Honey Bee Brood Research Articles

Hyperthermia treatment can kill immature and adult Varroa destructor mites without reducing drone fertility

In the last few decades, different hyperthermia devices have been developed to kill Varroa destructor mites in the colony. Intriguingly, effects of hyperthermia on Varroa destructor and honeybee brood have hardly been investigated. We exposed honeybee brood to temperatures of 41°C to 45°C to investigate effects on Varroa destructor in the hive and on drone fertility of treated colonies. Drone fertility is an important issue in keeping healthy and viable honeybee populations. We show that temperatures of 42°C for 3 hours or higher kill all the Varroa destructor but, unfortunately, also part of the honeybee brood. Temperatures below 42°C are ineffective against adult Varroa destructor. A temperature of 41°C and 2 hours duration is highly effective against immature Varroa destructor and thus interrupts their reproduction without harming the viability or fertility of drones, while longer durations or higher temperatures kill the spermatozoa of the drones.

Novel probiotic approach to counter Paenibacillus larvae infection in honey bees

American foulbrood (AFB) is a highly virulent disease afflicting honey bees (Apis mellifera). The causative organism, Paenibacillus larvae, attacks honey bee brood and renders entire hives dysfunctional during active disease states, but more commonly resides in hives asymptomatically as inactive spores that elude even vigilant beekeepers. The mechanism of this pathogenic transition is not fully understood, and no cure exists for AFB. Here, we evaluated how hive supplementation with probiotic lactobacilli (delivered through a nutrient patty; BioPatty) affected colony resistance towards a naturally occurring AFB outbreak. Results demonstrated a significantly lower pathogen load and proteolytic activity of honey bee larvae from BioPatty-treated hives. Interestingly, a distinctive shift in the microbiota composition of adult nurse bees occurred irrespective of treatment group during the monitoring period, but only vehicle-supplemented nurse bees exhibited higher P. larvae loads. In vitro experiments utilizing laboratory-reared honey bee larvae showed Lactobacillus plantarum Lp39, Lactobacillus rhamnosus GR-1, and Lactobacillus kunkeei BR-1 (contained in the BioPatty) could reduce pathogen load, upregulate expression of key immune genes, and improve survival during P. larvae infection. These findings suggest the usage of a lactobacilli-containing hive supplement, which is practical and affordable for beekeepers, may be effective for reducing enzootic pathogen-related hive losses.

Invasion pathway of the honeybee pest, small hive beetle, Aethina tumida (Coleoptera: Nitidulidae) in the Republic of Korea inferred by mitochondrial DNA sequence analysis

Abstract The small hive beetle (SHB), Aethina tumida, is native to the Sub-Saharan region of Africa, but it became invasive in many countries after its first introduction to the USA in 1996. The SHB is a destructive pest of the honey industry and can cause damage in apiaries due to feeding on the honey, pollen, honey bee brood and honey fermentation. SHB was recently found infesting honeybee colonies in the South-Eastern part of Korea, Miryang city in 2016. No inference of the origin or the pathway of the invasion into Korea has been made, so far. We analyzed partial cytochrome oxidase I gene of mitochondrial DNA to unveil the possible source of the invasive populations of SHB in South Korea. A Bayesian inference tree and median joining haplotype network revealed a strong relationship between South Korean and North American populations suggesting that the SHB in South Korea came from the USA. Low genetic variation among Korean populations suggests that the invasion might have occurred in a single event with small number of founders. In addition, a new global distributional map of SHB is provided.

Honeybee pupal length assessed by CT-scan technique: effects of Varroa infestation, developmental stage and spatial position within the brood comb

Honeybee pupae morphology can be affected by a number of stressor, but in vivo investigation is difficult. A computed tomography (CT) technique was applied to visualize a comb’s inner structure without damaging the brood. The CT scan was performed on a brood comb containing pupae developed from eggs laid by the queen during a time window of 48 hours. From the CT images, the position of each pupa was determined by recording coordinates to a common reference point. Afterwards, every brood cell was inspected in order to assess the developmental stage of the pupa, the presence of Varroa destructor, the number and progeny of foundress mites. Using data on 651 pupae, the relationships between varroa infestation status, developmental stage and spatial position of the pupa within the brood comb, and its length were investigated. Pupae at 8 post-capping days were shorter than pupae at 7 post-capping days. Pupae in infected cells were significantly shorter than those in varroa-free cells and this effect was linked both to mite number and stage and to the position in the comb. Overall, the results suggest that the CT-scan may represent a suitable non-invasive tool to investigate the morphology and developing status of honeybee brood.

