Insights into the metabolism and behaviour of Varroa destructor mites from analysis of their waste excretions

Abstract

Varroa destructor mites (Acari: Varroidae) are harmful ectoparasites of Apis mellifera honey bees. Female foundresses of wax-capped pupal host cells and their daughters feed on host fluids from open wounds on the host's integument. Details of V. destructor mite nutrition are forthcoming, and little is known about the potential physical effects on hosts from mite feeding. Chemical analysis of waste excretions can infer details of animals' nutrition. Here, chemical analysis by high-performance liquid chromatography/mass spectrometry (HPLC-MS/MS) of mite excretions showed that the purine content of V. destructor waste consists of guanine with traces of hypoxanthine. Traces of uric acid and caffeine were also detected. Concentrations of guanine attenuated over time and excretions collected from senescing mites did not contain detectable guanine. Non-reproducing individual female mites maintained in vitro, housed in gelatin capsules and provided a honey bee pupa, deposited an average of nearly 18 excretions daily, mostly on the host's integument rather than on the capsule wall. The weight and volume of excretions suggest mites can consume nearly a microlitre of host fluids each day. Compounded over 10 days, this together with open wounds, could lead to substantial water loss and stress to developing pupae.

Keywords: Apis mellifera; guanine; high-performance liquid chromatography; hypoxanthine; mass spectrometry; purines; xanthine.

Fig. 1.

Daily number and location of female Varroa mite excretory deposits. Mean (± s.e.m) daily deposits of waste excretions made by mites observed over 12 days on integument of host honey bee pupae (upper dark line) and on inner wall of gelatin capsule (lower light line) housing mites and hosts.

Fig. 2.

Weight of cumulative total, and estimated weights of individual excretory deposits from female Varroa mites. (A) Cumulative weight of excretory deposits collected over 12 days from integument of host pupae (dark bars) and total weight of deposits (light bars) including estimated weights of deposits on capsule walls. (B). Estimated total weight of individual excretory deposits made daily by individual mites. Textured bars in both (A) and (B) represent estimated weights (see text).

Fig. 3.

Volume of individual excretory deposits from female Varroa mites. (A) Box plot giving range, quartiles and median of excretory deposit volumes calculated from the measured diameter of N = 102 circular/semi-circular excretory deposits, and the frequency distribution of these calculated volumes. (B) Cumulative volume of excretory deposits from individual mites over 12 days, estimated from the mean calculated volume of individual deposits and number of deposits made daily per mite. Textured bars in (B) are estimated data extrapolated from measured excreta from days 1 to 10.

Fig. 4.

LC-MS analysis of purine standards and Varroa mite excreta samples amassed at day 3 (sample 1), day 6 (sample 2) and day 9 (sample 3), of a 12-day monitoring period. (A) LC-MS/MS chromatogram for MRM transition M/z 152.1→135.1 of sample 1 at 2.54 ppm. (B) sample 2 at 1.59 ppm, and (C) guanine standard at 0.25 ppm. Guanine was not detected in sample 3.