Nutritional indices of Orgyia trigotephras Boisduval, 1829 (Lepidoptera, Lymantriidae) on five shrub species
Abstract: Orgyia trigotephras is an important defoliator of oaks, mainly Quercus suber and Q. coccifera, but also of several other western Mediterranean shrub species. This work aims to study nutritional indices: relative ingestion rate (IRR), approximate digestibility (AD), efficiency of assimilated food (ECD), efficiency of conversion of ingested food (ECI) and relative growth rate (RGR) of O. trigotephras on five shrub species (Q. coccifera, Pistacia lentiscus, Erica multiflora, Erica arborea and Phylleria media). On each plant species, we conducted six repetitions. In each repetition, 10 larvae were used. A total of 300 fourth instar larvae was tested. Each larva was placed individually in a Petri dish (Ø = 9 cm) and weighed at the beginning and the end of the test. Two leaves from each shrub species were removed, weighed and placed in front the larva. After 3 days, the unconsumed foliage and the faeces produced by each larva were dried in an oven at 95 °C for 24 hours before being weighed. Results showed that IRI is high when larvae feed on Q. coccifera (0.4 mg/mg/h) and P. lentiscus (0.35 mg/mg/h) but lower when they feed on other shrub species. An important DA ratio was observed in all tested shrub species; 96% for larvae fed with Phylleria and 86% for those fed on Q. coccifera. High mortality (80%) was noted for larvae fed on Phylleria media and a lower one (28.33%) when fed on Q. coccifera. A better efficiency in conversion of ingested and digested food was observed when larvae feed on Q. coccifera (ECI = 10.56% et ECD = 12.37%) regardless to the other species (ECI between 2 et 4%, ECD between 2 et 5%). The important relative growth rate observed for larvae fed on Q. coccifera (0.031 mg/mg/h) confirms that O. trigotephras is an oak pest. However, on Ph. media, TRC = 0.005 mg/mg/h. This result showed that Orgyia has a relatively high plasticity to host plant changing. Studying the impact of secondary metabolites (tannins) on the development of O. trigotephras should provide important data to better understand the interaction between the insect and its host plants.