Adaptations for Symbiont-Mediated External Digestion in Sirex noctilio (Hymenoptera: Siricidae)

Abstract

Abstract Wood-feeding insects face a host of physical, chemical, and nutritional barriers in their food resource. Wood-boring Hymenoptera in the family Siricidae are associated with mutualistic basidiomycete wood-rot fungi, which assist colonization and provide nutrition for their insect partner, though functional properties of this symbiosis are poorly described. In this study, we document the behavioral and morphological adaptations of the globally invasive woodwasp, Sirex noctilio F., for foraging using its fungal symbiont Amylostereum areolatum (Chaillet ex Fries) Boidin. Larvae concentrated foraging near the border of fungal symbiont growth in pine xylem. Foraging larvae do not ingest bulk xylem tissue, but rather use specialized asymmetric mandibles to press xylem shavings and extract liquid fractions. Fluids drain toward the oral cavity via a sulcus on the occlusal surface of the left mandible. Processed shavings are expelled from the oral cavity without ingestion and passed along the underside of the larvae via peristaltic undulation to the rear of the feeding gallery. Larval midguts lack elaborated chambers typical in insects reliant on microbial fermentation of cellulose, and no xylem tissue was recovered from gut dissections. Larval behavior and functional morphology indicate larval S. noctilio do not ingest xylem, but instead use the fungus as an external gut for digestion of recalcitrant lignocellulosic compounds.