Survey of pollinators and beneficial insects 2020-2021

Wild flower strip sown between rows of plum trees in the NIAB Plum Demonstration Centre

The Plum Demonstration Centre offers the industry more than basic comparisons of production systems. The orchard offers an area where the results of research projects managed by NIAB and others can be demonstrated in a commercially relevant setting.

In AHDB Project TF 223, NIAB led a study on hastening the influx of beneficial insects into newly planted apple orchards, through the provision of wildflower strips in alleyways, earwig refuges (Wignests), and hoverfly attractants.

In general, these interventions had a positive impact over the two years of the project, by increasing numbers of beneficial insects including hoverflies, predatory spiders and anthocorids, whilst reducing numbers of pest species including aphid numbers in the Spring, and codling moth later in the season.

Although these findings were largely positive, the comparison was only made over two seasons, so growers were encouraged to exercise caution in interpreting the results.

Following the establishment of the plum variety trial at the PDC in the winter of 2019/20, it was decided to implement a similar demonstration to that employed in AHDB Project TF 223, to assess the impact on newly planted plum trees. With additional funding made available from the EU funded BEESPOKE project, NIAB entomologists were also able to undertake a survey of pollinators visiting the PDC.

The objectives of the entomologists’ work were to implement wildflower strips in alleyways, earwig refuges (Wignests) in trees, evaluate the establishment of the floral mix, and find out if the interventions increased the numbers of beneficial insects in the orchard. They also aimed to determine the main pollinators found in plum and investigate if the natural pollination could be improved.

Wildflower mixes were sown in September 2019 in the alleys in two areas of the orchard, one under the tunnel covered area and one in an open area. The mix included yarrow, common knapweed, lady’s bedstraw, oxeye daisy, musk mallow, sainfoin, salad burnet, selfheal, meadow buttercup and red campion.

Establishment of both mixes was disappointing in 2020, with 19% coverage of wildflowers in the open area and 25% in the tunnel area. However, as previously seen in apple orchards, wildflower coverage can continue to increase for up to three years if adequate management is undertaken. In fact, in 2021, the wildflower open area increased its coverage to 89% and the tunnel area to 90.2%.

In 2020, although aphid numbers were higher in trees without Wignests, there was no statistical difference. Ant and hoverfly numbers were significantly higher without Wignests, possibly as a result of higher aphid numbers. Predatory spider numbers were higher in trees containing Wignests, although the difference was not statistically significant. Of the beneficial insects recorded inside the Wignests, earwigs were the most numerous; over five earwigs per Wignest, although other beneficials such as predatory spiders, spider sacs and ladybirds were also found sheltering inside.

In 2021, there were no significant differences between the numbers of arthropods recorded between trees with and without Wignests. Similarly, there were no significant differences between earwig numbers counted at night time between control plots (without Wignests) and Wignest plots. However, interestingly the numbers of aphids per shoot found in tree plots with Wignests was significantly lower than the control plots.

In work to determine the principal pollinators working in the orchard, the entomologists found that honeybees, bumblebees, solitary bees and hoverflies were most commonly found. Surveys were done at blossom and while honeybees were the most abundant in 2020, solitary bees were the main pollinator in 2021.

This shift was probably due to the timing of the blossom. In 2020 plum blossom developed in March while in 2021, due to a colder start of the season, blossom developed in April. This demonstrates the importance of having a diverse community of pollinators to cover the interannual differences in weather. Scientists also looked at pollination deficit in Victoria and compared hand pollination versus open pollination through insect and wind. No yield or quality differences were found.