Aspergillus niger α-1,3-glucan acts as a virulence factor by inhibiting the insect phenoloxidase system.

•Insect PO is inhibited shortly after A. niger α-1,3-glucan injection into the hemocel.•Insect PO in hemolymph is inhibited shortly after injection of A. niger conidia.•α-1,3-Glucan and β-1,3/1,6-glucan affect insect PO activity differently in vivo.•A. niger α-1,3-glucan is a virulence factor that inhibits insect immune response.