The levels of natural Nosema spp. infection in Apis mellifera iberiensis brood stages

Nosema ceranae is the most prevalent endoparasite of Apis mellifera iberiensis and it is a major health problem for bees worldwide. The infective capacity of N. ceranae has been demonstrated experimentally in honey bee brood, however no data are available about its prevalence in brood under natural conditions. Thus, brood combs from 10 different hives were analyzed over two consecutive years, taking samples before and after winter. A total of 1433 larvae/pupae were analyzed individually and N. ceranae (3.53%) was the microsporidian most frequently detected, as opposed to Nosema apis (0.42%) which was more frequently detected in conjunction with N. ceranae (0.71%). The active multiplication of both microsporidians was confirmed by the expression (real-time-PCR) of the N. ceranae polar tube protein 3 gene and/or the N. apis RNA polymerase II gene in 24% of the brood samples positive for Nosema spp. Both genes are related to microsporidian multiplication. As such, N. ceranae multiplication was confirmed in 1.06% of the samples, while N. apis multiplication was only observed in co-infections with N. ceranae (0.07%). Brood cells were analyzed for the presence of Nosema spp., as those are the immediate environment where the brood stages develop. The brood samples infected by Nosema spp. were in brood cells in which that microsporidians were not detected, while brood cells positive for N. ceranae hosted brood stages that were not apparently infected, indicating that this is unlikely to be the main pathway of infection. Finally, the colonies with brood infected by N. ceranae showed higher levels (numbers) of infected adult bees, although the differences were not significant before (P = 0.260), during (P = 0.055) or after (P = 0.056) brood sampling. These results show that N. ceranae is a bee parasite ubiquitous to all members of the colony, irrespective of the age of the bee. It is also of veterinary interest and should be considered when studying the epidemiology of the disease.

Hyperspectral application for early diagnosis of American foulbrood disease in the honeybee (Apis mellifera L.) larvae

Non-destructive methods are of utmost importance for early diagnosis of American foulbrood (AFB) disease in honeybees’ larvae. This study discusses the potential application of spectroradiometric techniques for spectral discrimination of AFB disease in honeybees. AFB is a honeybees’ brood fully destructive bacterial disease caused by the spore forming bacterium; Paenibacillus larvae (P. larvae). It is important to detect the disease early as routine apiary administration and interchange of coloney components could spread it easily to healthy ones. Hyperspectral measurements of honeybee larval samples from 11 colonies were taken in range (350–2500 nm). The two spectral bands, Red and the Short-Wave Infrared (SWIR2), were of the promising results for differentiation between healthy and the different degree of honeybees’ larval infection. Correlation coefficient between Red and SWIR2 spectral reflectance data indicates that SWIR2 is the best spectral band to be used for discriminating between healthy and infected honeybee larvae. Confirmation of AFB was carried out by both cultivating smears of suspected larvae on agar plates and KAT-PCR technique for bacterial DNA confirmation. Both cultivation and PCR results agreed with hyperspectral results indicating that 6 colonies were of high loads of P. larvae, 2 of moderate bacterial loads and 1 was weak. Only 2 colonies appeared free from bacteria. Data analysed, discriminatory signatures the diseased samples in their various pathological stages. this research is an attempt to determine the feasibility of early diagnosis of AFB disease in honeybee brood through the application of hyperspectral technology.

Edible larvae and pupae of honey bee (Apis mellifera): Odor and nutritional characterization as a function of diet

Future protein demand is expected to rise with global population growth. In this study a comprehensive sensorial analysis of the odor of honey bee (Apis mellifera) larvae and pupae as function of their diet (with and without added sugar solution) was performed, as well as nutritional values and antioxidant activity analysis. Honey bee brood powder is a potentially valuable nutritional source with 20–25% protein (dry matter basis), high antioxidant activity and polyphenol content. Main volatile compounds detected using GC–MS with HS–SPME injection were odorless pheromones that represented differences between larvae and pupae. The determined active odor compounds were 2- and 3-methylbutanal, diacetyl, nonanal, dimethyl sulfide and ocimene. A trained sensory panel described honey bee brood aroma profile mainly with buttery and milky attributes, with different life stages and diets giving similar profiles. Such studies can be useful for future development of food products with desired nutritional and sensorial characteristics.

