IMPACT OF VARROA DESTRUCTOR N. SP. INFESTATION ON HONEY PRODUCTION OF APIS MELLIFERA L. COLONIES IN A MEDITERRANEAN CLIMATE

Prof. António Manuel Murilhas

Instituto de Ciências Agrárias Mediterrânicas, Univ. de Évora, 7000 Evora, Portugal Tel. + 315 266 760864, Fax + 351 266 711163, [email protected]

Summary This study aimed at investigating the impact of Varroa infestation on Apis mellifera carnica, A. m. iberica and A. m. ligustica honey productions of colonies kept, for approximately one year, in a Portuguese Mediterranean environment. By the end of the experiment, Varroa-free A. m. ligustica colonies stored the largest mean amounts of honey (≈ 67 kg), followed by A. m. carnica (≈ 44 kg) and A. m. iberica (≈ 29 kg). Within each subspecies, progressive reductions of honey stored in Varroa-infested colonies, compared with Varroa-free ones, became increasingly evident during spring and summer. By the end of the experiment, Varroa had apparently caused average reductions of ≈ 18 kg of honey per colony of A. m. iberica, ≈ 24 kg per colony of A. m. carnica and ≈ 54 kg per colony of A. m. ligustica.

Introduction Since Varroa destructor n. sp. (formerly Varroa jacobsoni Oud.) was first found in Portugal, it has been associated with both increased honey bee colony mortality and decreased honey production. However, this is the first study quantifying the impact of Varroa infestation on honey production of honey bee colonies in Portugal. Studies of this nature, conducted on a single environment and covering an entire annual colony cycle, have been rare (Echazarreta & Paxton, 1997). Furthermore, research assessing the impact of Varroa on productive performances of different subspecies of A. mellifera kept in the same environmental conditions are also rare (Büchler, 1992, 1994). In temperate climates, honey bee colonies show an annual cycle of growth and decline. The general pattern of colony growth throughout the year has been well described (Jeffree, 1955, 1956; Allen & Jeffree, 1956; Winston, 1981; Seeley, 1984; Calatayud & Verdú, 1992; Buhlmann, 1995; Fathy, 1996). In temperate climates, following several months without brood in autumn and early winter, honey bee colonies resume rearing brood in January or February (Seeley, 1984) as a response to increasing day-length (Kefuss, 1978). The daily rate of worker brood production continues to increase so that it reaches a maximum by May or June. If swarming occurs, an average temporary lack of brood in the colony may be expected for 10 to 20 days, associated with queen substitution and queen mating. Brood rearing then resumes, but then declines gradually throughout summer, ceasing in October (Seeley, 1984).

Proceedings of the 37th International Apicultural Congress, 28 October – 1 November 2001, Durban, South Africa APIMONDIA 2001 To be referenced as: Proc. 37th Int. Apic. Congr., 28 Oct – 1 Nov 2001, Durban, South Africa ISBN: 0-620-27768-8 Produced by: Document Transformation Technologies Organised by: Conference Planners In Mediterranean climates, the main qualitative difference to the described pattern of honey bee colony population dynamics is the presence of brood throughout the year (Marchetti et al., 1988; Prota, 1988; Cardenal Galván et al., 1989; Llorente Martinez, 1989; Vieira et al., 1990; Puerta et al., 1993; Ferrer-Dufol et al., 1994; Calatayud & Verdu, 1995), except perhaps for a short period associated with swarming. The milder winter conditions in Mediterranean climates and the availability of food during a considerable part of the winter account for this difference in the brood- rearing cycle, which, in turn, is relevant to the intrinsic growth rate of Varroa. Thus, this study aimed at investigating the relative impact of Varroa infestation on honey bee colonies kept in a common Mediterranean environment. As well as allowing for a quantitative approach to the evaluation of Varroa infestation impact on honey production on the local honey bee subspecies (A. m. iberica), this study also aimed at investigating the usefulness of importing A. m. carnica and A. m. ligustica subspecies as a strategy for alleviating the Varroa-related difficulties felt by beekeepers in terms of higher colony mortality and decreased gross income.

