Fungal Pathogen Infection by Metarhizium anisopliae Alters Climbing Behavior of Lymantria dispar with Tree-Top Disease Induced by LdMNPV

Certain parasites manipulate host behavior following infection to enhance their own dispersal and transmission. Lepidopteran larvae infected with baculoviruses exhibit increased locomotion, ascending to the apex of their host plant where they ultimately die in a characteristic inverted, liquefied posture suspended by their prolegs-a phenomenon termed "tree-top disease". Although numerous studies have investigated the underlying causes of this behavior, the precise mechanism governing tree-top disease formation remains unresolved. In this study, <i>Lymantria dispar</i> larvae were infected with <i>Metarhizium anisopliae</i> and Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). We compared symptom profiles across infection modes and assessed virulence, demonstrating that <i>M. anisopliae</i> infection alters the hyperactive state induced by LdMNPV in larvae exhibiting tree-top disease. Specifically, <i>M. anisopliae</i> promoted tree-top disease behavior during early infection stages but suppressed it during later stages. Furthermore, the symptomatology of larvae co-infected with both pathogens differed significantly from that observed in larvae infected with either <i>M. anisopliae</i> or LdMNPV alone. Co-infected larvae also exhibited accelerated mortality compared to those infected with a single pathogen. The above findings indicate that <i>L. dispar</i> larvae, when co-infected with LdMNPV and <i>M. anisopliae</i> may change behavioral responses that could further modulate the pathogenesis of LdMNPV-induced tree-top disease. Furthermore, a synergistic interaction between <i>M. anisopliae</i> and LdMNPV was observed in the biocontrol of <i>L. dispar</i>.