Testing naturally repulsive plant species against gypsy moth attacks

Abstract: The gypsy moth, Lymantria dispar, is known as an important agent of hardwood forest defoliation in the northern hemisphere. Field observations in Corsica showed that caterpillars did not attack three hardwood species: Olea europaea (olive), Fraxinus ornus (flowering ash) and Ficus carica (common fig). Bioassays were conducted in the laboratory to test the toxic and/or repellent effect of these tree species on gypsy moth caterpillars. Control species were oaks (Quercus ilex, pubescens and robur). Three larval instar groups were tested (L1-L2, L3-L4, and L5-L6). The first group was tested in boxes on fresh cut leaves, the others directly on young trees, the caterpillars being maintained in insect-proof system. Larval nutrition and development were recorded every two days. The results confirm the observations made in Corsica: larvae on anti-feedant species did not molt to the next larval stage and died of starvation (leaves were not consumed). This confirms that the anti-feedant tree species had a negative impact on gypsy moth caterpillars. Incidentally, it was also observed that larval development could be negatively affected when host plants are water stressed. Additional chemical studies should be conducted in order to identify and isolate the molecules responsible for this anti-feedant, repulsive and/or toxic effect.