Predicting the economic benefits and costs of introducing new biological control agents for Scotch broom Cytisus scoparius into New Zealand: how much will biological control of broom harm the New Zealand beekeeping industry?

A recent estimate of the potential benefits and costs of introducing additional biological control agents against the European shrub Scotch broom Cytisus scoparius (L.) Link (Fabaceae), henceforth broom, into New Zealand predicted an annual net benefit of $10.13 million, should broom be completely controlled. However, costs may be incurred by beekeepers and from non-target attack to tagasaste (tree lucerne, Cytisus proliferus L.f.), an exotic relative of broom used as a minor fodder crop and, occasionally, for erosion control on marginal hill country. Due to uncertainties regarding these costs, a risk-averse approach was adopted by assuming non-target damage to tagasaste will be similar to damage to broom and that no pollen sources suitable for beekeeping will replace broom stands. Work to refine these estimates by determining the potential impact of reduced broom pollen availability on honeybees was conducted in New Zealand and preliminary results are reported here. Pollen availability varied considerably between sites. At two of the three sites, only a minority of broom flowers were pollinated, indicating that even large reductions in pollen availability (>60% reduction in flowers) should not adversely affect honeybees. However, at one site 80% of broom flowers were pollinated, indicating even a minor reduction (25%) in broom pollen might affect honeybees. Nevertheless, at all sites, other sources of pollen growing in close proximity to broom were utilised by honeybees. If these alternative pollen sources increase in abundance, should biological control of broom succeed, this should mitigate the impacts of a reduction in broom pollen availability for honeybees. This study, therefore, indicates the potential impact of successful biological control of broom is likely to be less than previously supposed. Finally, the only species capable of pollinating broom flowers were introduced bees (mainly honeybees), indicating that introduced pollinators are at least partially responsible for the invasiveness of broom in New Zealand. Predicting the economic benefits and costs of introducing new biological control agents for Scotch broom Cytisus scoparius into New Zealand: how much will biological control of broom harm the New Zealand beekeeping industry?

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