Decrease in Apistan® efficacy used against Varroa disease in the honeybee (Apis mellifera)

Varroa disease is a very serious parasitosis of the bee, which induces important hive losses. Fluvalinate (Apistan®) has been one of the most widely used acaricides since 1989. In 1992, 1993 and 1994, ineffective treatments were observed in Italy and then France, and the hypothesis of a resistance was put forward. The therapeutic efficacy of fluvalinate was estimated in experiments performed in 1992, 1993 and 1994. Groups of colonies from different areas were thus treated for 32-34 d with 2 strips placed in contact with the cluster. Mite mortality was checked with fatted detector boards on hive bottom boards. A treatment with amitraz (Anti- Varroa Schering®) to determine the residual infestation of adult bees was performed the day following the strip withdrawal. Brood infestation was checked by examination of 100 capped cells, by entire brood hatching in an incubator, and by counting the mites on emerging bees. The results showed a decrease in efficacy in the Italian colonies in which fluvalinate therapeutic efficacy fell to 29.7% in 1994. The dosage of fluvalinate in the strips before and after treatment and their good positioning with regard to the bees did not allow us to incriminate a misuse of Apistan® strips. In France, the therapeutic efficacy will be examined at the Italian border during an investigation of all apiaries

Varroosis: demonstrating resistance of parasites to acaricides by means of the 'determination of mean lethal time' method

A simple method of evaluation of Varroa jacobsoni resistance to acaricides (amitraz, fluvalinate and acrinathrin) was developed by comparison of lethal times with a reference strain from CNEVA Sophia Antipolis. This method reproduced the field conditions of the bee colony and kept the host-parasite relationship. The acaricide was administered either as a small piece of plastic strip (Apistan®, Apivar®), or by impregnation of a piece of Whatmann paper no 3 with a known quantity of acaricide. The strip was stuck on the bottom of a Petri dish covered with a net. Five bees were put in contact with the acaricide for 2 hours (fig 1). After that, five mites were introduced onto the bees. The time for the parasites to fall off their host was determined and compared to controls without acaricide. According to the different varroa strains, the lethal time varied from 6-1218 min with fluvalinate. The highest lethal times were registered from mites coming from apiaries where treatments with Apistan® were ineffective (table I). A significant difference was noted with acrinathrin, coincident with the degree of resistance of the mites to fluvalinate. With amitraz, no difference was found. This simple method, named 'determination of average lethal time', can be applied to other acaricides and provide information on the development of resistance through time to beekeepers