Comparison between different fumagillin dosage and evaluation method in the apiary control of Nosemosis type C.

Fumagillin (dicyclohexylammonium) is an antibiotic with well-known microsporicidal activity widely used to control Nosema disease caused by Nosema apis in honeybees. Its use is permitted by law in Argentine and USA, but not in the EU countries, apart from specific cases under veterinary authorization. So far, the optimal dosage of this active ingredient in controlling Nosema ceranae has not been determined. The aim of the present work was to assess the efficiency of different fumagillin dosages on Apis mellifera colonies affected by different N. ceranae intensity. For this purpose, during April and May of 2007, in an apiary located near the Mar del Plata city (Buenos Aires Province, Argentine), forty eight A. mellifera colonies reared in Langhstroth hives, each containing 8 to 10 adult bee combs, were used in order to evaluate the effectiveness of different doses of fumagillin. The colonies were divided into those with mild or semi-severe disease intensity, based on the number of Nosema spores estimated in a sample of 60 returning bees (arb, spores abundance in returning bee). Then the colonies were randomly distributed forming 6 experimental groups (3 with mild intensity: one untreated left as control and the other two treated with 102 mg of fumagillin in two and three doses, respectively; 3 with semi-severe intensity: treated with 102 mg of fumagillin in two and three doses, and with 120 mg of fumagillin in four doses, respectively). Each dose was administered at intervals of 7 days. Before and during each administration, for a period of 35 days, the number of frames covered with adult bees was recorded in each hive and a sample of 100 workers per colony was collected from the hive entrance in order to determine the abundance (arb) and prevalence (prb) of spores. This alternative parameter (prb) was calculated by examining a sub-sample of 10 bees, crushing the propodeum of the bees in 0.5 ml distilled water, taking a drop of suspension and observing 20 fields under a optic microscope (450x) for spore absence (0) or presence (1); then the estimation of spores prevalence (prb) was recorded as percentage of spore presence in the 20 observed fields, and the average of two estimates were used. Moreover, spores from 5 randomly selected colonies were molecularly characterized to confirm the Nosema species. The sequencing results showed a 98% of homology with N. ceranae. The results obtained allow to establish that the efficiency of fumagillin in controlling Nosemosis type C caused by N. ceranae is affected by the intensity of the disease rather than from dosage or way of application. The best result was obtained when colonies were affected by a mild intensity and the drug was administered in two weekly doses of 51 mg per hive