Additive and synergistic interactions of entomopathogenic fungi with Bacillus thuringiensis for the control of the European grapevine moth Lobesia botrana (Denis and Schiffermuller) (Lepidoptera: Tortricidae)

Background The European grapevine moth, Lobesia botrana (Denis and Schiffermuller) (Lepidoptera: Tortricidae) is currently the most damaging pest in many viticultural regions across the Mediterranean basin and elsewhere. Its feeding activity also enhances the development of secondary infections by Botrytis cinerea - and other fungi - to wine grapes. The gram-positive bacterium Bacillus thuringiensis (Bt) has been reported to partially control larval populations of L. botrana, while it requires to be digested by the insect to cause infection. Entomopathogenic fungi (EPF) are possibly capable of acting synergistically with Bt to increase its efficacy against insect pests. Results The hypothesis of synergy or antagonism between Bt and EPF for the control of L. botrana was tested in two bioassays: A) Insects fed on Bt diet and subsequently some groups were sprayed by conidia of Beauveria bassiana or Paecilomyces fumosoroseus, and B) Grapes were sprayed by Bt, or B. bassiana, or combination of the two, and then untreated insects were placed to feed on the grapes. In both bioassays, combination treatments performed better than single treatments, indicating additive action or synergy. The Bt and B. bassiana combination treatment (Bt diet for 30 h and then sprayed with conidia of B. bassiana) resulted in 91% larval mortality while the single Bt and B. bassiana treatments caused 28% and 34% mortality respectively. Such results indicated synergism. Combination treatment on grapes also caused significantly higher mortality on L. botrana larvae, compared to single treatments. The median lethal time (LT50) was estimated as 8.43 days for the single Bt treatment, 7.87 days for the single B. bassiana treatment and 6.3 days for the combination Bt + B. bassiana treatment. Conclusions Absence of antagonism as well as additive action or synergy were indicated by the results. Larval populations of the pest can be effectively controlled by using microbial biocontrol agents. Further research is needed to investigate the biotic and abiotic factors that affect interactions between insect hosts and entomopathogenic organisms. However, the entomopathogens used in the present study showed remarkable action and may be included parallelly in control strategies against vineyard pests.