Reproduction of Distinct Varroa destructor Genotypes on Honey Bee Worker Brood.

Wenfeng Li,Richou Han,Yanping Chen,Zachary Y Huang,Cheng Wang

doi:10.3390/insects10110372

Abstract

Honey bees play important roles in pollination for many crops and wild plants, but have been facing great threats posed by various pathogens and parasites. Among them, Varroa destructor, an obligate ectoparasite of honey bees, is considered the most damaging. Within the last century, V. destructor shifted from the original host, the Asian honey bee Apis cerana to the new host, the European honey bee A. mellifera. However, the reproduction of Varroa mites, especially of different haplotypes in the two hosts, is still largely unknown. In this study, we first investigated the existing Varroa haplotypes in local colonies in southern China, and then compared the reproduction of different haplotypes on the worker brood of both the original and new hosts by artificial inoculation. We confirmed that there are two haplotypes of V. destructor in southern China, one is the Korea haplotype and the other is the China haplotype, and the two types parasitized different honey bee species. Although Varroa females from A. mellifera (Korea haplotype) are able to reproduce on the worker brood of both honey bee species, they showed better reproductive performance in the new host A. mellifera with significantly higher fecundity (number of offspring per mother mite) and reproductive rate (number of adult daughters per mother mite), suggesting that this parasite gains higher fitness after host shift. The data further showed that a short stay of Varroa females inside the A. cerana worker cells decreased their fecundity and especially the reproductive rate in a time-dependent manner, suggesting that the A. cerana worker cells may inhibit Varroa reproduction. In contrast, Varroa mites derived from A. cerana colonies (China haplotype) were entirely sterile in A. mellifera worker cells during two sequential inoculations, while the control mites from A. mellifera colonies (Korea haplotype) reproduced normally. In addition, all the infertile mites were found to defecate on the abdomen of bee pupae. We have revealed that two haplotypes of V. destructor exhibit differential reproduction on the worker brood of the original and new host honey bees, providing novel insights into the diversity and complexity of the reproduction of V. destructor.

Highlights

The honey bee is one of the most important economic insects
These findings suggest that the genetic variation among Varroa mites themselves might determine their reproductive capability on worker brood hosts, and mites of different haplotypes show varying reproductive capability on the worker brood of the same honey bee species, but further studies are needed to confirm this
We found that A. mellifera derived Varroa mites (Korea haplotype) reproduced on both A. cerana and A. mellifera worker brood, but had better fecundity on the latter, while Varroa mites from A. cerana colonies (China haplotype) were completely sterile on A. mellifera worker brood in two sequential inoculation experiments

Summary

Introduction

By providing the essential pollination services for crops and wild plants, honey bees play a vital role in the sustainable development of modern agricultural industries and the global ecosystem, and produce a wide range of benefits for human well-being [1,2]. There is mounting evidence indicating that the honey bee colony numbers in Western Europe and the US are in decline, which results in a severe pollination crisis and threatens. V. destructor had been misclassified to be another honey bee Varroa parasite, V. jacobsoni Oudemans for decades until 2000 [10]. V. destructor originally parasitizes the Asian honey bees, Apis cerana, but successfully switched its host to the European honey bees, A. mellifera likely in the Far East in the first half of the last century [11]. There are at least seven haplotypes of V. destructor identified based on a 458 bp fragment of the mitochondrial CO-I gene, including Korea, Japan/Thailand, Nepal, Vietnam, China, China 2, and Sri Lanka haplotypes [10,15], and much greater genetic variation among

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