Efficacy of strips coated with Metarhizium anisopliae for control of Varroa destructor (Acari: Varroidae) in honey bee colonies in Texas and Florida

Strips coated with conidia of Metarhizium anisopliae (Metschinkoff; Deuteromycetes: Hyphomycetes) to control the parasitic mite, Varroa destructor (Anderson and Trueman) in colonies of honey bees, Apis mellifera (Hymenoptera: Apidae) were compared against the miticide, tau-fluvalinate (Apistan) in field trials in Texas and Florida (USA). Apistan and the fungal treatments resulted in successful control of mite populations in both locations. At the end of the 42-day period of the experiment in Texas, the number of mites per bee was reduced by 69-fold in bee hives treated with Apistan and 25-fold in hives treated with the fungus; however mite infestations increased by 1.3-fold in the control bee hives. Similarly, the number of mites in sealed brood was 13-fold and 3.6-fold higher in the control bee hives than in those treated with Apistan and with the fungus, respectively. Like the miticide Apistan, the fungal treatments provided a significant reduction of mite populations at the end of the experimental period. The data from the broodless colonies treated with the fungus indicated that optimum mite control could be achieved when no brood is being produced, or when brood production is low, such as in the early spring or late fall. In established colonies in Florida, honey bee colony development did not increase under either Apistan or fungal treatments at the end of the experimental period, suggesting that other factors (queen health, food source, food availability) play some major role in the growth of bee colonies. Overall, microbial control of Varroa mites with fungal pathogens could be a useful component of an integrated pest management program for the honey bee industry.     

Field trials using the fungal pathogen, Metarhizium anisopliae (Deuteromycetes: Hyphomycetes) to control the ectoparasitic mite, Varroa destructor (Acari: Varroidae) in honey bee, Apis mellifera (Hymenoptera: Apidae) colonies

The potential for Metarhizium anisopliae (Metschinkoff) to control the parasitic mite, Varroa destructor (Anderson and Trueman) in honey bee colonies was evaluated in field trials against the miticide, tau-fluvalinate (Apistan). Peak mortality of V. destructor occurred 3-4 d after the conidia were applied; however, the mites were still infected 42 d posttreatments. Two application methods were tested: dusts and strips coated with the fungal conidia, and both methods resulted in successful control of mite populations. The fungal treatments were as effective as the Apistan, at the end of the 42-d period of the experiment. The data suggested that optimum mite control could be achieved when no brood is being produced, or when brood production is low, such as in the early spring or late fall. M. anisopliae was harmless to the honey bees (adult bees, or brood) and colony development was not affected. Mite mortality was highly correlated with mycosis in dead mites collected from sticky traps, indicating that the fungus was infecting and killing the mites. Because workers and drones drift between hives, the adult bees were able to spread the fungus between honey bee colonies in the apiary, a situation that could be beneficial to beekeepers.     

Virulence and site of infection of the fungus,Hirsutella thompsonii,to the honey bee ectoparasitic mite,Varroa destructor

The Varroa mite, Varroa destructor, is recognized as the most serious pest of both managed and feral Western honey bee (Apis mellifera) in the world. The mite has developed resistance to fluvalinate, an acaricide used to control it in beehives, and fluvalinate residues have been found in the beeswax, necessitating an urgent need to find alternative control measures to suppress this pest. Accordingly, we investigated the possibility of using the fungus, Hirsutella thompsonii, as a biocontrol agent of the Varroa mite. Among the 9 isolates of H. thompsonii obtained from the University of Florida and the USDA, only the 3 USDA isolates (ARSEF 257, 1947 and 3323) were infectious to the Varroa mite in laboratory tests. The mite became infected when it was allowed to walk on a sporulating H. thompsonii culture for 5 min. Scanning electron micrographs revealed that the membranous arolium of the mite leg sucker is the focus of infection where the fungal conidia adhered and germinated. The infected mites died from mycosis, with the lethal times to kill 50% (LT(50)s) dependent on the fungal isolates. Thus, the LT(50)s were 52.7, 77.2, and 96.7h for isolates 3323, 257, and 1947, respectively. Passage of H. thompsonii through Varroa mite three times significantly reduced the LT(50)s of isolates 257 and 1947 (P<0.05) but not the LT(50) of isolate 3323.The fungus did not infect the honey bee in larval, prepupal, pupal, and adult stages under our laboratory rearing conditions. Our encouraging results suggest that some isolates of H. thompsonii have the potential to be developed as a biocontrol agent for V. destructor. However, fungal infectivity against the mites under beehive conditions needs to be studied before any conclusion can be made.     

