Grain waste as a promising substrate for growing Hermetia illucens L. (Diptera, Stratiomyidae) larvae

Currently, the possibility of using insects in the feeding of farm animals is widely discussed. Feed products from insects will successfully replace fish meal and other components of animal origin, high-protein ingredients of plant origin based on soybean and other leguminous crops. Hermetia illucens L. larvae are able to successfully consume a variety of wastes. In the presented work, we have established for the first time the effect of grain wastes feeding on zootechnical parameters of H. illucens larvae rearing, dynamics of parameters controlled in the feed substrate (temperature, activity of hydrogen ions), assimilation and yield of nutrients, nutrition, including amino acid and fatty acid composition of larvae. The aim of the work was to evaluate the prospects and establish practical aspects of using grain wastes in rearing larvae of Hermetia illucens L. to obtain feedstuffs with high nutritional value. Experiments were conducted in 2024 at the Federal Research Centre of Animal Husbandry named after academician L.K. Ernst. Larvae aged 8 days with an average live body weight of 12.45±1.53 mg were used. Six batches of 12.45 g larvae (approximately 1000 individuals in each) were selected and divided into three groups (three replicates in each). Larvae were kept in plastic containers with dimensions 17×26×14.5 (area 442 cm2) at a planting density of 2.26 individuals/cm2. The grain waste was a mixture of substandard grain littered with soil, dust, and weed seeds. Forage preparation (by weight of ingredients) was carried out according to the scheme: for group I - mixture of wheat (80 %) + bran (20 %), water was added until the moisture content of 64 % (C); for group II - mixture of components wheat (40 %) + bran (10 %) + vegetables (Peking cabbage) (50 %), water was added until the moisture content of 64 % (C+V); for group III, water was added to the grain waste until the moisture content of the mixture was 64 % (GW). All components were ground before preparation of the mixture. The total weight of the fed mixtures per container was 1000 g for C, 1000 g for C + V, and 1200 g for GW. The first feeding was carried out at the rate of 1/2 of the total amount of each feed variant (C and C+V groups - 500 g each, GW group - 600 g of fodder substrate); the second feeding - on the 7th day of the experiment (500 and 600 g, respectively). Temperature and hydrogen ion activity in the medium of substrates with larvae were recorded every day during the experiment using pH-meter Testo 205 (Testo, China). The feeding substrate was stirred every day. When observation showed the presence of darkened individuals in the total mass, the containers were prepared for resettlement and removal from the experiment. Larvae were counted and a sample from each container was weighed separately, recording the mass of every 100 individuals. Their total mass and mass per individual were then determined. Bioconversion and feed conversion ratio (FCR) were estimated when the first pre-pupae appeared. Larvae were processed by drying them in a Chirana HS62A oven (Chirana, Czechoslovakia) at t = 65 °С for 24 h. After the end of the experiment, averaged samples of feed, larval waste, and dried larvae samples were subjected to chemical analysis according to generally accepted methods. The apparent digestibility of nutrients was calculated, and the balance of N, Ca, P was calculated. The total amino acid (AA) content of larval meal was determined by ion-exchange chromatography with post-column derivatization of samples with ninhydrin. The lipid portion was obtained by direct pressing of dried larvae at 70 °С on an Akita JP AKJP-500 (Akita JP, China). Fat samples from larvae were tested for fatty acid content on a Shimadzu GC 2010 gas chromatograph (Shimadzu, Japan). It was found that live body weight per individual was higher (p 0.05), indicating that comparable results in final live weight with group II can be achieved, and the values of feed intake and consumption should be taken into account when developing a feeding scheme for larvae on grain wastes. Bioconversion processes in groups I and II were characterized by similar dynamics of pH and temperature changes. In group III the pH of the medium was similar in the first phase of feeding to the values of group I, but after 7 days the pH of the medium increased significantly. Similar to groups I and II, the temperature values of the feeding substrate of group III were similar during the period up to 4 days, after which a strong temperature spike occurred. Data analysis indicates a significant (p 12:0), especially it was expressed in comparison with group II. Overall, we found that feeding grain wastes can be an effective fattening technique for larvae of H. illucens . The addition of vegetables (e.g. cabbage) can be effective in improving larval conversion of feeds based on different nutrient sources.