Regular Dorsal Dimples on Varroa Destructor -- Damage Symptoms Or
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Apidologie 40 (2009) 151–162 Available online at: c INRA/DIB-AGIB/ EDP Sciences, 2009 www.apidologie.org DOI: 10.1051/apido/2009001 Original article Regular dorsal dimples on Varroa destructor – Damage symptoms or developmental origin?* Arthur R. Davis Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2 Received 6 June 2008 – Revised 29 October 2008 – Accepted 18 November 2008 Abstract – Adult females (n = 518) of Varroa destructor from Apis mellifera prepupae were examined by scanning electron microscopy without prior fluid fixation, dehydration and critical-point drying. Fifty-five (10.6%) mites had one (8.1%) or two (2.5%) diagonal dimples positioned symmetrically on the idiosoma’s dorsum. Where one such regular dorsal dimple existed per mite body, it occurred on the left or right side, equally. Mites with at least one regular dorsal dimple were 3.4% longer, but neither idiosomal width nor elliptical area differed significantly from mites lacking dimples. Dimple length was normally distributed in the population, and averaged 462 ± 9.2 μm(s.e.;n= 68 dimples). Internally, each regular dorsal dimple aligns with a series of obliquely-situated, dorso-ventral muscles in the opisthosoma. It is concluded that regular dorsal dimples are faults originating during mite ontogeny and should be considered separately from damage to Varroa destructor inflicted by honeybees or predatory arthropods. developmental fault / idiosoma / regular dorsal dimple / scanning electron microscopy / Varroa destructor 1. INTRODUCTION hive’s worker bees. The procedure has as- sociated risks, in that naturally-fallen mites Since spreading to colonies of the European which die may subsequently be damaged by honey bee (Apis mellifera L.), much interest house-cleaning workers (Rosenkranz et al., has been directed at identifying colonies that 1997; Bienefeld et al., 1999)ormaybein- actively defend themselves against infestation jured or go missing owing to activities of a by parasitic mites of Varroa spp. (reviewed by hive’s non-bee inhabitants (Szabo and Walker, Boecking and Spivak, 1999). Commonly these 1995; Lodesani et al., 1996; Thakur et al., mites are groomed and bitten by worker bees 1997; Bienefeld et al., 1999). Moreover, it of their natural host species, A. cerana Fabri- is questionable whether all irregularities on cius (Peng et al., 1987; Büchler et al., 1992; the mite body should be regarded as handling Fries et al., 1996), a behaviour also exhibited damage from arthropods. To be able to accu- in certain colonies of A. mellifera (reviewed by rately quantify the frequency and understand Boecking and Spivak, 1999; Corrêa-Marques the types of injuries received by V. destructor, et al., 2000; Stanimirovicetal.,´ 2005). it is essential for surveys of mite morphology To identify honey bee colonies that injure to carefully discriminate damage symptoms Varroa destructor, mites fallen to the bot- from deformities of a developmental nature. tom board frequently are sorted to charac- Upon the idiosoma of fallen mites of V. d e - terize individuals potentially damaged by a structor, many studies have distinguished dor- sal indentations from other body abnormali- Corresponding author: A.R. Davis, ties (Morse et al., 1991; Ruttner and Hänel, [email protected] 1992; Lodesani et al., 1996; Rosenkranz et al., * Manuscript editor: Peter Rosenkranz 1997; Bienefeld et al., 1999; Harbo and Harris, Article published by EDP Sciences 152 A.R. Davis 1999a, b; Corrêa-Marques et al., 2000, 2002; damage by arthropods versus faults in V. d e - Zaitoun et al., 2001; Stanimirovicetal.,´ 2005). structor development, to specifically address But few have questioned whether these types the following objectives: to (i) quantify the fre- of body imperfections may arise during mite quency of one and two regular dimples, and ontogeny (Lodesani et al., 1996; Rosenkranz their length, on the dorsum of the mite idio- et al., 1997). The “single or double hollow on soma; (ii) compare the size of mites possessing dorsal shield (Figs. 2, 4a–c)” and in Table II, regular dorsal dimples with those lacking dim- “hollow of the dorsal shield” were identified ples; (iii) relate the precise location of regular as types of damage, but later discussed as pos- dorsal dimples to the mite’s internal structure; sible “accidental events in the growth stages and (iv) resolve confusion from the literature of the nymphs or during the sclerotization pro- by providing clear images and a standardiza- cess” (Lodesani et al., 1996). Rosenkranz et al. tion of terminology for reference. (1997) reported “regular dimples on the dor- sal shield” of living mites and attributed them to “disturbances during the nymphal develop- 2. MATERIALS AND METHODS ment”, citing the previous paper. Related de- 2.1. Source and treatment of mites scriptions, recognized as mite damage, have also been given. Morse et al. (1991) found Most mites of Varroa destructor (Anderson