The Oomen bee brood feeding test – revision of the method to current needs and developments

The Oomen bee brood feeding test is a long‐standing, frequently used test for investigating potentially adverse effects of plant protection products on honey bee brood. Based on new methods, new regulatory requirements, technical improvements as well as current experience and results from the evaluation of acute (single) and chronic (multiple) honey bee brood feeding tests, a revision of the method is advised. This paper describes recent approaches suited to current needs and developments and fulfils new risk assessment requirements.

Seasonal timing in honey bee colonies: phenology shifts affect honey stores and varroa infestation levels.

Increasingly frequent warm periods during winter, which are associated with climate change, may cause mismatches between the colony phenology of the western honey bee, Apis mellifera L., and their floral resources. Warmer winter periods can also affect colony brood rearing activity and consequently the reproduction of the invasive brood parasite Varroa destructor Anderson and Trueman. Until now little is known about the effects of climate change on biotic interactions in such a multitrophic system comprising flowering plants, a pollinator, and its parasite. We performed a reciprocal translocation experiment with honey bee colonies to simulate climate change-induced phenology shifts. Honey bee brood phenology was highly sensitive to environmental conditions in late winter. Colonies in which phenology was experimentally delayed had smaller worker populations in early spring and reduced amounts of stored honey during the following months. During summer, the varroa load in colonies with non-shifted phenology was three times higher than in colonies with delayed phenology. High varroa loads during summer were negatively correlated with worker population growth. Despite a remarkable resilience of colony development to phenology shifts, our results show that mismatches between the phenology of honey bee colonies and flowering plants can affect the build-up of resource stores. Further, an advanced onset of brood rearing activity after hibernation can reinforce the negative impact of the brood parasite V. destructor. We conclude that trade-offs between synchronisation with earlier flower phenology and prolonged brood phases with build-up of varroa populations might constrain the honey bees' capability to adapt to climate warming.

Effect of transient exposure to carbaryl wettable powder on the gut microbial community of honey bees

Bees are important pollinators in agriculture. The bee population has recently begun to decline possibly due to pesticides. The bee gut microbiota strongly influences the health of bees. The gut microbiota of bees is composed of distinct members belonging to selective taxa. Chemicals like pesticides can alter the gut microbiota. The present study investigated the effect of carbaryl pesticides on gut microbiota of honey bees, which had come in contact with rapeseed plants (Brassica napus) sprayed with carbaryl wettable powder during the honey bee brood test under semi-field condition. Molecular techniques (conventional and quantitative polymerase chain reaction (PCR), clone library method, and DNA sequencing) were employed to analyze changes in the microbial communities between the pesticide-exposed and unexposed bees. Phylogenetic analysis of 16S rRNA genes of the clones from both groups, showed differences in their respective compositions of core and non-core bacteria. Both groups contained carbohydrate-degrading bacteria such as Gilliamella apicola and Lactobacillus. However, the unexposed bees harbored Alphaproteobacteria, which were absent in the exposed bees. Microorganisms found in honey bee guts such as Snodgrassella alvi and L. kullabergensis, however, were observed only in the exposed bees, but not in the unexposed bees. The difference between the two groups was distinctly recognized when copy numbers of 16S rRNA genes were compared by quantitative PCR. Results showed that the average gene copy number for the unexposed bees was higher than that for the exposed bees. This may indicate the toxic effect of pesticides on bees and gut microbiota.

High-resolution maps of Swiss apiaries and their applicability to study spatial distribution of bacterial honey bee brood diseases.

Honey bees directly affect and are influenced by their local environment, in terms of food sources, pollinator densities, pathogen and toxin exposure and climate. Currently, there is a lack of studies analyzing these data with Geographic Information Systems (GIS) to investigate spatial relationships with the environment. Particularly for inter-colonial pathogen transmission, it is known that the likelihood of a healthy colony to become infested (e.g., Varroosis) or infected (e.g., American foulbrood—AFB, European foulbrood—EFB) increases with higher colony density. Whether these transmission paths can actually be asserted at apiary level is largely unknown. Here, we unraveled spatial distribution and high-resolution density of apiaries and bacterial honey bee brood diseases in Switzerland based on available GIS data. Switzerland as ‘model country’ offers the unique opportunity to get apiary data since 2010 owing to compulsory registration for every beekeeper. Further, both destructive bee brood diseases (AFB and EFB) are legally notifiable in Switzerland, and EFB has an epizootic character for the last decades. As governmental data sets have to be ameliorated, raw data from the cantonal agricultural or veterinary offices have been included. We found a mean density of 0.56 apiaries per km2, and high resolution spatial analyzes showed strong correlation between density of apiaries and human population density as well as agricultural landscape type. Concerning two bacterial bee brood diseases (AFB, EFB), no significant correlation was detectable with density of apiaries on cantonal level, though a high correlation of EFB cases and apiary density became obvious on higher resolution (district level). Hence, Swiss EFB epizootics seem to have benefited from high apiary densities, promoting the transmission of pathogens by adult bees. The GIS-based method presented here, might also be useful for other bee diseases, anthropogenic or environmental factors affecting bee colonies.

