Evidence of group size dependent suppression of entomopathogenic bacteria, Bacillus thuringiensis subspecies thuringiensis and israelensis, by formosan subterranean termites (Isoptera: Rhinotermitidae)

Two entomopathogenic bacteria, Bacillus thuringiensis subspecies israelensis (Bti) and thuringiensis (Btt), were used as models to study the effect of the Formosan subterranean termite, Coptotermes formosanus, against microbial pathogens. C. formosanus with different group sizes (50, 25, 10 and no termites (control)) were placed on well-grown Bti or Btt agar plates. On day 1, the mean diameter of Bti or Btt colonies from lowest to highest was: 50-termite < 25-termite < 10-termite < control. This group size dependent suppression was even more distinct on day 2. On day 5, clear zones were observed in all of the three treatments with termites where Bti or Btt originally grew. The viability of Bti and Btt cells from these clear zones was checked. Bti and Btt colonies regrew on new plates after transfer from 25- and 10-termite treatments as did the control, but no regrowth was observed after transfer from 50-termite treatment. The results showed that the presence of C. formosanus can suppress the growth of Bti and Btt and the suppression effect enhanced with the increase of group size. Moreover, antagonistic tests showed that natural bacteria carried by termites play a role in this group size dependent suppression of Bti and Btt.