Abstract :
[en] In 2013, alarming outbreaks of the cedar – web spinning sawfly Cephalcia tannourinensis Chevin (Hymenoptera : Pamphiliidae) have been threatening the survival of the Lebanese cedars, Cedrus libani Richard (Pinales : Pinaceae) in Tannourine nature reserve. Since chemical control is no longer accepted for forest applications, an attempt of a biological control treatment was suggested. This approach aligns with the development of a short – term management strategy aimed at the control of this insect forest pest population via inundative treatments using entomopathogenic agents. An entomopathogenic fungus, Beauveria bassiana Vuillemin (Hypocreales : Cordycipitaceae) and entomopathogenic nematodes (EPNs) of the species Heterorhabditis bacteriophora Poinar (Rhabditida : Heterorhabditidae) were studied.
However, before proceeding with in situ inundative treatments, it was necessary to verify the natural occurrence of EPNs in the forest. A survey was conducted in cedar – suitable habitats using Galleria mellonella Linnaeus (Lepidoptera : Pyralidae) baits. Morphometric and molecular analyses identified a local strain of the species Steinernema feltiae Filipjev (Rhabditida : Steinernematidae), found at a single forest site characterized by sandy soil and a reduced presence of C. tannourinensis.
Subsequent in vitro experiments aimed to determine the most virulent entomopathogenic agent against C. tannourinensis larvae, between B. bassiana and the EPNs, and evaluated its host – seeking capacity. Tests where both agents were applied together aimed to identify competitive or cooperative interactions. Larval mortality rates were between 70 and 76 % with B. bassiana at doses of 50 and 500 spores respectively, and between 90 and 93 % with EPNs (for H. bacteriophora and S. feltiae, respectively). Simultaneous application of both agents achieved mortality rates of 90 to 100 %, suggesting an additive effect against C. tannourinensis. However, only one entomopathogen was recovered from the cadavers reflecting a competition between the agents.
The host – seeking capacity was more efficient in EPNs, with mortality rates between 53 to 100 % compared to 0 to 46% for B. bassiana. These results highlight a higher parasitic success of EPNs compared to B. bassiana due to their ambush strategy.
Thus, EPNs were preferred as entomopathogenic agents for controlling C. tannourinensis populations in situ and the commercial strain of H. bacteriophora, being available in large quantities, was identified as the most suitable candidate. Its pathogenicity was evaluated under real forest environmental conditions through inundative treatments. Pots containing G. mellonella larvae were also included in this experiment to confirm the EPN virulence in situ. Mortality rate of 85 % were obtained for both C. tannourinensis and G. mellonella larvae. The parasitic success of H. bacteriophora was confirmed only in G. mellonella larvae (77 %) with no emergence from C. tannourinensis cadavers. Instead, 3 % emergence rate of opportunistic nematodes belonging to the Acrobeloïdes Thorne (Rhabditida : Cephalobidae) group were identified through molecular analyses.
Considering these results, large – scale inundative treatments were applied forest sites in Tannourine between 2014 and 2016. Eight sites were designated. Treatment efficacy was evaluated by monitoring C. tannourinensis underground population with soil samples. Aerial populations were estimated by counting adults captured on yellow sticky traps while the percentage of defoliation was assessed based on the number of damaged cedar buds. The year 2015 witnessed an increase in the pest’s dynamic across all sites. No significant difference was detected between treated and control sites for underground larval density nor the emerging adults. Although foliar damage was significantly reduced in treated sites, it does not confirm the efficacy of inundative EPN treatments in controlling C. tannourinensis populations in situ.
To evaluate initial insights into the potential of ecological impacts of inundative treatments in such environment, the EPN persistence in the soil and their interactions with the local B. bassiana strain naturally present in the forest were analysed. This impact was monitored using G. mellonella traps to measure the parasitic success of EPNs and B. bassiana. The impact of inundative treatments on non – target insects was followed using pitfall – traps. No emergence of H. bacteriophora but an unexpected emergence of opportunistic Acrobeloïdes nematodes exclusively in EPNs treated sites (between 14 and 33 % emergence rate), confirmed competitive interactions between exotic EPNs and opportunistic nematodes. Finally, the presence of B. bassiana and the abundance of non – target insect families was not impacted by EPN treatments, except for a positive effect observed on Carabidae and Formicidae.
In conclusion, biological control using EPNs against C. tannourinensis populations in situ remains experimental and cannot yet be recommended as a short – term control method against alarming outbreaks. It also emphasizes on the need to optimize the monitoring of an insect pest and the importance of preventive measures in maintaining the pest’s population at a controllable threshold.
