Effect of Apis mellifera (Apidae) honeybee brood amount on Oxavar® acaricide efficacy against the mite Varroa destructor (Varroidae)

Efecto de la cantidad de cría de abeja Apis mellifera (Apidae) sobre la eficacia del Oxavar® para el control del ácaro Varroa destructor (Varroidae)

Marcangeli, Jorge* and María del Carmen García**

* Laboratorio de Artrópodos. Fac. Cs. Exactas y Naturales. Universidad Nacional de Mar del Plata. Funes 3350. 7600 Mar del Plata, Argentina; e-mail: jamarca@mdp.edu.ar
**Centro de Extensión Apícola. Ruta 2 Km 342. Coronel Vidal, Argentina; e-mail: ceapicultura@yahoo.com.ar

ABSTRACT. The aim of this work was to evaluate the effect of honeybee brood on acaricide efficacy of Oxavar® to control the ectoparasitic mite Varroa destructor (Anderson & Trueman). Work was done at Centro de Extensión Apícola experimental apiary located at Coronel Vidal, province of Buenos Aires. Ten Langstroth hives were selected and divided in two groups: a) hives containing three honeybee combs full of brood and b) hives containing six honeybee brood combs. Both groups received five ml of Oxavar® (Laboratorio Apilab, Argentina; 64.6 g/l oxalic acid in destilled water) by comb covered by adult honeybees in three doses at seven days period. Weekly, dead mites were collected from special floors to avoid mite removal by adult honeybees. Then, two Apistan® (Laboratorio Roteh, Argentina) strips were placed in each colony to kill remant mites in colonies and the acaricide efficacy was calculated. Results showed that Oxavar® efficacy in the first group (85.6% ± 1.4) was significantly higher than in the second one (75.7 ± 1.7). These differences were tested on the basis of total number of mites killed by Oxavar® and Apistan® in both groups (p< 0.05). Results show a good efficacy of this product, being application recomended in colonies with reduced honeybee brood.

KEY WORDS. Varroa destructor. Apis mellifera. Oxavar®. Oxalic acid.

RESUMEN. El objetivo del presente trabajo fue evaluar la eficacia acaricida del Oxavar® en el control del ácaro ectoparásito Varroa destructor (Anderson & Trueman). El trabajo fue realizado en el apiario experimental del Centro de Extensión Apícola ubicado en Coronel Vidal, provincia de Buenos Aires. Se seleccionaron diez colmenas tipo Langstroth que fueron divididas en dos grupos: a) cinco colmenas con tres cuadros cubiertos completamente de cría en desarrollo y b) cinco colmenas con seis cuadros cubiertos por cría. Ambos grupos recibieron cinco ml of Oxavar® (Apilab, Argentina; 64,6 g/l de ácido oxálico en agua destilada) por cuadro cubierto por abejas adultas en tres dosis a intervalos de siete días. Semanalmente, se colectaron los ácaros muertos de los pisos especiales provistos a las colmenas de estudio con el objeto de evitar su remoción por parte de las abejas. Una vez concluido el tratamiento, en cada colmena se introdujeron dos tiras plásticas de Apistan® (Roteh, Argentina) para eliminar los ácaros remanentes y poder así calcular la eficacia acaricida del Oxavar®. Los resultados mostraron que la eficacia del Oxavar® en el primer grupo (85,6% ± 1,4) resultó significativamente superior a la registrada en el segundo grupo (75,7 ± 1,7). Estas diferencias fueron testeadas a partir del número total de ácaros eliminados por el Oxavar® y Apistan® en ambos grupos de colmenas (p< 0,05). Los resultados muestran una buena eficacia del producto, siendo recomendada su aplicación en colonias con una reducida cantidad de cría en desarrollo.

PALABRAS CLAVE. Varroa destructor. Apis mellifera. Oxavar®. Acido oxálico.

INTRODUCTION

At present, Varroa destructor (Anderson & Trueman, 2000), is the major problem to beekeeping development in all countries. Year by year, an increased number of honeybee colonies died as consequence of this parasite (Imdorf et al., 2003).

Several acaricide products have been utilized to reach an effective control. Pyrethroids as Apistan® (Fluvalinate) and Bayvarol® (Flumetrine) seemed to be the answer. However, these products are not utilized in different countries as a consequence of resistant mite populations (Lodesani et al., 1995; Milani, 1995; Lodesani, 1996).

In recent years, new substances to control Varroa mites avoiding mite resistance mechanism and reduced residues in wax and honey were analyzed. In this sense, organic acids as formic and lactic have been used. Formic acid presented the best results, reaching mites on adult and brood honeybees (Eguaras et al., 2001a).