Material and Methods 1. Colony establishment and management Fieldwork was started during 1992 and ran until late 1993. At the beginning of this experiment, 14 young (< 2 month old) A. m. iberica mated queens were randomly selected from local colonies and 12 mated queens each of A. m. carnica and A. m. ligustica were imported from the USA. All experimental queens were wing-clipped, colour coded (on their scuta) and introduced into 1 kg (± 50 g) packages of A. m. iberica workers, that had been previously treated with Folbex VA (bromopropylate). After queen introduction, these packages of bees were kept in 5 frame Langstroth hives and received 6 l of sucrose solution (50 %, w/v) during the 4 weeks after colony set-up (which occurred during early May). In June, these colonies were moved into 10 frame Langstroth hives and allowed to develop for approximately 8 weeks before being evaluated (mid July), so that the progeny of the introduced queens could replace most of the initial A. m. iberica workers. By this time, 32 honey bee colonies existed (7 of A. m. carnica, 13 of A. m. iberica and 12 of A. m. ligustica), all kept at the Research Station of Évora University (southern Portugal) and all morphometrically authenticated concerning their respective subspecies (Murilhas & Graça, 1994; Graça, 1996), confirming that the introduced queens had replaced most of the initially provided A. m. iberica workers with their own offspring. Colony management was conservative, meaning that no strategies or management techniques were followed to improve colony productivity or population growth. The major concern was to prevent swarming during the spring, mainly by making areas of honey storage and egg laying space (drawn comb) available approximately two weeks before it was probably needed. No prophylactic or therapeutic actions were carried out, other than the one applied to colonies intended to be Varroa-free. Also, no feeding was carried out during the experimental period and no honey was removed from the experimental colonies. 2. Artificial Varroa infestation Within each studied subspecies, colonies were randomly selected to be kept virtually Varroa- free (3 in A. m. carnica, 6 in A. m. iberica and 6 in A. m. ligustica, hereafter referred to by "control" colonies) or to be artificially infested with Varroa (4 in A. m. carnica, 7 in A. m. iberica and 6 in A. m. ligustica, hereafter referred to by "treatment" colonies). To ensure comparable initial infestation levels (in relative terms), the number of Varroa received by each treatment colony ranged from 42 to 108 (75.6 ± 4.4, x ± s.e.m.) and was kept directly proportional (2.91 ± 0.005 %) to the number of capped worker brood cells that these colonies had at that time (July). Varroa were collected from honey bees of highly Varroa-infested colonies (by dusting with powdered sugar) and pooled together before being randomly distributed to experimental colonies previously selected to be artificially infested with Varroa. Only dark-brown motile Varroa were selected for artificial infestation. They were introduced into colonies at sunset, onto frames with open and capped worker brood. 3. Maintenance of control colonies Control colonies were continuously treated with 2 strips of Apistan (Fluvalinate, 800 mg per strip) to kill most incoming Varroa before they could start reproducing (Milani et al., 1993). Apistan strips were replaced approximately every 8 weeks, according to manufacturer's instructions.

4. Data collection Colonies were assessed, and data were, collected approximately every 3 weeks (except during the months of November, December and January). All comb areas occupied with capped honey were drawn onto transparent acetate sheets and were later digitised by computer assisted image analysis [using a digitising table (Genitizer GT- 1812), connected to a personal computer, and operated by the software package Sigma Scan]. All measured areas, in every colony frame side, were summed per experimental colony and observation date. The weights of stored honey were then calculated by considering that each square centimetre of registered capped honey held 1.79 ± 0.06 g of riped honey. Loss of bees by swarming was appraised by checking for the presence of the young colour- coded queens during the regular visits to the colonies. Colonies where loss of queens occurred (either by swarming, supersedure or accident) were removed from the experiment and not considered here. 5. Statistical analyses Assumptions inherent to parametric statistics, namely variables' random distribution of errors and experimental group (subspecies x Varroa status) homoscedasticity (Sokal & Rohlf, 1981; Fry, 1993), were confirmed before carrying out statistical tests. Honeybee colony amounts of capped honey were analysed by a three-way repeated-measures ANOVA, where (i) honey bee colonies represented the subjects upon which measures were repeated, (ii) honey bee colony subspecies and Varroa status were the between-subject effects and (iii) the observation date was the within-subjects effect.

