Beobachtungen zum Auftreten beschädigter Varroamilben im natürlichen Totenfall bei Völkern von Apis mellifera carnica

Abstract - Observations on the occurrence of damaged Varroa mites in natural mite fall of Apis mellifera carnica colonies
Natural mite mortality reflects the population growth and the reproductive potential of Varroa and also provides information on bees active defence mechanisms. Wallner (1989; 1990a,b; 1991; 1992) was the first to postulate active defence mechanisms based on his observations of mites with damaged legs and cuticle of the idiosoma. These findings were confirmed by Ruttner (1991) and Ruttner and Hänel (1992). In the present study the incidence of damaged mites was recorded in 111 colonies of Apis mellifera carnica at 4 locations in lower Austria from 18th August-18th September 1991. Dead mites were collected via wire-covered inlays in the floor board and counted 3 times during the investigation period. Mites were registered as undamaged or damaged (legs partly or completely amputated, cuticle of idiosoma or cuticle and legs damaged). The percentage of damaged mites was correlated with bee- and brood infestation level, origin of queens, and the number of Varroa found after treatment with pyrethroid strips. Details on observed hives have been given in table I. To estimate total mite population, a treatment with Apistan or Bayvarol followed immediately after the last determination of natural mite fall. Data on bee and brood infestation rate were determined once at the beginning of September. Out of 8 452 mites (= total sum of all colonies) 1 601 (18.9%) showed injuries (table II). Light coloured mites (= immature adult females) were injured significantly more frequent (27.1 %) than dark mites (9.5%; table II). Figure 1 presents data on injury, which is divided into 9 percentage classes and shows the distribution of colonies according to these particular classes. Only 7.4% of the colonies (= last 3 classes) had > 30% damaged mites in their natural mite fall. No significant differences in percentage of damaged mites was observed between the 4 locations or 4 groups of queens from different origins. Natural mite mortality and the number of damaged mites showed a highly significant linear correlation. These findings are in agreement with those of Ruttner and Hänel (1992). The percentage of natural mite mortality was significantly correlated with bee- and brood infestation rate. A significant correlation was found between percentage of light-coloured damaged mites and brood infestation rate (%), but not with bee infestation rate (%). The percentage of dark, damaged mites was significantly correlated both with brood and bee infestation rate (figs 2, 3). The significant negative correlation between percentage of damaged mites and number of mites found after chemical treatment as well as the negative correlation with bee- and brood infestation rate gives an indication of the influence of active defence behaviour by the bees on Varroa population growth (fig 4). If this behaviour could be increased by breeding and selection measures, it would probably constitute an important step towards a Varroa-resistant Carniolan honey bee.