EN
The studies were done in 1987 and in 1988 on 1062 worker bees. The extensiveness of Varroa jacobsoni invasion was low in June and July (E.i.=0.1—5.0%) but in August, September and October the extensiveness of Varroa invasion reached almost 15.0% in most bee colonies. A mean body weight of worker bees from parasitized colonies was in June and July higher by about 0.7—1.1 mg, and lower in August by 2.9%, in September by 1.6 mg and in October by 11.2 mg than that in normal colonies. The effect of the invasion on body weight is mostly pronounced in colonies in which more than 5% of bees is infested. In August and September the effect of Varroa invasion is not so strong (weight of workers from infested colonies is lower by 1.8—4.8 mg in comparison to weight of bees from normal colonies) but in October a body weight of worker from colonies infested in 5.1—10% is lower by 9.0 mg, from colonies infested in 10.1—15% by 19 mg comparing to bees from normal colonies. Such large diminishion of body weight must affect further development of a bee colony.