Making a pest beneficial: Fungus gnats [Bradysia impatiens (diptera: Sciaridea)] as potential vectors of microbial control agents to suppress pathogens they also spread

Abstract

The fungus gnats, Bradysia impatiens (larvae and adults), were either separately or concurrently exposed for 6 h to growing spores of Fusarium oxysporum and Pythium aphanidermatum, and a powder formulation of Clonostachys rosea on leaf discs of strawberry to determine their capacity in laboratory trials for vectoring these three microbes. We determined the number of spores for each fungus species that individual larval or adult fungus gnats carried or ingested. We also assessed the capacity of fungus gnats to carry or ingest the three kinds of spores simultaneously, and to evaluate suppression of growth of F. oxysporum and P. aphanidermatum by C. rosea (a known biological control agent against numerous plant pathogens) 14 days after sampling. When the fungus gnats were exposed to single fungus, larvae ingested about 21 times more spores than did adults. The adults carried twice as many spores externally as did the larvae. When the insects were exposed simultaneously to all three kinds of spores both larvae and adults carried externally more or less the same numbers of each kind of spore. However, when both larvae and adults were exposed to all three kinds of spore simultaneously, the larvae had ingested about ten times the number of spores as the adults. When ingested mixtures of spores were cultured, Clonostachys reduced Fusarium and Pythium sporulation only when the spores were ingested by larvae. That result reflects that adults ingested very few spores of the three microbes, probably too few for suppression of Fusarium or Pythium by Chlonostachys to be expressed. Our results indicate that both larval and adult B. impatiens are indeed likely effective vectors of infections of both F. oxysporum and P. aphanidermatum but may be able to be exploited to vector C. rosea alone and simultaneously for potential control of the two pathogens.