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Rural Industries Research & Development Corporation
Control of chalkbrood disease with natural products
by Dr Craig Davis and Wendy WardDecember 2003
RIRDC Publication No 03/107 RIRDC Project No DAQ-269A
Chalkbrood is a highly contagious disease of the honeybee Apis mellifera caused by the heterothallic fungus Ascosphaera apis. It was first identified in Queensland in 1993, and, since that time, the disease has spread throughout Australia. Although not usually fatal, the disease causes reduced honey production. The effects of chalkbrood can be controlled by improved management techniques such as strengthening colonies with bees or hatching brood and enlarging colony entrances to aid ventilation.
While chalkbrood infections have been related to stress factors, sensible management practices can reduce the numbers of spores of Ascosphaera apis in infected hives and hive equipment. Some hives appear to be more resistant to chalkbrood disease than others due to the ability of their adult bees to uncap and remove affected brood. The disease appears to be most prevalent in the spring when the brood area is increasing. The presence of chalkbrood in a colony can prevent normal colony growth and can seriously affect the honey production of the hive. While a broad range of chemicals have been used either in hives or in the laboratory to control chalkbrood, no chemicals for the treatment of chalkbrood have been registered for use in Australia and no specific strategy has been universally adopted or accepted by beekeepers around the world. Chalkbrood-resistant bees have been shown to exist naturally in Australia, but the large-scale production of such bees either overseas or in Australia has been slow. The thermal destruction (time/temperature) parameters have been determined for Ascosphaera apis in honey and the sensitivity of Ascosphaera apis to ã-irradiation using Cobalt 60 has been estimated. Such treatments of honey or other bee products can reduce the spread of the disease.
There has been an increased interest in the investigation of alternative controls strategies. A compound for control of chalkbrood should have the following three characteristics. First, it must completely control the disease, or more realistically, keep it below the natural infection rate. Second, the control must be convenient to use, since practices such as applying chemicals and cleaning the bottom boards of colonies every week are not practical for commercial beekeepers with large numbers of colonies.
Third, the control must not be more expensive than the natural loss due to the disease.
This project has investigated the antifungal efficacy of over 50 natural products and found that a number of essential oils were particularly efficacious at controlling the in vitro growth of Ascosphaera apis. Of these, citral-containing oils were the most active, with growth inhibition at 250 ppm. These findings need to be progressed to field studies to evaluate product efficacy in the hive and to determine whether residues are a problem with this form of disease control. The most active antifungal test agents in this study were Nepalese Lemon Grass oil, Lemon Scented Eucalyptus (Eucalyptus citrodora) oil, Lemon Scented Tea Tree (Leptospermum petersonii) oil and a particular fraction of a New Zealand Manuka (Leptospermum scoparium) oil. All of these agents presented with a Minimal Fungicidal Concentration against Ascosphaera apis of 250 ppm. Citral is the major component of the former three oils while the active chemical in the New Zealand Manuka (Leptospermum) oil is a unique terpenoid agent (leptospermone). Interestingly, a number of other oils which should have contained significant levels of citral (East Indian Lemon Grass oil, cold-pressed Lemon oil, Natural Citral and lemon essential oil) returned negative MFC scores at 1000 ppm or greater. Several other oils exhibited moderate antifungal activity (active at 500 ppm) against Ascosphaera apis (Citricidal, several Tea Tree oils, a number of other New Zealand Leptospermum oils, a ginger oil and a lavender oil). The remaining essential oils were shown to be ineffectual against Ascosphaera apis in this in vitro test system.
This project has also proposed a field test system to assess the efficacy of the most active antifungal agents identified in the in vitro assay system presented in this report. There are few reports in the literature of field trials of natural products against bee diseases, and even fewer investigating the efficacy of natural products against the causative agent of chalkbrood in an apiary system.
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