and dead mites with “one or two indentations in the Trueman, 2000) examined in this study were those dorsal surface” and attributed these to worker from earlier experiments (Davis et al., 2007a). bee mandibles. Some young adult females had Briefly, mites were collected during May–July 2005 “dents on their idiosoma” (Harbo and Harris, from brood frames of two queenright colonies of 1999a) or “dents in the dorsal surface of the id- hybrid Italian honey bees (Apis mellifera ligus- tica Spinola) from the same apiary (N52◦ 03.741’ iosoma” (Harbo and Harris, 1999b) upon cell ◦ departure as progeny; these depressions were W106 38.704’). The colonies had not been treated reported to have been impossibly caused by chemically in any way. Mites were harvested immediately prior to ex- the mandibles of grooming bees but rather, perimentation. Because immature drones were in- possibly from leg movements of the host bee frequent, more than 85% of mites came from sealed while inside the brood cell (Harbo and Harris, worker cells, from multiple frames. All mites there 1999a). Moreover, a “depression (hollow) in were dark brown adult females (prior to oviposi- the dorsal shield” (Tab. 3 of Corrêa-Marques tion) which adhered to the visually-healthy, mo- et al., 2000), an “indentation or hollow in the tionless prepupae (neither mature larvae nor pupae) dorsal shield” (Tab. 2 of Corrêa-Marques et al., when withdrawn using forceps. From the prepu- 2002), and “depression or hollow in the dor- pae, individual mites latching onto a new child’s sal shield” (Tab. 2 of Stanimirovicetal.,´ 2005) paint brush or forceps were gently transferred un- have been identified, but the accompanying il- til ten mites had been introduced to each 60 mm lustrations (Figs. 2, 4 of Corrêa-Marques et al., petri-dish arena lacking ants (controls) or hold- 2000; Fig. 4-B5 of Stanimirovicetal.,´ 2005) ing five ants; trials were performed separately for show features different from those displayed each of three species (Formica fusca group, La- by Lodesani et al. (1996). sius neoniger, Tapinoma sessile). Sealed arenas, not supplied with additional food or water, were held While investigating the impact of different ◦ in darkness at 24 C. Arenas either allowed con- ant species as natural agents causing mortal- tact between the five ants and ten mites, or only ity of V. destructor in the laboratory (Davis their vapours to pass. Once sealed, mites and ants et al., 2007a), a morphological abnormality of remained in arenas until death of all arthropods. Tri- occasional incidence but predictable location als were replicated five to usually seven times and and certainly distinct from damage symptoms lasted up to 132 h. caused by ant mandibles and chemical sprays Additional to the original study (Davis et al., (Davis et al., 2007b; in preparation), was dis- 2007a), ten mites harvested in exactly the same way covered in mites whether exposed to ants or before placement into contact arenas lacking ants, not. Accordingly, the purpose of this study was were instead immediately frozen. Four trials were to address the standing question of inflicted performed. Regular dorsal dimples on Varroa destructor 153 2.2. Examination of mite morphology (Fig. 1). However, in 8.7–12.9% of mites from various treatments, including 9.8% of those With the intent to document how published in- without any exposure to ants (i.e., control) juries to mites of V. destructor inflicted by worker (Tab. I), the idiosoma possessed one (Fig. 3) honey bees might vary from those due to ants or a maximum of two (Fig. 4) depressions (Davis et al., in preparation), all mites were gently on the dorsum in a regular location. Overall, gathered from arenas. Each of the 518 dead mites 10.6% (55) of the 518 total mites examined was observed preliminarily using a stereomicro- had dorsal dimples; 8.1% (42 of 518) had a × scope (40 ) to detect macroscopic irregularities be- single, and 2.5% (13 of 518) had double dim- fore examination by scanning electron microscopy ples, respectively (Tab. I). (SEM). Without any of the conventional steps of When viewed dorsally, such dimples were fluid fixation, dehydration or critical-point drying, slanted toward the idiosoma’s midline of max- naturally air-dried individuals simply were mounted imum length, almost always without merging, in appropriate orientation on double-sided tape ad- hering to labeled aluminum stubs. Stubs containing typically yielding a bilaterally-symmetrical mites were stored in closed specimen boxes pro- appearance in a high proportion of that minor- tected from dust. After gold coating (Edwards sput- ity of mites (23.6%; 13 of 55) which possessed ter coater S150B), mites were examined at 30 kV two dimples (Figs. 4, 9; Tab. I). Especially by using a Philips 505 SEM and photographed using tilting to view in posterior-dorso perspective, Polaroid 665 P/N film, or high-density printing pa- however, it was evident that the degree of dim- per (UPP-110HD Type II) on a Sony UP-860 Video pling could vary on the same mite (Fig.