Results In the first evaluation (mid July), no statistically significant differences were found (regarding honey bee colony populations, areas occupied with capped worker brood or amounts of capped honey) that could be associated with subspecies or Varroa status (Subspecies or Varroa, P > 0.05, Tab I). Table I. Two-way ANOVAs of capped worker brood cells, colony population and stored honey in A. m. carnica, A. m. iberica and A. m. ligustica (Subspecies) control or treatment colonies (Varroa), at the first evaluation (15 July 1992)

Variable Source of variation Sum of Degrees of Mean F Significance squares freedom square of F (P) Capped worker Subspecies 780435 2 390218 0.820 0.452 brood cells Varroa 4240 1 4240 0.009 0.926 Subspecies x Varroa 2495 2 1247 0.003 0.997 Error 12375045 26 475963 - - Colony Subspecies 10650297 2 5325148 0.724 0.494 population Varroa 196667 1 196667 0.027 0.871 Subspecies x Varroa 1803098 2 901549 0.123 0.885 Error 191282411 26 7357016 - - Stored honey Subspecies 5693001 2 2846501 2.186 0.133 Varroa 83753 1 83753 0.064 0.802 Subspecies x Varroa 2191579 2 1095790 0.842 0.442 Error 33848332 26 1301859 - -

Results obtained throughout the experimental period are presented in Fig 1 and 2, where the apparent Varroa infestations of worker brood, drone brood and adult honey bees in control colonies were respectively maintained under 4, 1 and 2 % across the entire experiment. All subspecies showed similar temporal patterns of honey storage, where considerable increases in stored honey were only observed from early spring onwards, mainly in control colonies (Fig 1). By the end of this experiment, control A. m. ligustica colonies stored the largest mean amounts of honey (≈ 67 kg), followed by control colonies of A. m. carnica (≈ 44 kg) and A. m. iberica (≈ 29 kg).

In Fig 2, a within-subspecies comparison between average honey stored in control and treatment colonies is presented, for all tested commercial subspecies and according to the formula  SHt   −1 ×100 (1)  SHc  where SHt = mean honey stored in treatment colonies and SHc = mean honey stored in control colonies. Within each subspecies, a progressive reduction on honey stored in treatment colonies, compared with control ones, became increasingly evident during spring and summer. In all subspecies, Varroa infestation reduced considerably the amount of stored honey. By the end of the experiment, A. m. carnica colonies were the least affected by Varroa (treatment colonies held 45.2 % of the honey stored in control colonies), contrarily to A. m. ligustica colonies that, in this respect, were the most affected ones (treatment colonies retained only 19.4 % of the honey stored in control colonies). This means that Varroa apparently caused average reductions of ≈ 24 kg of honey per colony of A. m. carnica, ≈ 18 kg per colony of A. m. iberica and ≈ 54 kg per colony of A. m. ligustica.

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Mean ± s.e.m. of capped honey 20 A. m. iberica 10

0 A. m. ligustica 15.Jul 26.Aug 09.Oct 26.Feb 29.Apr 10.Jun 28.Jul 05.Aug 18.Sep 30.Oct 06.Apr 20.Mai 30.Jun

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Mean ± s.e.m. of capped honey 20 A. m. iberica 10

0 A. m. ligustica 15.Jul 26.Aug 09.Oct 26.Feb 29.Apr 10.Jun 28.Jul 05.Aug 18.Sep 30.Oct 06.Apr 20.Mai 30.Jun

Observation date

Figure 1. Amount of stored honey (mean ± s.e.m. of honey, in kg) observed in control and treatment colonies of A. m. carnica, A. m. iberica and A. m. ligustica, across the experiment (Observation date)

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-100 A. m. ligustica 15.Jul 26.Aug 09.Oct 26.Feb 29.Apr 10.Jun 28.Jul 05.Aug 18.Sep 30.Oct 06.Apr 20.Mai 30.Jun

Observation date Figure 2. Average relationship of Varroa infestation with amounts of stored honey in colonies of A. m. carnica, A. m. iberica and A. m. ligustica (mean reduction, in %), across the entire experiment (Observation date), and according to formula (1)

