Dietary influence of iron formulation on the development of gypsy moth (Lepidoptera: Lymantriidae) in laboratory colonies

Colonies of gypsy moth, Lymantria dispar (L.), reared on the standard wheat germ diet have been experiencing periods of unexplained slow asynchronous development. The cause of the variability in development has been identified to be the variable nature of ferric phosphate provided in the diet. As a separate ingredient or in a salt mixture, ferric phosphate is a mixture of the amorphous and crystalline forms of this compound. Slow, asynchronous development occurred when ferric phosphate containing inadequate quantities of the amorphous form (6 mg/liter) was used in the preparation of diet. When the amount of amorphous ferric phosphate was higher, 35-49 mg/liter, larvae developed normally. Slow, asynchronous development did not occur when diet containing sufficient amorphous ferric phosphate was supplemented with extra crystalline ferric phosphate. Thus, the presence of crystalline ferric phosphate did not cause the developmental problems. Insects fed diet containing ferric citrate as a substitute for ferric phosphate, or Reck's mineral supplement containing ferrous sulfate, developed synchronously and faster than siblings fed diet containing inadequate amorphous ferric phosphate, indicating that the iron component of ferric phosphate was the nutrient affecting synchrony and speed of development. The possibility of an iron deficiency caused by differences in bioavailability of amorphous and crystalline ferric phosphate is discussed.