Giorgio Della Vedova

Synergistic Parasite-Pathogen Interactions Mediated by Host Immunity Can Drive the Collapse of Honeybee Colonies

The health of the honeybee and, indirectly, global crop production are threatened by several biotic and abiotic factors, which play a poorly defined role in the induction of widespread colony losses. Recent descriptive studies suggest that colony losses are often related to the interaction between pathogens and other stress factors, including parasites. Through an integrated analysis of the population and molecular changes associated with the collapse of honeybee colonies infested by the parasitic mite Varroa destructor, we show that this parasite can de-stabilise the within-host dynamics of Deformed wing virus (DWV), transforming a cryptic and vertically transmitted virus into a rapidly replicating killer, which attains lethal levels late in the season. The de-stabilisation of DWV infection is associated with an immunosuppression syndrome, characterized by a strong down-regulation of the transcription factor NF-κB. The centrality of NF-κB in host responses to a range of environmental challenges suggests that this transcription factor can act as a common currency underlying colony collapse that may be triggered by different causes. Our results offer an integrated account for the multifactorial origin of honeybee losses and a new framework for assessing, and possibly mitigating, the impact of environmental challenges on honeybee health.

Octanoic acid confers to royal jelly varroa-repellent properties

The mite Varroa destructor Anderson & Trueman is a parasite of the honeybee Apis mellifera L. and represents a major threat for apiculture in the Western world. Reproduction takes place only inside bee brood cells that are invaded just before sealing; drone cells are preferred over worker cells, whereas queen cells are not normally invaded. Lower incidence of mites in queen cells is at least partly due to the deterrent activity of royal jelly. In this study, the repellent properties of royal jelly were investigated using a lab bioassay. Chemical analysis showed that octanoic acid is a major volatile component of royal jelly; by contrast, the concentration is much lower in drone and worker larval food. Bioassays, carried out under lab conditions, demonstrated that octanoic acid is repellent to the mite. Field studies in bee colonies confirmed that the compound may interfere with the process of cell invasion by the mite.

(Z)-8-Heptadecene from infested cells reduces the reproduction of Varroa destructor under laboratory conditions

The parasitic mite Varroa destructor Anderson and Trueman, the most serious threat to apiculture in many countries of the world, reproduces inside honeybee brood cells. Previous research, using artificial cells for rearing the mite on an Apis mellifera larva, indicated that semiochemicals affecting the reproduction of Varroa destructor are released into such cells. In order to isolate these semiochemicals, infested artificial cells were extracted with hexane and the extract fractionated twice. Several unsaturated hydrocarbons were identified in the active fraction; some of them were released in higher amounts in case of infestation and were, therefore, bioassayed for their effect on the mites reproduction. Of five alkenes tested under laboratory conditions, (Z)-8-heptadecene, caused a 30% reduction in the mean number of offspring of mites reared in cells treated with this compound.

Determination of the LC50 of chlorfenvinphos in Varroa destructor

Chlorfenvinphos is an organophosphorus insecticide ((EZ)-2-chloro-1(2, 4dichlorophenyl) vinyl diethyl phosphate) authorised in some countries for agricultural and veterinary uses but not for apiculture. Nevertheless, in recent years it has been increasingly employed to control Varroa destructor Anderson & Trueman, as indicated by the growing percentage of wax samples found to be positive for residues (17% in 2003: Costa et al., 2006). A possible lack of efficacy of treatments with chlorfenvinphos has been claimed as a cause of bee losses. We developed a bioassay to evaluate the toxicity of this organophosphorus acaricide to V. destructor, to provide a technique for the early detection of resistance. A technique similar to that described in Milani and Della Vedova (1996) was used. Adult females of V. destructor were sampled in three areas of Northern Italy: in Udine, in an apiary never treated with organophosphorus acaricides; and in two districts of Emilia, from heavily infested colonies, in an area where chlorfenvinphos had been widely and repeatedly used. Mites were taken from brood cells and assayed within 90 min. Mites from brood of different ages: spinning larvae (l5); white eyed pupae (pw); pupae with dark eyes and white or pale body (pd) were assayed separately. Those from pupae with pigmented bodies and adult bees were not used (Milani, 1995). Shallow capsules, consisting of two glass discs (62 mm dia.) and two stainless steel rings (56 mm inner dia.) were prepared. Their interior, including the rings, was coated with paraffin wax (Merck 7151; melting point 46-48°C), containing chlorfenvinphos (Riedel-de Haën 36551), as described in Milani and Della Vedova (1996). The concentrations used were: 0 (control); 0.02; 0.05; 0.1; 0.2; 0.5; 1; 2; 5 mg/kg. The capsules were used within 12 days of preparation. About 15 females mites were introduced into each capsule. After 4 h they were transferred to a clean glass Petri dish (60 mm dia.) with three worker larvae taken from cells 0-24 h after capping. The mites were observed under a dissecting microscope 4 h (when transferred to the Petri dish), 24 and 48 h after the introduction into the capsule, and classified as mobile, paralysed (knocked down) or dead (as in Milani, 1995). The assays were carried out at 33°C and 70% RH; a series of tests was carried out at 26°C. As a rule, the assay was repeated 2 or 3 times per brood stage and origin of the mites, so that 30-45 mites per concentration were assayed. As in previous work, more mites were assayed at concentrations around the median lethal concentration. The data were analysed using probit transformation, according to Finney (1971). As in previous work, observations at 48 h gave the most reliable results, as paralysed mites were less than 0.5%, while natural mortality did not exceed 2%. The results of the replications were consistent and a good fit using the probit regression was found. The variance of the lethal concentrations was low, resulting in steep