Duration and spread of an entomopathogenic fungus, Beauveria bassiana (Deuteromycota: Hyphomycetes), used to treat varroa mites (Acari: Varroidae) in honey bee (Hymenoptera: Apidae) hives

A strain of the fungus Beauveria bassiana (Balsamo) Vuillemin (Deuteromycota: Hyphomycetes) isolated from varroa mites, Varroa destructor Anderson & Trueman (Acari: Varroidae), was used to treat honey bees, Apis mellifera L. (Hymenoptera: Apidae), against varroa mites in southern France. Fungal treatment caused a significant increase in the percentage of infected varroa mites compared with control treatments in two field experiments. In the first experiment, hives were treated with a formulation containing 0.37 g of B. bassiana conidia per hive and in the second experiment with a dose of 1.0 g of conidia per hive. The percentage of infected varroa mites also increased in the nontreated (control) hives, suggesting a movement of conidia, probably via bee drift, among the hives. Mite fall was significantly higher among treated hives compared with control hives on the sixth and eighth days after treatment in the first experiment. These days correspond to previously published data on the median survivorship of mites exposed to that fungal solate. The interaction of treatment and date was significant in the second experiment with respect to mite fall. Increases in colony-forming unit (cfu) density per bee were observed in all treatments but were significantly higher among bees from treated hives than control hives for at least a week after treatment. The relationship between cfu density per bee and proportion infected was modeled using a sigmoid curve. High levels of infection (>80%) were observed for cfu density per bee as low as 5 x 102 per bee, but the cfu density in hives treated with 0.37 g generally dropped below this level less than a week after treatment.     

A scientific note on the detection of honeybee viruses using real-time PCR (TaqMan) in Varroa mites collected from a Thai honeybee (Apis mellifera) apiary

Bee parasitic mite syndrome is a disease complex of colonies simultaneously infested with Varroa destructor mites and infected with viruses and accompanied by high mortality. By using real-time PCR (TaqMan), five out of seven bee viruses were detected in mite samples (V. destructor) collected from Thailand. Moreover, the results of this study provide an evidence for the co-existence of several bee viruses in a single mite. This is also the first report of bee viruses in mites from Thailand.     

Expression and characterization of the chitinases from Serratia marcescens GEI strain for the control of Varroa destructor, a honey bee parasite

Serratia marcescens GEI strain was isolated from the gut of the workers of Chinese honey bee Apis cerana and evaluated in the laboratory for the control of Varroa destructor, a parasite of western honey bee A. mellifera. The supernatant and the collected proteins by ammonium sulfate from the bacterial cultures showed a strong miticidal effect on the female mites, with 100% mite mortality in 5 days. Heat (100 °C for 10 min) and proteinase K treatment of the collected proteins destroyed the miticidal activity. The improved miticial activity of this bacterial strain on chitin medium indicated the involvement of chitinases. The expressed chitinases ChiA, ChiB and ChiC1 from S. marcescens GEI by recombinant Escherichia coli showed pathogenicity against the mites in the laboratory. These chitinases were active in a broad pH range (5-9) and the optimum temperatures were between 60 and 75 °C. Synergistic effects of ChiA and ChiB on the miticidal activity against V. destructor were observed. The workers of both honey bee species were not sensitive to the spraying and feeding chitinases. These results provided alternative control strategies for Varroa mites, by formulating chitinase agents and by constructing transgenetic honey bees.     

