A series of experiments were conducted to compare the susceptibility of P. regina larvae reared in isolation or in groups to the effects of diet-borne metabolic inhibitors: chlorogenic acid (CGA) and mimosine. Larvae were presented with diets containing 0.4 mM CGA or 0.4 mM CGA in combination with 22 mM lysine or methionine or with diets containing 1.5, 15 or 30 mM concentrations of mimosine. Methionine and CGA caused significantly reduced pupal weights when compared with larvae presented with lysine and CGA. All concentrations of mimosine resulted in 100% mortality with larvae unable to successfully complete pupation even at the lowest concentration. In general, larvae reared in groups were less susceptible to the toxic effects resulting from diet-borne metabolic inhibitors. The results are discussed in relation to the chemical factors that result from the feeding activity of saprophagous dipterans.