The effect of diet, temperature and diapause on the number and identification of larval instars in the oriental fruit moth Grapholita molesta (Busck) (Lepidoptera : Tortricidae)

Abstract

It has been demonstrated that the Oriental fruit moth (Grapholita molesta) has 5 larval instars when reared on an artificial diet at various constant temperatures ranging from 11-28 °C, on apple and on peach at 25 °C and on an artificial diet and apple under field conditions. Instars may be distinguished by head capsule width on any one medium, but mean widths vary significantly between media. Insects feeding on peach shoots in the field in New Zealand and France showed only 4 larval instars. The distribution of head capsule widths from all the above experiments conform to Dyar’s rule. Larvae feeding on peach fruit in the field showed a complex mixture of 4 and 5 instar forms, whereas larvae reared under diapause inducing photoperiods had consistently larger mean head capsule widths for a given instar than those reared under different conditions. Mean head capsule size increased slightly with rearing temperature to a maximum of 25 °C then declined slightly to 28 °C. In addition to head capsule width, the colouration of body parts is used to provide a practical method of separating instars.