Heat-induced aggregation and gelation of proteins from edible honey bee brood (Apis mellifera) as a function of temperature and pH

Edible insects offer a huge potential as a new source of proteins for food and feed and as a promising functional agent. In this regard, our study aimed to evaluate the effect of pH and temperature on aggregation and gelation of honey bee brood (Apis mellifera). Aggregation of soluble proteins was dependent on temperature and pH value and the highest coagulation was reached at 85 °C at pH 5 and 7 (73.7 and 68.4%, respectively). Changes in protein surface hydrophobicity, net charge and amount of available and buried sulfhydryl groups demonstrated the contribution of either covalent or non-covalent intermolecular interactions resulted in aggregation at various pH. The least gelation concentration of raw powder from honey bee brood varied from 5 to 11% on raw powder basis at pH 7 and 3, respectively. Moreover, the effect of pH on rheological and textural properties, as well as gel microstructure from honey bee brood is discussed. Therefore, freeze dried raw powder of honey bee brood has rich nutritional value and demonstrated gelation concentration comparable to conventional protein sources that reveal its potential as an ingredient for insect-based gel product.

Morphometric analysis of some external and internal body characteristics of honey bee Apis mellifera queens treated with Paenibacillus larvae larvae

ABSTRACT The present investigation has been conducted to characterize morphological changes in virgin honey bee Apis mellifera queens challenged, in 4th larval instar, with a sub-lethal dose of Paenibacillus larvae larvae, causative agent of American foulbrood (AFB) the most threatening bacterial disease of honey bee brood. It provides direct tests to assess the quality of treated queens in terms of their external physical characters of the body (body weight, body length, abdomen length, thorax width, fore wing length and head width) and internal body characteristics (ovary size and number of ovarioles per ovary) in comparison with normal ones.The estimated LD20 was 1.07 x102 CFU/queen. This dose was enough to immunize queens and did not cause high mortality rate, so it was used in the subsequent tests during this study. The results indicated that the bacteria-treated queens didn't undergo any significant changes in their external body characteristics than normal ones. This reflects the difficulty of discrimination between normal and disease-tolerant queens in the field. Additionally, the number of ovarioles per ovary didn't show any significant changes between normal and treated queens. In contrast, ovaries of treated queens showed an increase of their sizes as compared with normal queens. These results indicate that the bacterial treatment has no impact on the development of queen`s ovary.

Risk assessment and predation potential of Stratiolaelaps scimitus (Acari: Laelapidae) to control Varroa destructor (Acari: Varroidae) in honey bees.

The biocontrol of the honey bee ectoparasite Varroa destructor is an underexploited but promising avenue that would benefit from being integrated in a Varroa management program. Our study aimed to investigate the potential of the predatory mite Stratiolaelaps scimitus to control Varroa infestations in honey bees. Tests on safety and predation were carried out to: (1) assess the risk of predation of the honey bee brood by S. scimitus under laboratory conditions and within the colony, and (2) evaluate the predation potential of S. scimitus on phoretic Varroa mites. Under laboratory conditions, S. scimitus was able to feed upon free Varroa mites, but also attacked every unprotected honey bee brood stages with a strong preference for bee eggs. When introduced inside colonies, however, S. scimitus does not have negative effects on the survival of the bee brood. Moreover, observations made in the laboratory revealed that S. scimitus does not attack Varroa mites when they are attached to the body of bees. However, all Varroa mites that had naturally fallen from the bees were predated upon by S. scimitus and died in less than 24h. This study provides evidence that S. scimitus does not represent a significant threat to the bee brood, but also suggests that its effect in Varroa control will probably be limited as it does not attack phoretic Varroa mites. Our results represent a first step in assessing the potential of S. scimitus to control V. destructor and provide novel information about the predator’s behavior inside the honey bee colony.

Hygienic behaviour selection via freeze-killed honey bee brood not associated with chalkbrood resistance in eastern Australia.

