Nutritional aspects in the development of Bruchidius sahlbergi (Coleoptera: Bruchidae) in seeds of Acacia erioloba

Abstract

Seeds of Acacia erioloba, a savanna tree of great potential for forestry and an excellent browse for game and livestock, is heavily infested with the bruchid beetle Bruchidius sahlbergi. The development from egg to beetle, and the uptake and turnover of food by the various developmental stages were investigated. After oviposition, the first-instar larva developed outside the host seed, with the egg mass as the only energy source. From the second instar onwards, development took place inside the host seed: this was followed by a non-destructive gravimetric method. During development from the second to the fifth instar, the larva consumed only a small part of the embryonic seed tissue: in single-infested seeds this was between 9 and 38% depending on the seed mass, but 100% in very small seeds. With increasing larval instars the number of starch grains in the faeces strongly decreased, but the size of the remnant grains increased. From the mineral nutrients of the food, copper and zinc were highly concentrated and passed to the beetle with a nutrient use efficiency of >90%. The nutrient use efficiency of calcium, magnesium, manganese and potassium was below 20%. Nitrogen utilization was only of the order of 34–41%, with a high concentration of nitrogen present in the frass. The approximate digestibility of the food was between 85 and 98%. The efficiency of conversion of ingested and digested food was larva-specific and varied between 10 and 45%. The impact of seed tissue consumption is discussed in relation to multiple seed infestation by this bruchid beetle.