Valorization of hop leaves for development of eco-friendly bee pesticides

Abstract

The bacterium Paenibacillus larvae and the mite Varroa destructor are two of the most severe biotic stressors affecting honeybees and are responsible for American foulbrood and varroosis respectively. To control these pathogens, beekeepers regularly apply synthetic acaricides or antibiotics to parasitized hives. However, antibiotic and acaricide overuse over time leads to resistance in bacteria strains and mite populations respectively, not to mention the residual contamination of bee products with these chemicals. The development of alternative and effective control methods of bee diseases is therefore crucial. In recent years, natural substances from plant extracts have emerged as the basis of suitable control methods to treat bee colonies parasitized by both P. larvae and V. destructor . Our aim was to evaluate the bioactivity of ethanolic and methanolic hop leaf extract (species: Humulus lupulus L, varieties: Victoria, Spalt, and Cascade) against P. larvae , V. destructor , and A. mellifera . The bactericidal activity against P. larvae was evaluated by the broth microdilution method. Topical administration protocols were used to determine the bioactivity of hop extracts on V. destructor and A. mellifera . Total polyphenols, flavonoids, saponins, and antioxidant capacity were determined for each hop leaf extract tested. The Victoria extract had the highest concentration of phenolic compounds, whereas Cascade and Victoria extracts had higher concentrations of the glycoside saponin. All hop extracts presented low toxicity against A. mellifera bees after 48 h of topical administration (except for Cascade ethanolic extract which reached a maximum of 36% of bee mortality). Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values ranged from 0.69 to 2.75 mg/kg for the Cascade variety, 1.38 to 5.5 mg/kg for the Spalt variety, and 5.5 to 11 mg/kg for the Victoria variety. After 48 h, the acaricidal activity for the ethanolic extract of the Victoria variety reached a value close to 80%, while the methanolic extract of Cascade showed an acaricidal activity close to 70%. The results reported in this study support the potential use of methanolic and ethanolic extracts of hop leaves from Argentina as promising natural alternatives for varroosis and American foulbrood control.Paenibacillus larvae / Varroa destructor / Apis mellifera / natural extracts / Humulus lupulus / chemical composition

Varroa destructor from the Laboratory to the Field: Control, Biocontrol and IPM Perspectives—A Review

“…Finally, another plant extract, relying on hop leaves, was shown to contain polyphenols with high miticide effect and low acute toxicity for bees [ 161 , 162 ]. An advantage of hop extracts comes from their antioxidant activity for honeybees, benefiting them if administered orally [ 33 ].…”

“…However, preventing them from reproduction or right orientation, as supported by Soroker et al, (2019) [81], would reduce the selective pressure in favour of the most resistant mites. Other works also explained that the use of natural substances could grant a low level of V. destructor infestation, keeping honeybee colonies alive [33]. An integrated pest management strategy should thus, combine all the above ideas.…”

“…Insects 2021, 12, x FOR PEER REVIEW 9 of 26 resistant mites. Other works also explained that the use of natural substances could grant a low level of V. destructor infestation, keeping honeybee colonies alive [33]. An integrated pest management strategy should thus, combine all the above ideas.…”

“…Other solutions were explored too, like drone removal, brood interruption, or breeding programs [ 30 , 31 ]. Soft acaricides such as thymol, hop leaves or acids seem promising for some [ 32 , 33 ] and already trouble making for others [ 34 , 35 ].…”

Hop Leaves as an Alternative Source of Health-Active Compounds: Effect of Genotype and Drying Conditions

“…The authors also provided some suggestions to improve the composting of hop leaves, such as mixing them with other materials (i.e., cow manure and wheat straw; [ 14 ]). Recently, Iglesias et al [ 15 ] suggested the use of alcoholic extracts of hop leaves to produce an eco-friendly bee pesticide and underlined the influence of genotype on their secondary metabolite content. These biological activities are related to the bioactive compounds present in hop leaves, such as polyphenols [ 16 ].…”