Scientists from the USDA’s Aquatic Animal Health Research Unit looked at how adding frass from black soldier fly larvae to the diets of channel catfish (Ictalurus punctatus) impacted growth, feed utilisation and body composition.
Frass includes larval waste, exoskeleton sheds and residual feed ingredients and was obtained from the larvae of black solder flies fed distillers' dried grains with solubles and had a protein and fat content of 216 and 60 g/kg, respectively.
Over the course of 10 weeks, five diets containing 0, 50, 100, 200 and 300 g frass per kg diet were fed to channel catfish (5.24 ± 0.04 g) in quadruplicate aquaria to apparent satiation twice daily.
The researchers noted that frass helped growth rates, noting: “Final weight gain was significantly increased in fish fed diets containing frass at levels from 100 to 300 g/kg.”
They also noted a marked increase in the palatability of feeds containing higher levels of the insect watste.
“Fish fed diets without frass, and with 300 g/kg frass, showed the lowest and highest feed intake, respectively,” they observed.
On a less positive note, they recorded that feed and protein efficiencies were significantly lower in fish fed frass at levels of 200 g/kg or above, compared to the control diet.
However, survival, whole‐body composition and mineral content were not affected by the level of frass included.
They conclude that: “In summary, black soldier fly larval frass has potential as a protein source or just an ingredient for enhancing palatability of catfish diets.