Three-way repeated measures ANOVA revealed consistent statistically significant differences between mean amounts of stored honey as a result of Varroa status (P < 0.001, Tab II), where control colonies tended to store considerably more honey than treatment ones. Both the subspecies effect and the interaction term subspecies x Varroa were found not to be so relevant (P > 0.03, Tab II), meaning that relatively weak relationships were found between these two sources of variation and the amount of honey stored in the colonies. The within-subject observation date effect was highly significant (P < 0.001, Tab II) reflecting the substantial variations in overall mean amounts of honey stored in the colonies across the experimental period. The interaction term subspecies x observation date was statistically significant (P < 0.001, Tab II), suggesting slightly different behaviours of honey storage among colonies of different subspecies as time passed by. In this case, very similar slow decrease mean rates of stored honey were observed in all studied subspecies until the 26th of February. However, from this observation date onwards, considerably different mean increase rates of stored honey were observed among subspecies, where A. m. ligustica colonies performed considerably better than A. m. iberica ones. On the other hand, the association of stored honey with the interaction term Varroa x observation date was also highly significant (P <0.001, Tab II), meaning that the overall pattern of stored honey in control colonies was different from the one observed in treatment colonies. In this case, considerable mean differences in the amount of stored honey between colonies of different Varroa status were only observed from March 1993 onwards, where a considerable higher mean increase rate of stored honey was observed in control colonies. The interaction term subspecies x Varroa x observation date was found to be highly significant (P = 0.001, Tab II), suggesting that across the experiment the amounts of honey stored in the experimental colonies did vary with subspecies as Varroa status changed.

Table II. Three-way repeated-measures ANOVA of observed amounts of honey stored in A. m. carnica, A. m. iberica and A. m. ligustica (Subspecies) control or treatment colonies (Varroa), throughout the experiment (Observations) Source of variation Sum of Degrees of Mean F Significance squares freedom square of F (P) Subspecies 791836743 2 395918371 3.76 0.037 Varroa 1974544417 1 197454441 18.73 < 0.001 7 Subspecies x Varroa 582372429 2 291186215 2.76 0.082 Within + Residual 2740928034 26 105420309 - - Observation date 9042527799 12 753543983 55.24 < 0.001 Subspecies x Observations 1372077084 24 57169879 4.19 < 0.001 Varroa x Observations 3121902636 12 260158553 19.07 < 0.001 Subspecies x Varroa x Observations 731770304 24 30490429 2.24 0.001 Error 4255839327 312 13640511 - -

