Ex-hive trials on the efficacy of hop beta acids on the control of the honey bee parasite Varroa destructor
Author(s)
Date Issued
2023
Type
conferenceObject
Volume
22
Abstract
Recently, interest in α- and β-acids of hop (Humulus lupulus)
has increased for their acaricidal effects. In fact, some components
of hop β-acids have been found to be effective on the ectoparasitic
varroa mite (VM) (Varroa destructor), with a low impact
on its host, the western honey bee (HB) (Apis mellifera). The
present contribution (PRIMA project 1812/PLANT-B) aimed to
assess the HB tolerability of hop β-acids (HBA) and their possible
acaricidal effect.
Three consecutive trials under controlled conditions (34.3 ± 1 °C;
72.0 ± 1.5% RH) were performed. Young emerging HB (24 h old),
34.8 ± 1.2 per cage, were submitted to an experimental scheme
including: 6% oxalic acid (OA) in water (positive control) and tap
water as negative control; the treatments, namely HBA at 0.125%,
0.25%, 0.5%, 1%, 2% were dissolved in acetic acid (MetCOOH) or
alternatively in ethanol (EtOH), with relative negative controls.
Three replicates for each treatment were set up. After 24 h of
acclimatation, a volume of 1.6 μL of treatment/control per bee was
spread on the HB. Cages were daily checked and the consumption
of water, 66% sugar solution (SS66), VM death (DVM) and HB
death (DHB) were recorded.
The treatment that had the highest impact in terms of DVM was
EtOH/HBA 2% (31.3 ± 4.7%), comparable to OA (32.7 ± 3.8%;
p > 0.05), with a dose-dependent increase compared to untreated
bees (p < 0.01). The EtOH-dissolved treatments showed no statistical
differences for DHB except for EtOH/HBA 0.25% (p < 0.01)
that was the highest. The SS66 and water consumption resulted
the highest for the HB exposed to EtOH/HBA 0.125 and 0.25% (p
< 0.01). MetCOOH-dissolved treatments showed no effect on DVM but
were ineffective compared to OA (32.7 ± 4.6%) (p < 0.01).
Conversely, the highest dosages MetCOOH/HBA 1% (38.3 ± 5.4%),
2% (34.6 ± 3.8%) and OA (37.0 ± 3.2%) had higher DHB than
lower dosages (p < 0.01) or controls. The SS66 and water consumption
resulted positively affected (p < 0.01) at the dosages
of MetCOOH/HBA 0.125% and 1% (108.0 ± 22 μL and
101.5 ± 31.5 μL); furthermore, the negative control MetCOOH/W
elicited higher consumption of SS66 (83.8 ± 18.1 μL) and water
(27.4 ± 7.3 μL) in comparison to the EtOH/W counterparts
(46.9 ± 13.4 μL and 13.7 ± 2.0 μL).
The results showed that HBA in EtOH gave DVM and DHB levels
comparable to classical treatment with OA; oppositely HBA in
MetCOOH seemed to be less effective as acaricide. Further investigations
are needed to explain such differences and to test the
applicability as hive level.