The value of Mopane worms: quantifying ecosystem services in terms of nutrient cycling in semi-arid regions of Limpopo

Abstract

Mopane worms are the late-instar stage caterpillars of the mopane moth, Imbrasia belina/ Gonimbrasia belina, found in southern Africa virtually following their hosts' trees, Colophospermum mopane, Terminalia seriscea, and Sclerocarya birrea, among others. The caterpillars are harvested for consumption and sale, thus crucial for rural livelihoods. Due to their importance to the livelihood of people and the rise in the human population, coupled with poverty, there is growing concern about harvest rates which might pose a risk to their sustainability. It is thus essential that research focused on aspects related to sustainable harvest rates is done alongside research focusing on the general species ecology. This study has two parts; firstly, the study aimed to do a systematic review to determine the current state of knowledge and research on mopane caterpillars in southern Africa. Secondly, the study aimed to assess the contribution of mopane caterpillars’ frass to nutrient cycling by determining the nitrogen mineralization rates of frass compared to that of mopane leaves. We used search engines such as African Journals Online (AJOL), Google Scholar, and Web of Knowledge. The online databases were searched using the following keywords, 'mopane worms', 'mopane caterpillar', 'Imbrasia belina', 'Imbrasia belina caterpillars', and 'Gonimbrasia belina'. All the publications were compiled and evaluated based on title, keywords, and abstract to select only studies focusing directly on mopane caterpillars. Only peer-reviewed publications were included and did not include grey literature, such as reports, presentations, dissertations, or book chapters. A total of 104 publications were found that fit the assessment criteria. The temporal distribution showed that studies on mopane caterpillars began in the 1960s, and there was a steady increase from 1991 onwards. The spatial distribution of publications closely followed the mopane caterpillars distribution in southern Africa. Two main themes emerged from the papers: resource use and ecology. Most papers (79 papers; 76%) focused on resource use, while only 25 papers (24%) focused on the species' ecology. Furthermore, spatially there was a lack of studies in many countries where mopane caterpillars occur, highlighting the spatial bias in studies. The review highlighted that research on mopane caterpillars is biased to resource use, with little attention toward the ecological role the species play in the ecosystem. We conducted an incubation experiment to measure mineralization rates between mopane caterpillars’ frass and mopane leaves. The results showed that frass had high mineralizable N due to the high NH4⁺ and NO3⁻ concentration compared to the whole and fine (ground) leaves. Overall, frass released more inorganic N than leaves; thus, it seems that mopane caterpillars’ frass has high mineralization potential and is essential in aiding nutrient cycling in the ecosystem. From this, we recommend more field and lab studies to establish the mineralization potential of mopane caterpillar frass and the rate at which the frass release nutrients. We also recommend more studies that will focus on consumption of leaves and the release of frass by mopane caterpillars to determine the biomass consumed and consequently returned to the ecosystem by mopane caterpillars.