A sand dilution assay was developed to study how composting affects the nutritional value of stored laying hen manure for larvae of the house fly, Musca domestica L. Equal numbers of eggs were inoculated into graded amounts of stock manure and incubated under standardized moisture conditions. Survival and mass per emerging adult diminished with progressively lower supplies of manure per larva, whether the manure was diluted into clean, white sand or placed on top of an equal volume of sand. Mass of adults per original egg was an increasing linear function of log₂ manure supply, with extrapolated lower supply threshold, S_L = 0.06 g per egg. It is proposed that S_L is a measure of a substrate’s nutritional value—the greater the threshold, the lower its value. Dilution of the same stock manure in loam or sandy loam reduced the manure’s apparent nutritional value, and dehydration of the stock manure to 20% water before rehydration to 70% also reduced nutritional value. Assays of bulk samples from replicated piles of laying hen manure mixed with sunflower hulls indicated the mixture was nutritionally equivalent to the stock manure, but that 3–4 wk of subsequent aerobic, thermophilic composting reduced it to ≈10% of its initial value. These results suggest that composting may be a useful technique for reducing the fly breeding potential of laying hen manure and other substrates that must be stored before spreading and incorporation on crop land.