Does orchard netting affect insect visitation to apple flowers in Stanthorpe, Queensland?Research & Extension
Jeremy Jones, Stephen Tancred, Charlie Bailey, Karen Bezerra da Silva Santos, Erandi Subasinghe and Romina Rader
This article originally appeared in the Winter 2021 edition of AFG.
Watch Jeremy Jones present the findings of this study here.
In apple growing regions throughout Australia, permanent nets are commonly employed by growers to reduce the risk of hail and bird damage to fruit. While these nets can provide good protection against these risks, we know little about how protective crop covers affect insects that visit flowers.
In apples, like many other fruit crops, optimal fruit quality is closely linked to the transfer of pollen from compatible cultivars by insects during bloom. As the number of permanently netted orchards across Australia is growing, the need to understand how these structures might impact pollinating insects is becoming more and more important.
In particular, little is known about how insect pollinators such as honeybees respond to net height. The variation in tree height under the net may impact the ability of pollinators to navigate and move across orchards to visit flowers. To better understand how orchard netting structures potentially impacts insect visitation to apple flowers, we undertook research in Stanthorpe apple orchards, QLD. We asked two main questions:
(i) Does the amount of space between netting and apple trees impact the activity of insect pollinators in apple orchards?
(ii) Does opening the inter-row seam of pitched nets during the flowering period increase the activity of insect pollinators on apple flowers under the opened rows?
Pink Lady is a widely-grown apple in the Stanthorpe region, therefore we targeted blocks with these cultivars. Overhead net height and heights of trees varied between apple blocks in Stanthorpe. The relative difference between tree height and net height means blocks with low trees and high ceilings are very spacious, with clear gaps above trees, while in orchards with trees that grow to the height of the nets, the blocks are dense with foliage with little room for insects to fly.
The nets used for crop protection in the Stanthorpe region are commonly called hail nets, and in Stanthorpe, each has a particular colour, weave style or mesh size. The two main types of overhead nets employed in the Stanthorpe region were flat nets and pitched nets. Flat nets are continuous sheets of netting which are capable of stretching under heavy hail loads, returning to their original shape as the hail melts. Under extreme hail loads, these flat nets may collapse and damage crops and infrastructure. Pitched nets are designed to overcome the support limitations of flat nets. Pitched net systems consist of segmented nets positioned at angles to allow heavy hail loads to open up the net, dumping accumulated hail onto the inter-row gaps between trees. The splits created by hail loads are easily re-joined with plastic clips. This complex net design reduces the risk of structural failure under high hail loads, and also provides a simple mechanism allowing growers to purposefully open these nets.
We visited orchards during the apple flowering seasons of 2017, 2018 and 2019. To measure the impact netting has on insect visitation rates to apple flowers, we conducted surveys in orchards with and without overhead netting, and then analysed the results to assess if there were differences in visitation rates between the two types of orchards.
During each insect survey, we performed one-minute observations of 20–30 apple trees in each study block, recording the number of visits made by insects, including bees, flies and beetles, to apple flowers. We repeated these surveys two to three times per day for each block over a two-week period each year.
In Stanthorpe in 2019, some apple growers opened the pitched net seams during apple flowering, in an effort to improve flower visitation by providing an opening for honeybees to move in and out of netted orchards. For each survey we conducted in 2019, we recorded if the overhead net had been opened above the row of surveyed apple trees under pitched nets. We also recorded the height of any nets and the average height of the trees located under the net in order to examine if insect visitation to apple flowers was affected by the amount of overhead space present in netted orchards.
All of the overhead nets we were able to identify in the Stanthorpe region had mesh sizes of less than 6 mm. These tight weaves are likely to act as barriers to movement of honeybees – the main pollinators of apple in the Stanthorpe region. Provision of gaps in nets may facilitate the movement of honeybees and other insects to and from the surrounding environment. Honeybees are likely to benefit from access to alternative forage sources outside the nets, and while this may distract some bees away from orchards, healthier and stronger hives may benefit both growers and beekeepers. Beekeepers may benefit from reduced hive losses while growers may benefit by having reduced loss of worker bees and higher forager numbers, which translates to more bees being available to visit crop flowers.
Over the three years of research, we recorded a total of 4672 insect visits to apple flowers. Honeybees were the most frequent visitors to apple flowers, representing 94 per cent of all visits, with the remainder from a diverse range of native bees, flies, wasps, beetles and ants.
The results of our surveys between 2017–2019 indicate that the presence of orchard netting reduces the visitation rate of insects to apple flowers in comparison to orchards without netting.
We also identified a positive relationship between the overhead clearance gap – the gap between the tops of the apple trees and the maximum height of the nets – and insect visitation to apple flowers. Netted orchards with more space between the tops of the apple trees and the overhead netting received more visits by insects, and this effect increased with more clearance space. Our investigation of the effect of opening overhead rows on insect visitation to apple flowers located under these openings tentatively suggests that this management strategy may increase visitation to apple flowers, however these results need to be repeated in more orchards before we can conclusively say this practice is effective for other orchards and is consistent between years.
This study also suggests that limiting tree height to have a clear gap under netted orchards may increase pollinating insect activity. We have yet to understand the mechanism behind this finding but having a clear space above trees may enable bees and other insects to more easily navigate their way across orchard blocks. Identifying effective orchard management practices that increase insect visitation rates to crop flowers is an important goal towards securing pollination services to agriculture. These activities will not only benefit growers, but also beekeepers and the wild insects that have an essential role in pollinating other crops and native plants.