Discussion At the initiation of this experiment, statistically significant differences were not found between honey bee subspecies or Varroa status as far as numbers of capped worker brood, honey bee colony population and amounts of stored honey. Apparently, it appears reasonable to consider the subsequent results of this experiment not to have been considerably affected by the relatively small differences observed among colonies at experiment set-up. The very low apparent Varroa infestations observed on both brood and adult honey bees of control colonies were lower than the ones achieved in a similar type of study (Calatayud & Verdú, 1992). In the environment in which this experiment was run, and considering that all experimental colonies were to be kept in as much a "common environment" as possible, in practice control colonies were only possible to maintain by repeated use of chemotherapy (fluvalinate). Thus, control colonies are expected to have been slightly weakened both by vestigial Varroa infestation as well as by the negative effect of successive fluvalinate applications (Barbattini et al., 1989; Slabezki et al., 1991; Liu, 1992; Ben Hamida, 1996). Therefore, one may consider that, had it been possible to maintain absolutely Varroa-free colonies in the same environment without the need of acaricidal applications, the magnitude of the results' differences between control and treatment colonies might have been greater. On the other hand, although colonies placed in the same environment do not necessarily exploit the same resources (Seeley, 1995), potentially, colonies used in this experiment had the same possibilities to collect food, and the similar observed patterns of honey storage do not suggest that considerably different food resources were explored by colonies of different subspecies or Varroa status. Hence, it seems acceptable to consider that colonies were subjected to a "common" environment, reducing the effect of environmental variance on the observed phenotype variance. Given that laboratorial analyses for possible secondary unapparent infections were not carried out (namely for acute paralysis virus, slow paralysis virus or deformed wing virus), the results obtained in this experiment should be looked at within a framework of Varroa infestation global impact in this environmental conditions, rather than an exclusive consequence of Varroa population dynamics. Considerable quantitative differences were found among colonies in the amount of stored honey they held, mainly as a result of colonies' Varroa status. However, honey stores followed a similar annual pattern, independently of subspecies or Varroa. Indeed, the dynamic of this variable was typical of honey bee colonies kept in temperate climates (Seeley, 1984, 1995; Morse & Hooper, 1985; Winston, 1987), with net increases in honey during spring / summer and decreases in honey over winter. The main exception observed in my study was that much less drastic honey loss was observed during autumn and winter. Milder overwinter environmental conditions and the possibility for some nectar or honeydew collection during autumn- and winter-time in southern Portugal may reduce a colony's reliance on stored honey. By the end of this experiment, and within each studied subspecies, artificially Varroa infested colonies had considerably less stored honey than control ones. Considering that traditionally beekeeping in southern Portugal is generally practised with an exclusive view to honey harvest, significant reductions in annual colony honey production are expected to lead to severe decreases in the gross economic income from this activity. In this situation, it seems reasonable to accept from this study that alternative beekeeping with any of the other tested subspecies (besides A. m. iberica) is not, per se, a solution to avoid gross income losses due to Varroa infestation. Indeed, from this study it may be roughly estimated that most beekeepers following traditional management practices, as well as typical regional Varroa control policies (one annual fluvalinate treatment after the honey harvest, generally occurring from July until September) with A. m. iberica are loosing ≈ 64 % of their beekeeping income due to Varroa infestation, without considering the higher colony mortality associated with Varroa in this region. By the end of this experiment, A. m. ligustica control colonies stored an average of ≈ 68 kg of honey, compared with ≈ 29 kg of honey stored by control A. m. iberica colonies. A. m. ligustica treatment colonies only stored ≈ 13 kg of honey, compared with ≈ 11 kg of honey stored by A. m. iberica colonies of the same Varroa status. Therefore, although A. m. ligustica had the highest potential for honey production in the southern part of the country, in the present regional beekeeping reality it seems of little interest to suggest its use by most of our beekeepers without very significant changes in management practices and Varroa control policies. Without these, A. m. ligustica colonies tend to perform about as badly as A. m. iberica ones. Considering that most regional beekeepers frequently only first become aware of Varroa infestations by "suspecting" of the development of their colonies (particularly in terms of honey bee population and amounts of stored honey), this experiment suggests that smaller amounts of honey stored in treatment colonies, compared with control ones, tend to become evident in apiary inspections only as colonies approximate collapse. Thus an effort for earlier monitoring of Varroa population dynamics must bee strongly recommended to regional beekeepers, if colony mean honey productivity and survivorship is to be improved.

References

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Harvard University Press; Massachusetts (USA); 281 pp. IMPACT OF VARROA DESTRUCTOR N. SP. INFESTATION ON HONEY PRODUCTION OF APIS MELLIFERA L. COLONIES IN A MEDITERRANEAN CLIMATE

Prof. António Manuel Murilhas

Instituto de Ciências Agrárias Mediterrânicas, Univ. de Évora, 7000 Evora, Portugal Tel. + 315 266 760864, Fax + 351 266 711163, [email protected]

Curriculum vitae

Name: António Manuel de Oliveira Coelho Murilhas Date of birth: 21. Ago. 1962 Nacionality: Portuguese Institutional address: Universidade de Évora Departamento de Zootecnia (Sector de Apicultura) 7000 ÉVORA Portugal Academic degrees, fields of study, institutions, dates: Doctor of Philosophy, Applied Biology - University of Wales (UK), Out. 1998 Apiculture, Master of Philosophy, Applied Biology - University of Wales (UK), Ago. 1991 Apiculture, Pos-grad. Diploma, Apiculture, University of Wales (UK), Jul. 1989 Licenciatura, Animal Product. University of Évora (Pt), Jul. 1986 Engineering,

Present position, institution, starting date: Professor, University of Évora, 01 Nov. 1998

Previous positions, institutions and dates: Assistant, University of Évora, From 1987 to 1998

Main scientific areas of research: Beekeeping and bee products (production strategies and QA/QC) Bee pathology (mainly Varroosis)