Hygienic behaviour is a social immune response in honey bees shown to help provide resistance to honey bee pests and diseases. A survey of hygienic behaviour and brood diseases was conducted on 649 colonies in eastern Australia to initiate a selective breeding program targeting disease resistance and provide a level of resistance to Varroa (Varroa destructor Anderson and Trueman and V. jacobsoni Oudemans) mites should they become established in Australia. The test population showed a remarkably high baseline level of hygienic behaviour with 17% of colonies meeting or exceeding breeding selection thresholds. Colonies belonging to a breeding program were 5.8 times more likely to be highly hygienic and colonies headed by queens raised from hygienic queen mothers were 2.2 times more likely. Nectar availability (nectar yielding flowering plants within honey bee forage range) influenced hygienic behaviour expression but was not a significant predictor of level of hygienic behaviour. Surprisingly, hygienic behaviour was not a significant predictor of the presence of infection of the honey bee brood disease chalkbrood (Ascosphaera apis) and was not influential in predicting severity of chalkbrood infection in surveyed honey bee colonies. This study, along with reports from commercial beekeepers that chalkbrood infection is on the rise, warrants a deeper exploration of the host-pathogen relationship between Apis mellifera and Ascosphaera apis in Australia.

Glyphosate affects the larval development of honey bees depending on the susceptibility of colonies.

As the main agricultural insect pollinator, the honey bee (Apis mellifera) is exposed to a number of agrochemicals, including glyphosate (GLY), the most widely used herbicide. Actually, GLY has been detected in honey and bee pollen baskets. However, its impact on the honey bee brood is poorly explored. Therefore, we assessed the effects of GLY on larval development under chronic exposure during in vitro rearing. Even though this procedure does not account for social compensatory mechanisms such as brood care by adult workers, it allows us to control the herbicide dose, homogenize nutrition and minimize environmental stress. Our results show that brood fed with food containing GLY traces (1.25–5.0 mg per litre of food) had a higher proportion of larvae with delayed moulting and reduced weight. Our assessment also indicates a non-monotonic dose-response and variability in the effects among colonies. Differences in genetic diversity could explain the variation in susceptibility to GLY. Accordingly, the transcription of immune/detoxifying genes in the guts of larvae exposed to GLY was variably regulated among the colonies studied. Consequently, under laboratory conditions, the response of honey bees to GLY indicates that it is a stressor that affects larval development depending on individual and colony susceptibility.

Extraction, characterization and functional properties of soluble proteins from edible grasshopper (Schistocerca gregaria) and honey bee (Apis mellifera)

In our research soluble proteins from edible grasshopper (S. gregaria) and honey bee brood (A. mellifera) were exposed to defatting, alkaline, and sonication-assisted extractions. New nitrogen-to-protein conversion factors based on amino acid analysis were estimated for both insects: 4.5 for adult grasshopper, and 4.9 and 5.6 for pupae and larvae of honey bee respectively, in contrary to 6.25 commonly used for insects. All fractions were characterized by their composition, yield, color, protein solubility, and functional properties in comparison to whey protein concentrate. Besides an increase in protein content up to 57.5 and 55.2% for grasshopper and honey bee respectively, protein-enriched fractions showed improved foaming and emulsifying abilities. The highest emulsion stability after 120 min was determined for grasshopper powder extracted with sonication (85.5%) and whey proteins (89.8%). The protein-enriched fractions of both insects had significantly higher foaming stability (74.1% for grasshopper fraction after alkaline extraction and 55.5% for sonication-assisted honey bee fraction) than raw and defatted powder. All fractions obtained from honey bee brood showed significantly higher protein heat coagulation than grasshopper and whey proteins. Changes in protein functionality were found related to alteration in protein charge, surface hydrophobicity, and distribution of proteins according to their molecular weight. Therefore, our results showed that S. gregaria and A. mellifera have a potential for future applications for food, feed, or insect-based dietary supplements.

Comparative Genomics and Description of Putative Virulence Factors of Melissococcus plutonius, the Causative Agent of European Foulbrood Disease in Honey Bees.

