Invasive pathogen–insect symbioses have been extensively studied in many
different ecological niches. Whether the damage of symbioses in different introduced regions
might be influenced by other microorganisms has, however, received little attention. Eight
years of field data showed that the varied levels of the nematode and beetle populations and
infested trees of the invasive Bursaphelenchus xylophilus–Monochamus alternatus symbiosis
were correlated with patterns in the isolation frequencies of ophiostomatoid fungi at six sites,
while the laboratory experiments showed that the nematode produced greater numbers of
offspring with a female-biased sex ratio and developed faster in the presence of one native
symbiotic ophiostomatoid fungus, Sporothrix sp. 1. Diacetone alcohol (DAA) from xylem
inoculated with Sporothrix sp. 1 induced B. xylophilus to produce greater numbers of
offspring. Its presence also significantly increased the growth and survival rate of M.
alternatus, and possibly explains the prevalence of the nematode–vector symbiosis when
Sporothrix sp. 1 was dominant in the fungal communities. Studying the means by which
multispecies interactions contributed to biogeographical dynamics allowed us to better
understand the varied levels of damage caused by biological invasion across the invaded range.