Field Evaluation of Composted Black Soldier Fly Frass as a Soil Amendment for Restoration of Dodonaea madagascariensis (Sapindaceae) in Madagascar

Madagascar’s Central Highlands have experienced extensive deforestation and soil degradation, limiting the success of reforestation efforts. Poor soil fertility, particularly nitrogen limitation, constrains early seedling growth in degraded landscapes. This study evaluated the field performance of composted Black Soldier Fly frass (CBSFF) as a soil amendment for the native pioneer tree Dodonaea madagascariensis within the Ambohitantely Special Reserve. Four treatments were compared across four sites using a randomized complete block design: unfertilized control, cattle manure (4 g N), CBSFF one-fold (4 g N), and CBSFF two-fold (8 g N). The experiment was conducted on seedlings aged 16 months at the start of the study, and their growth was monitored over a six-month period. Growth responses were analyzed using generalized linear mixed-effects models with site included as a random factor. Seedling survival remained near 100% across all treatments, indicating no phytotoxic effects of composted frass under field conditions. Fertilization significantly enhanced both basal stem diameter and height growth. When standardized by nitrogen input, cattle manure and CBSFF produced comparable growth responses, indicating that nitrogen availability, rather than fertilizer identity, primarily drove early seedling performance. Height growth exhibited a clear dose-dependent response, with the double-dose CBSFF treatment producing the greatest increase. Planting method had a modest effect on height but did not alter the relative performance of fertilizer treatments. These findings demonstrate that composted BSF frass functions as an effective nitrogen source for early tree establishment in degraded tropical soils and performs comparably to traditional manure under field conditions. By validating insect-derived fertilizer within a restoration context, this study supports the integration of circular nutrient systems into sustainable reforestation strategies in biodiversity-rich yet resource-limited landscapes.