In Europe, approximately 84% of cultivated crop species depend on insect pollinators, mainly bees. Apis mellifera (the Western honey bee) is the most important commercial pollinator worldwide. The Gram-positive bacterium Melissococcus plutonius is the causative agent of European foulbrood (EFB), a global honey bee brood disease. In order to detect putative virulence factors, we sequenced and analyzed the genomes of 14 M. plutonius strains, including two reference isolates. The isolates do not show a high diversity in genome size or number of predicted protein-encoding genes, ranging from 2.021 to 2.101 Mbp and 1589 to 1686, respectively. Comparative genomics detected genes that might play a role in EFB pathogenesis and ultimately in the death of the honey bee larvae. These include bacteriocins, bacteria cell surface- and host cell adhesion-associated proteins, an enterococcal polysaccharide antigen, an epsilon toxin, proteolytic enzymes, and capsule-associated proteins. In vivo expression of three putative virulence factors (endo-alpha-N-acetylgalactosaminidase, enhancin and epsilon toxin) was verified using naturally infected larvae. With our strain collection, we show for the first time that genomic differences exist between non-virulent and virulent typical strains, as well as a highly virulent atypical strain, that may contribute to the virulence of M. plutonius. Finally, we also detected a high number of conserved pseudogenes (75 to 156) per genome, which indicates genomic reduction during evolutionary host adaptation.

ICP-PR Bee Brood Working Group – Variability of brood termination rates in reference to validity criteria and limited effectiveness of method improvement in honeybee semi-field studies (OECD GD 75)

OECD Guidance Document 75 (2007) outlines a test method to assess effects of plant protection products (PPPs) on honeybee brood under semi-field conditions. The assessment of bee brood development is conducted by mapping cells containing eggs and following their development until emergence. Evaluated parameters are: brood termination rate (BTR), brood compensation index (CI) and brood index (BI). Due to high variability of BTRs within treatments and high control mortality in a number of studies no definite conclusions regarding effects on brood were possible in the past and studies needed to be repeated (Pistorius et al. 2012). To address this variance, effort was taken by ICP-PR and AG Bienenschutz to improve the method by further analyzing current and historical data considering possible influencing factors (Pistorius et al. 2012, Becker et al. 2015) to give recommendations for future testing. The main findings were that reliability of the test method was questionable and that further method improvement and data evaluation was required. Therefore in this paper data evaluation of studies conducted between 2014 and 2017 is carried out and potential key parameters influencing outcome of studies are given. To evaluate the improvement of the OECD 75 test method following the recommendations from 2015, a data analysis of 86 studies conducted in Germany, France, Spain and US was performed. The mean BTR value in the control group was 30.2% for studies conducted in Germany (mean of 61 studies), 19.4% in France (mean of 3 studies), 41.8% in Spain (mean of 5 studies) and 50.6% in US (mean of 17 studies). Results from Spain and US displayed higher BTRs in control compared to data from Germany. Evaluation of BTRs for Germany displayed only a slight improvement (historical value of 32.9%). Analysis of data shows a limitation of options to improve the method as no main driver for high variability of BTRs in the control group was found. The cause for low precision may be multifactorial and driven by “caging effect”. There are alternative test methods available to observe bee brood development, without confinement in the tunnels, under field conditions (Oomen et al. (1992), OECD GD 75 field test design). Therefore it is necessary to investigate differences between these open field methods and semi-field testing with regard to routes of exposure, residues in brood and brood mortality, to choose the most reliable and adequate testing method assessing potential effects of PPP on honeybee brood development.

Towards integrated control of varroa: 5) monitoring honey bee brood rearing in winter, and the proportion of varroa in small patches of sealed brood cells

Oxalic acid is used by beekeepers to control varroa mites, Varroa destructor. Oxalic acid is effective against mites phoretic on adult bees but not against mites in sealed brood cells. In Britain and other temperate areas, brood rearing diminishes in winter and may even fall to zero, providing a natural window of opportunity for varroa control with oxalic acid. We monitored brood rearing from September to March for four years in hives in Sussex, southern Britain. We also sampled phoretic mites on adult bees and in brood cells to determine the proportion protected from oxalic acid in sealed brood cells. Our results showed that December is consistently the month with the least sealed brood, with 17 times less than November or January, on average. However, winter reduction in brood rearing varied among years, and even in December some colonies still had sealed brood (range 9–52%, n = 4 winters). Although the amounts were low, even a small patch of c. 500–600 sealed cells (300 cm2) contains approximately 14% of the mites in a colony, enough to halve the duration of control provided by an oxalic acid treatment. Our results indicate that beekeepers should check their hives in winter if they plan to control varroa with oxalic acid. This will show which month(s) have the least capped brood, whether there is period when all colonies are broodless, and if not which hives have patches of capped brood to remove.