Uncovering the localised impacts of bushfires on apple pollination

Published on May 24, 2023

Research & Extension

Research shows that orchard design should be revisited following major environmental disruptors to ensure flora used to attract and support wild pollinators has not been lost. 

Key points:  

• Researchers examined how bushfires affect pollination in the post-fire recovery phase. 
• Different mixes of floral resources and visiting insects were observed before and after bushfires. 
• As well as major events like bushfires, seasonal differences in temperature can also influence insect visitation. 
• The impact of bushfires should be considered in orchard design and pollination strategies. 

Australia’s landscapes have a remarkable ability to regenerate after major bushfire events, with imagery of green shoots emerging from scorched country a poignant reminder of how ecosystems endure bushfires. 

Despite the appearance of recovery, we don’t really understand the baseline ecology of native pollinators (bees and hoverflies), let alone the impact of bushfires on the prevalence of these unmanaged native insects or feral honey bees, and especially what impact disruption to these populations may have on pollination service to apple orchards.   

Some Australian native bees that visit apple flowers arose tens of millions of years ago (Tierney et al. 2008) and are fire adapted (renewal of nesting substrates depend on fire events), so there are unlikely to be long-term effects on species’ survival over geological timescales, but there may be highly localised effects influencing horticulture in the short term. 

This was the impetus for a Hort Frontiers project closely examining the long-term impact of bushfires in the Bilpin region of New South Wales on pollinators and the flora which helps attract them. 

Funnily enough, the project exists almost by chance. Professor James Cook and his team at Western Sydney University had spent 2017–19 examining wild pollinator populations in Bilpin when bushfires ravaged the Blue Mountains area in December 2019. 

Out of the devastation, a scientific opportunity arose. The team now had a scientifically robust control dataset which would enable it to track how the Bilpin region’s flora and pollinator populations were affected by this event. 

A new ecosystem

According to James, one of the most significant impacts to the ecosystem following the fires was a substantial change to the floral resources found in plots established by Dr Amy Gilpin in bush plots bordering six participating orchards in the region. 

“Of the 71 species of plant observed flowering either before or after the event, only 17 of those occurred in both periods,” James said. “Eighteen species of plant observed in the area before fires have not been recorded following the bushfire, while 35 species were only detected after the fire.”  

James said that although the emergence of a somewhat different landscape after bushfires is to be expected, there were still important lessons for growers. 

“The vegetation-altering properties of bushfires are well known, although few studies have focused on floral resources as we did. While our findings are not completely surprising, they do remind us that orchard design should be revisited after extreme events like fires to ensure that flora used to attract and support wild pollinators has not been lost,” he said. 

Meanwhile, preliminary analysis of crop pollinator data collected by Dr Simon Tierney from the first post-fire crop flowering season in 2020 suggests that numbers of both honey bees and stingless bees (the two main pollinator species) were slightly reduced following the bushfires, but not outside the upper and lower bounds of the 2017–19 baseline dataset collected by Simon and his team. 

In contrast, hoverflies increased in abundance by an order of magnitude; however, pollinator efficiency metrics developed by Simon (based on amount of pollen carried/deposited and likelihood of contacting reproductive flower structures) suggests hoverflies are unlikely to compensate for any decreases in bee populations.  

While the project aimed to focus specifically on the impact of bushfires on the pollinator ecosystem, some curiosities were observed recently as researchers continued to examine insect visitors to apple flowers. 

“The control set of data (pre-fire) told us that insect visitors were typically dominated by two species: European honey bees and Australian native bees (mostly stingless bees). The dominance of these two species meant that in most years, [more than] 90 per cent of insect visitors were from these two species,” James said. 

“However, in 2022 a curious phenomenon was observed – although honey bees continued to represent a major proportion of apple flower visitors, stingless bee visitations were comparatively extremely low. In fact, only a handful of stingless bees were observed throughout the entire survey period.”  

But the researchers do not currently think that this reflects the effects of the fires. Instead, the drastic decline in stingless bee numbers is likely a consequence of lower daily maximum temperatures during the apple flowering period. The baseline dataset shows that stingless bee numbers are very low on survey days when the temperature is below 18°C and the 2022 flowering season was unusually cool compared to spring 2017–21. 

This is useful information for growers as they seek to determine their pollination strategies. When a colder season is forecast, it may be unfeasible to rely on wild bee pollination and more important that apple growers enlist the support of honey bee apiarist service providers. 

While the main effects of climate change are likely to be detrimental to growers, and the honey bee population is now threatened by the recent establishment of Varroa mite in NSW, increases in mean temperatures may lead to a relative increase in contributions from native bees to apple pollination in this and other regions. 

Next steps

The research is ongoing and the team expects that future stingless bee visits to apple flowers will continue to be reflective of springtime daily maximum temperatures, regardless of other major environmental disruptors such as bushfires, floods and pestilence. 

Some aspects of the project have been hampered by years of pandemic-related lockdowns. For example, the team was unable to collect data during spring 2021 and, most recently, the Varroa mite incursion in NSW meant they were unable to transport experimental bee hives to study sites in spring 2022. However, owing to the important role this research will play in the future of orchard design and pollination strategies, the team hopes to continue parts of the program next season. 

More information on this project and how orchard design should be considered in bushfire-prone areas will be released throughout this time.  

In the meantime, a series of planting guides using this project’s research has been made available for growers via the Wheen Bee Foundation. You can access these guides here: www.wheenbeefoundation.org.au/our-work/projects/powerful-pollinators/ 

References

Tierney SM, Smith JA, Chenoweth L and Schwarz MP (2008) ‘Phylogenetics of allodapine bees: a review of social evolution, parasitism and biogeography’, Apidologie, 39:3–15. 

Acknowledgements

The Loss of horticultural pollination services from wild insects following bushfires (PH20002) project is funded through the Hort Frontiers Pollination Fund, part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation, with co-investment from Western Sydney University and contributions from the Australian Government. 

 

This article was first published in the Autumn 2023 edition of AFG.

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