|Fungus gnats, including the common species Bradysia impatiens Johannsen (Diptera: Sciaridae) are one of the most common pests in greenhouse production systems, mainly due to the direct feeding damage by larvae on the roots and stems. Environmental pollution, human health concerns and a high risk of insecticides require new, environmentally safe and effective strategies for the management of fungus gnats in greenhouses. Specifically, organic production systems need to fall back on biological control strategies for fungus gnat control. Several microbial products and biorationale insecticides have been developed and evaluated as pest control alternatives, with varying efficacies. In a first series of bioassays we evaluated the efficacy of entomopathogenic fungi on the survival rate and adult emergence of fungus gnats. In a second series of bioassays, “attract and kill” formulations for fungus gnat control by combining a biopesticide (Metarhizium brunneum Cb15 and Spinosad) and a botanical compound, formulated in a slow release capsule were studied. In subsequent laboratory bioassays, the application of biopesticides (M. brunneum, Bacillus thuringiensis Bt and Spinosad) alone or in combination with the botanical capsule were tested on attraction and oviposition of the fungus gnat B. impatiens. Furthermore, the effectiveness of the two biopesticides (M. brunneum and Spinosad) individually or in combination with the botanical capsules were assessed against fungus gnats using three different host plants (basil, parsley and poinsettia), under greenhouse conditions. These combined formulations were assessed to improve the control efficiency of fungus gnats aiming at an appropriate practical control strategy for fungus gnat B. impatiens control in greenhouse production systems.
1. The effect of the entomopathogenic fungi EPF (isolates of two Metarhizium brunneum, three Beauveria bassiana and one Acremonium strictum) as pure fungal diets or as spore suspensions mixed with the substrate were studied on fungus gnat larvae B. impatiens.
a. Fungus gnat larvae are able to survive even on pure entomopathogenic fungal diets. Larval stages of fungus gnats were more susceptible to M. brunneum in comparison to other fungal strains, which resulted in a reduced number of emerging adults. Larvae fed on fungal strains B. bassiana and A. strictum had a high percentage of survivorship.
b. Less larvae of B. impatiens survived, when EPF spore suspensions were mixed with the substrate. Hence, fewer numbers of adults emerged from larvae exposed to the substrate treated with M. brunneum and A. strictum.
c. M. brunneum was therefore selected as a suitable EPF to control fungus gnat larvae.
2. The effectiveness of two different formulations of botanical compounds containing mint oil (Mentha and Minx) and the combination of a biopesticide with these botanical compounds was assessed against larvae of B. impatiens.
a. Minx capsules showed an attractant effect for fungus gnat larvae.
b. The biopesticides M. brunneum or Bt individually reduced the emergence rate of fungus gnat adults.
c. The application of M. brunneum or Bt combined with the botanical capsules Minx resulted in a higher reduction of adult emergence, although not significantly, as compared to the single treatments interpreted as an additive effect on the efficacy of biopesticide.
d. The combined application of biopesticides (M. brunneum or Bt) and the botanical capsules (Minx) points towards an “attract and kill” strategy.
3. The egg-laying behavior of fungus gnats when exposed to M. brunneum and two biopesticides (Bt or Spinosad) individually or in combination with Minx capsules was tested in laboratory experiments. Moreover, the effect of combining a biopesticide (M. brunneum, Neem Azal or Spinosad) and the botanical capsules (Minx) was evaluated in an “attract and kill” approach targeting fungus gnat larvae in laboratory experiments.
a. Fungus gnat females preferred to lay more eggs in Spinosad treatments individually or in combination with the botanical capsules.
b. Eggs laid by fungus gnat females in Spinosad treatments did not successfully finalize their life cycle to adults, resulting in an efficient reduction of adult emergence, either when applied alone (99.81%) or in combination with the botanical compound (99.93%) as compared to the other treatments. Hence, Spinosad in combination with Minx capsules might be used as a “trap crop strategy”.
c. Fungus gnat larvae showed a positive interaction with the Minx capsules. Moreover, fungus gnat females exhibited a strong ovipositional preference for the substrate treated with the botanical capsules vs. the control substrate.
d. The combination of biopesticides (M. brunneum and Spinosad) with the Minx capsules, specifically Spinosad, resulted in an additive effect in reducing the emergence of adults.
4. The combined application of two biopesticides (M. brunneum or Spinosad) and Minx capsules was tested against the fungus gnats species on three different host plants (basil, parsley and poinsettia), under semi-greenhouse conditions.
a. Fungus gnat females laid more eggs on parsley and poinsettia plants soils treated with Spinosad alone or in combination with Minx capsules as compared to the other treatments. Using basil plants, females preferred to lay more eggs in treatments with Minx capsules or the control in comparison to other treatments.
b. In all of three host plant species tested, Spinosad treatments alone or in combination with Minx significantly reduced the number of emerging adults.
The results compiled so far still require further improvements to make these co-formulations (e.g. biopesticides M. brunneum or Spinosad in combination with the botanical capsules) practically feasible for greenhouse producers to manage fungus gnat populations.