Russian honey bee (Hymenoptera: Apidae) colonies: Acarapis woodi (Acari: Tarsonemidae) infestations and overwintering survival

Honey bee, Apis mellifera L. (Hymenoptera: Apidae), colonies infested by parasitic mites are more prone to suffer from a variety of stresses, including cold temperature. We evaluated the overwintering ability of candidate breeder lines of Russian honey bees, most of which are resistant to both Varroa destructor Anderson & Trueman and Acarapis woodi (Rennie), during 1999-2001. Our results indicate that Russian honey bee colonies (headed by original and supersedure queens) can successfully overwinter in the north, even during adverse weather conditions, owing to their frugal use of food stores and their resistance to tracheal mite infestations. In contrast, colonies of Italian honey bees consumed more food, had more mites, and lost more adult bees than Russian honey bees, even during unusually mild winter conditions.     

Evaluation of Metarhizium anisopliae (Deuteromycota: Hyphomycetes) for control of broad mite Polyphagotarsonemus latus (Acari: Tarsonemidae) in mulberry

A study on 12 entomopathogenic fungi for controlling broad mite (Polyphagotarsonemus latus (Banks)) in mulberry found that Metarhizium anisopliae CKM-048 was the most virulent strain in controlling both larvae and adult broad mites at the concentration of 2 x 10(8) conidia/ml. There was no ovicidal effect when tested with broad mite eggs. Median lethal concentrations (LC(50)) of M. anisopliae in killing larvae and adults were 8.7 x 10(6) and 1.3 x 10(7 )conidia/ml, respectively. Median lethal times (LT(50)) of larvae and adults were 2.4 and 3.8 days, respectively, at the concentration of 2 x 10(8) conidia/ml. The fungus was found to produce protease and chitinase. Scanning electron microscope (SEM) studies were done to monitor the infection steps of the fungus on broad mites. A greenhouse test on mulberry trees revealed that M. anisopliae could reduce the broad mite population within 4 days after treatment. However, after 7 days, its efficacy was decreased significantly.     

Direct effect of acaricides on pathogen loads and gene expression levels in honey bees Apis mellifera

The effect of using acaricides to control varroa mites has long been a concern to the beekeeping industry due to unintended negative impacts on honey bee health. Irregular ontogenesis, suppression of immune defenses, and impairment of normal behavior have been linked to pesticide use. External stressors, including parasites and the pathogens they vector, can confound studies on the effects of pesticides on the metabolism of honey bees. This is the case of Varroa destructor, a mite that negatively affects honey bee health on many levels, from direct parasitism, which diminishes honey bee productivity, to vectoring and/or activating other pathogens, including many viruses. Here we present a gene expression profile comprising genes acting on diverse metabolic levels (detoxification, immunity, and development) in a honey bee population that lacks the influence of varroa mites. We present data for hives treated with five different acaricides; Apiguard (thymol), Apistan (tau-fluvalinate), Checkmite (coumaphos), Miteaway (formic acid) and ApiVar (amitraz). The results indicate that thymol, coumaphos and formic acid are able to alter some metabolic responses. These include detoxification gene expression pathways, components of the immune system responsible for cellular response and the c-Jun amino-terminal kinase (JNK) pathway, and developmental genes. These could potentially interfere with the health of individual honey bees and entire colonies.     

Honey bee colony collapse disorder is possibly caused by a dietary pyrethrum deficiency

Industrialized farming relies on bee keepers transporting hives to the vicinity of large areas of mono-crops for crop pollination. Hives are typically moved multiple times per growing season to satisfy the pollination need. A phenomenon wherein colonies of honey bees collapse in large numbers has been threatening crops in North America. Honey bees are hosts to at least two pathogenic mites; Varroa destructor and Acarapis woodi (a tracheal mite). Pyrethrums are a group of flowering plants which include Chrysanthemum coccineum, Chrysanthemum cinerariifolium, Chrysanthemum marschallii, and related species. These plants produce potent insecticides, also named pyrethrums, which are powerful mite toxins. We believe that a honey bee dietary deficiency of pyrethrums and other micro-nutrients from pyrethrum producing plants allows parasitic mites to either kill the honey bees directly or reduce honey bee resistance to other pathogens. Intermittent feeding of honey bees on pyrethrum producing plants might reverse or prevent colony collapse disorder.