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.

-Hygienic behaviour of bees towards mite infested brood is involved in the tolerance of some bee strains to Varroa destructor. The stimuli triggering hygienic behaviour are olfactory cues emanating from cells containing infested brood but their identity is still unknown. After confirming the capacity of bees to detect and empty mite-infested cells, we studied the volatile chemicals released by artificially infested worker brood cells by means of SPME-GC-MS. The identified chemicals were then bioassayed by comparing the bees' hygienic behaviour towards treated cells into which 1 µg of each compound was injected and control cells which received the solvent alone. Z-(6)-pentadecene significantly increased the number of cells emptied by the bees. Varroa destructor / semiochemicals / hygienic behaviour / removal / hydrocarbons

The parasitic mite, Varroa destructor, is the most important threat for apiculture in most bee-keeping areas of the world. The mite is carried to the bee brood cell, where it reproduces, by a nurse bee; therefore the selection of the bee stage by the parasite could influence its reproductive success. This study investigates the role of the cuticular hydrocarbons of the European honeybee (Apis mellifera) in host-selection by the mite. Preliminary laboratory bioassays confirmed the preference of the varroa mite for nurse bees over pollen foragers. GC-MS analysis of nurse and pollen bees revealed differences in the cuticular hydrocarbons of the two stages; in particular, it appeared that pollen bees have more (Z)-8-heptadecene than nurse bees. Laboratory experiments showed that treatment of nurse bees with 100 ng of the pure compound makes them repellent to the varroa mite. These results suggest that the mite can exploit the differences in the cuticular composition of its host for a refined selection that allows it to reach a brood cell and start reproduction. The biological activity of the alkene encourages further investigations for the development of novel control techniques based on this compound.

-The reversion of resistance to pyrethroids in Varroa destructor Anderson & Trueman was studied in Friuli (northern Italy), where resistance was detected in 1995 and pyrethroids had not been used since. Mites were sampled in seven localities each year between 1997 and 2000 and assayed in the laboratory for the resistance to fluvalinate by using paraffin coated capsules. Survival at the diagnostic concentration, expected to kill all susceptible mites (200 mg/kg), decreased in all the localities by about ten times in three years, from 19-66% to 1.3-7.8%. Thus, the disadvantage associated with the resistance to pyrethroids in V. destructor is small, as usual when resistance is due to monooxygenases. Its impact on the selection of resistant mites during annual application of treatments is negligible; appreciable effects of reversion can be expected only over many generations of the mite.Varroa destructor / reversion / resistance / pyrethroids INTRODUCTIONIn recent years, populations of Varroa destructor Anderson & Trueman resistant to pyrethroids have been detected in several countries (Lodesani et al., 1995;Trouiller, 1998;Elzen et al., 1998) and increasing attention has been paid to this problem. Several laboratory assays have been developed (e.g., Milani, 1995;Faucon et al., 1996;Colin et al., 1997;Elzen et al., 1998) and used to monitor the spread of resistant populations (Trouiller, 1998).The biochemical mechanism of resistance has been investigated; monooxygenases of the P450 system are involved, at least in the strain of V. destructor that originated in Italy and later spread through the Old World, while esterases do not play a significant role (Hillesheim et al., 1996;Mozes Koch et al., 2000). Fast selection and spread of resistant mites make it unlikely that resistance is polygenic; this is a general rule with insects and mites (Roush and McKenzie, 1987).Resistant genotypes usually are at some fitness disadvantage in the absence of pesticides (Roush and Daly, 1990;Denholm and Rowland, 1992), because of unbalanced or unregulated physiological processes. This makes the frequency of resistant mites decline when the acaricide is not used (a phenomenon usually called reversion). No information is available on the decrease in fitness associated with resistance to pyrethroids in V. destructor. More data on this aspect would be useful to elaborate theoretical models of the development of resistance and to develop integrated resistance 417

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