Interactions among Varroa jacobsoni mites, acute paralysis virus, and Paenibacillus larvae larvae and their influence on mortality of larval honeybees in vitro

Abstract

The mortality of honey bee larvae and pupae reared in vitro caused by various combinations of oral inoculation with Paenibacillus larvae larvae spores, acute paralysis virus (APV), and infestation with Varroa jacobsoni was studied. The effect of the mite itself and the mite acting as a vector of APV on the mortality of larvae and pupae was investigated. Mortality caused by P. l. larvae ranged from 25 to 55% depending on spore dose. Oral inoculation with APV caused 9% mortality, which was not additive to the mortality caused by P. l. larvae. P. l. larvae did not induce the activation of APV infection. The mortality caused by V. jacobsoni itself was 25%, and by mites transmitting APV, 55%. Neither the mites themselves or the mites transmitting APV had an additive effect on mortality caused by P. l. larvae. The study suggests that APV transmitted by mites is the most significant cause of mortality of the treatment combinations tested. The results do not suggest that the mite itself or the mites transmitting APV act as a stress factor provoking clinical symptoms of American foulbrood (AFB) in individual larvae in the in vitro rearing system.

The Honeybee Disease American Foulbrood — An African Perspective

“…Moreover, the number of spores causing clinical symptoms is highly dependent on susceptibility of the individual colony. The appearance of clinical symptoms of AFB is influenced by factors such as the age and resistance of larvae, the hygienic behaviour of adult bees that remove infected larvae as well as environmental factors such as temperature, quality and quantity of pollen and nectar (Sutter et al 1968;Rinderer & Rothenbuhler 1969;Rinderer & Rothenbuhler 1974;Brødsgaard et al 2000). AFB not only kills the infected individuals but can be lethal to the colonies if present in large quantities (Ratnieks 1992;Genersch 2008).…”

“…According to Brødsgaard et al (2000) there seems to be a correlation between varroa infestations and the expression of AFB symptoms by infected colonies. In a further study by De Rycke et al (2002), Varroa destructor was shown to carry viable P. larvae spores and it was concluded that it may act as a vector of AFB.…”

Number and position of wounds on honey bee (Apis mellifera) pupae infested with a single Varroa mite

“…The limited number of wounds made by Varroa mites may be an adaptive trait, because these perforations in the pupal cuticle are routes for microbial infection (Brødsgaard et al, 2000), which will eventually kill the bee, because only towards the end of the pupal phase will a scar form over the wounds (Kanbar & Engels, 2003). If the adult bee does not emerge from its cell, the mites are trapped in the sealed brood cell, as is observed in heavily infested drone brood of A. cerana (Boecking, 1999).…”

Scent or movement of Varroa destructor mites does not elicit hygienic behaviour by Africanized and Carniolan honey bees

“…'Mite-only removal' without damaging the host may be an adaptive strategy to minimise the loss of brood. If the V. destructor female is removed shortly after invasion of the brood cell, even secondary infections transmitted by the mite (Brødsgaard et al, 2000) may be prevented. The complexity of the mite-specific removal behaviour may have consequences for selective breeding practices.…”