The impact of post-defoliation foliage of Pinus halepensis Mill. on the larval performance of Thaumetopoea pityocampa and its relationship with the tree-induced defense

Induced defense in trees refers to the increased production of specific substances in response to herbivore attacks, which negatively affects the herbivore. This type of resistance can contribute to the sustainable management of herbivorous insect problems in forest ecosystems. The Aleppo pine, a drought-resistant forest tree species, holds great ecological and socioeconomic importance. One of its predators is the Pine Processionary Moth (PPM) Thaumetopoea pityocampa. This study explores the impact of new foliage that emerges following severe defoliation caused by the PPM on the subsequent year’s larval performance. Moreover, we investigate whether there is a difference in the chemical composition of the foliage of defoliated and undefoliated trees in terms of terpenes (monoterpenes, sesquiterpenes, and diterpenes), total phenols, nitrogen (N), carbon (C), and the C/N ratio. The aim is to determine whether these components act as an induced defense mechanism. The results indicate that the performance of L1 larvae was negatively affected by new foliage after defoliation. Larvae feeding on new foliage experienced high mortality rates and were significantly shorter than those that fed on the foliage of undefoliated trees. The chemical composition analysis revealed that only the new foliage after defoliation contained bornyl acetate (monoterpene), and the concentration of p-cymene (another monoterpene) was higher in this new foliage compared to that of undefoliated trees. Furthermore, two diterpenes, methyl dehydroabietate and dehydroabietal, showed a stronger correlation with defoliation. These compounds have demonstrated insecticidal activity against other insects and could potentially serve as starting components for a bio-insecticide against the PPM. These findings suggest the presence of an induced defense in Aleppo pine, as the tree produces foliage containing specific terpenes that are unfavorable to larvae. This is one of the factors contributing to the reduction of the PPM population. However, due to the current climate change situation, further research is necessary to better understand how these conditions impact the susceptibility of trees to defoliation.