity increases measurably only if the water content of the biomass is less than 10%, says Dr Sabine Grube. She is currently in charge of project management at the Baltic Sea Foundation.
Author: Dirk Heinrich