Insect-parasitic phases in the development of the mycetophagous wood nematode, Bursaphelenchus trypophloei, in its vector, an aspen bark beetle Trypophloeus asperatus

Abstract

Summary Bursaphelenchus trypophloei is a common wood nematode associated with the poplar bark beetle, Trypophloeus asperatus, in European aspen, Populus tremula. The nematode develops and reproduces in larval galleries of T. asperatus and is transmitted to new breeding trees by adult beetles during their flight and oviposition. Its dispersal third-stage juveniles (DJIII) are unique because of their exceptionally early penetration into the haemocoel of the second- or third-larval stage of the future vector/host and long-term (up to 10 months) occupation of this environment. Detailed morphological/anatomical, and bionomic study revealed that while living in the vector’s haemocoel the nematode dispersal/dauer juveniles pass through a series of intermittent changes with three distinct developmental phases. The sequence of these phases corresponded to the process of diapause observed in many plant and animal parasites. Throughout the period of living in the haemocoel, the nematodes remained at their dispersal juvenile stage (DJIII) but clearly increased in size and changed their morphometrics and shape of such anatomical/morphological characters as the metacorpus and tail terminus. Moreover, the juveniles apparently ingested the food from the vector’s haemolymph. Specifically, in the insect haemocoel, the body of dispersal/parasitic juveniles became less transparent with most organs being obscured by an increasing amount of refractive nutrient storage granules. After penetration into the insect the process of diapause was irreversible, irrespective of the availability of food (i.e., Leptographium spp. fungus). The nematodes could resume their further development only after the natural emergence from the vector beetle to its new galleries or a few days earlier, when mature third phase DJ (DJIII-3) juveniles were dissected from the beetle and transferred to a fresh fungal culture. The data indicate that B. trypophloei is exclusively mycetophagous when free-living in the larval galleries of its insect vector, T. asperatus, and parasitic in the haemocoel of this insect.