Formic acid is a natural component of honey and present a strong acaricide effect (Ritter & Ruttner, 1980; Feldlaufer et al., 1997; Eguaras et al., 2001b). However, this product, as currently used, has two problems: a) high variability in efficacy against the mite and b) hazard to the beekeepers due to corrosive effect.

Another organic substance used in recent years was oxalic acid. This acid is prepared in water solution (30 g/l) and applied to reach adult honeybees (Imdorf et al., 1997). This agent was effective but mortality and toxicity on adult bees were reported (Higes et al., 1999). Charriere (1997) observed that this kind of treatment is negatively influenced by increased brood area, being not recomended in climates where honeybee brood is present in great proportion throughout the year.

The aim of this work was to evaluate the effect of honeybee brood on the acaricide efficacy of oxalic acid under its new formulation Oxavar® (Apilab, Argentina).

 MATERIAL AND METHODS

Work was done at Centro de Extensión Apícola experimental apiary located at Coronel Vidal (57° 44'W; 37° 27'S), province of Buenos Aires during October and November 2002.

Ten Langstroth honeybee hives, Apis mellifera (L. 1778), formed by one brood chamber naturally infested by Varroa destructor were selected. In each colony, a mobile bottom board was installed with wire screen to count dead mites and to avoid mite removal by honeybees. Colonies were divided into two groups: a) five colonies with three combs full of brood and b) five colonies with 6 combs full of brood. Both groups received Oxavar® in destilled water solution (64.6 gr/l), applying five ml of solution into a bee ways of bee occupied combs in three doses at seven days period.

Mite mortality was registered weekly on hive bottoms. In order to evaluate total mite population two Apistan® strips were applied to the colonies. Apistan® strips were removed from the colonies after 4 weeks and dropped dead mites were counted.

Treatment efficay was calculated for each colony as follows:

Treatment efficacy was evaluated using Student test between number of mites killed by Apistan® in both groups.

RESULTS

Results obtained are shown in Table I. Significant differences in total number of mites presented in both groups were not found (t_obs= 1.17; t_crit= 2.3; p= 0.27; d.f.= 8), showing all colonies similar Varroa infestation levels. Acaricide efficacy does not present high differences within groups, with average values of 85.6% ± 1.4 and 75.7% ± 1.7 for group 1 and 2 respectively. Higher efficacy was registered in colonies with 3 brood combs. Average number of dead mites collected for the first group was 541.8 ± 58.3 and 847.2 ± 67.8 for the second (t_obs= 2.5; t_crit= 2.3; p= 0.03; d.f.= 8; Table I). These results suggest that Oxavar® efficacy is variable and influenced negatively by increased honeybee brood area.

Dead bees pupae and adult were not observed on the bottom board or in front of colonies during the treatment period. This would indicate that Oxavar® does not present mortality or toxicity effect.

Table I: Acaricide efficacy of Oxavar® in Apis mellifera colonies with different amount of brood.

DISCUSSION

Results presented in this work show that Oxavar® is effective for Varroa destructor control, killing a large number of mites from the colonies. Average values agree with those previously reported by Barbero et al. (1997) and Marcangeli et al. (2003), who found an efficacy ranging between 75-80%. However, these results are lower than observed in other works by Mutinelli et al. (1997) and Higes et al. (1998). These differences could be explained by higher oxalic acid concentration and different application method used in these experiences.

According to Charriere (1997), the presence of large brood area would be one of major factors which reduce the acaricide efficacy. In this sense, a large number of mites invading brood cells are protected from this acaricide agent. These observations are clearly supported by the results presented in this work. Increasing brood area from 3 combs to 6 combs treatment efficacy presented a significative reduction of 10% (Table I).

Results presented would support the normal development and survival of colonies treated with Oxavar®. However, its application would be only recomended in colonies with reduced number of brood. In this way, oxalic acid would represent an effective agent to control Varroa destructor mites, particulary during early spring or late autumn treatments when queen oviposition is reduced and the great proportion of mites are present on adult bees. Oxavar® treatment with large number of brood will be not effective and colony survival is reduced as a consequence of large number of mites in brood cells protected from the acaricide. In this sense Gregorc & Planinc (2001) reported that oxalic acid in water solution (2.9%) is only effective during the broodless period (99.44%) and less effective when applied to colonies with capped brood (52.28%).

Higes et al. (1999) reported significant honeybee brood and queen mortality after oxalic acid treatment. However, no mortality effect was observed in this work. In this way, Oxavar® is a new formulation of oxalic acid as contact agent which avoid adult honeybee intoxication.

ACKNOWLEDGEMENTS

We wish to thank Universidad Nacional de Mar del Plata grant. Also we wish to thank Mr. Damián and Mr. Eugenio Ferrara.

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Recibido: 10-XI-2003
Aceptado: 20-VII-2004