Growers from the British Hop Association are investigating the application of predatory mite Amblyseius andersoni in hops as a non-chemical control method to reduce damage from a key hop pest.
Many hop growers are losing the battle against a key pest of hops – two-spotted spider mite, which damages the cones of the plants, reducing crop yield and quality.
A very limited range of chemical control products are available and control can be difficult with warmer summers encouraging higher levels of the pest. However, recent research has shown that a naturally occurring predatory mite, Amblyseius andersoni, is capable of keeping spider mite under control in other outdoor crops, e.g. cherry (UK) and apple (NL).
In this field lab, seven growers are working with researchers from Hutchinson Ltd and NIAB EMR to release predatory mites at two points in the year - first in autumn, after harvest to ‘mop up’ remaining spider mites before they hibernate overwinter, which they hope will reduce subsequent numbers of spider mite the following spring. They will then apply the predatory mites in the spring at the growing stages and assess two-spotted spider mite populations, compared with control plots.
Final report published
Field lab launch meeting
Virtual meeting to co-design trials
Field lab group formed
The field lab triallists are 6 of the most engaged hop growers who are based in Kent, Herefordshire. The field lab idea emerged through numerous discussions between the Hutchinson’s and the hops growers they work closely with, followed by a meeting in May to design a field lab. The growers in this trial have all tried to use predatory mites at different times in their hop crops with varying degrees of success. They wish to take a more systematic approach to assess the impact of post-harvest introductions as this approach can require the least changes to current crop protection practice in the crop.
The group includes researchers from NIAB EMR and representatives of Bioline Agrosciences, who supply the predatory mites.
Background to the trials
There is a growing preference and need for integrated pest management (IPM) strategies for two-spotted spider mite in hops, and chemical controls are becoming less available.
Previous experience with the use of the predatory mite Phytoseiulus persimilis in hops has been very variable and the crop architecture of traditional tall hops is not well suited to effective use of this predator. P. persimilis can penetrate the webbing created by spider mite but only feed on this spider mite and are often not in high enough numbers to keep spider mite under control. P. persimilis is often used as remedial treatment when spider mite begins to become out of control. Additionally, sulfur sprays applied to hops to control powdery mildew can be damaging to predatory mites, exacerbating the proliferation of spider mite in the crop.
An alternative approach would be to make augmented releases of predatory mites in the autumn to ‘mop up’ remaining spider mites before they find refugia to overwinter, reducing subsequent numbers of spider mite the following spring. Ideally the predatory mite would also overwinter so that it can re-emerge and feed on remaining spider mites in the spring.
Recent research has shown that a naturally occurring predatory mite, Amblyseius andersoni, is capable of keeping spider mite under control in other outdoor crops, e.g. cherry and apple. It can feed on other food sources (e.g. pollen from flowering weeds) when spider mite is less abundant. This means that it can be released and retained in crop over much longer periods and is able to feed on spider mites before they build up to damaging levels in the crops. This native species can even overwinter in UK conditions. It is active at a broader temperature range (6-40ºC) than P. persimilis, so can be introduced much later and earlier in the growing season than other predatory mites. However, to date, grower efforts on trying A. Andersoni have been uncoordinated and hence existing data is variable.
This field lab will bring together a group of UK hop growers to systematically test the release of A. andersoni in the autumn so that they are retain in the crop overwinter to ‘clean up’ spider mites before they overwinter and then again in the spring as they emerge, thereby reducing the initial population in the crop the following year and resulting in less reliance on chemical sprays.
The grower group will be advised by scientists from NIAB EMR (formerly East Malling Research, with a track record in hops research), and Bioline Agrosciences, a leading producer of A. andersoni in a dispensable formation.
The principal outcome from this project would be sufficient knowledge and experience with this method of control to give the majority of UK hop growers the confidence to use this approach in their crops. The BHA (representing all the hop growers) are keen to encourage more sustainable methods of pest control and are very supportive of this approach. They hope that the ‘early adopters’ trialling this approach will provide others confidence to try out the use of predatory mites or alternative, sustainable control methods.
Trial design
At the farms involved, sites with spider mite problems in 2021 will be identified. One area of the crop (minimum 0.25 ha) will be treated initially with loose (bulk bags) of Anderline (10 per sq m (100,000 per ha)) post-harvest with application made to the crown of the plant and any remaining foliage after harvest. Anderline contains the adult predatory Amblyseius andersoni and will ‘clear up’ the remaining two-spotted spider mite post-harvest, before overwintering. An area of crop (minimum 0.25 ha) adjacent to the A. andersoni treated area will be left untreated for comparison.
Once temperatures are >6ºC in the spring slow-release Gemini sachets containing A. andersoni will be applied to hop plants and poles. Gemini sachets contain all stages of the A. andersoni life cycle so that the predator is active throughout the early part of the growing season.
It won’t be possible differentiate the extent to which post-harvest (autumn) vs spring applications of A. andersoni contributed to the success of suppressing two-spotted spider mite because they will still be overwintering prior to spring application of the Gemini sachets.
The Hutchinson agronomist and/or a NIAB technician will assist the hop growers onsite on the release of the predatory mites in the correct manner and how to assess the impact of the predatory mite releases. Assessment will be counting numbers of mites (both pest and predators). Additional leaf samples will be sent to NIAB EMR to more accurately count numbers of spider and predatory mites and confirm the identification of A. andersoni in the spring in the crop from the treated and untreated areas.
Final results
In this field lab, growers and researchers investigated whether autumn and spring releases of predatory A.andersoni can suppress two-spotted spider mite populations the following spring. This involved two experiments: A fully replicated small plot trial at one farm assessed by NIAB East Malling staff. 6 grower trials assessed by Hutchinsons agronomists and analysed by NIAB.
KEY FINDINGS:
• Autumn and spring applications of A. andersoni did not increase predatory mite numbers in the hop plants and therefore did not reduce spider mites on hop leaves in the spring
• Predatory mites were not significantly higher in the treated compared to the control plots. It is possible that there were already sufficient numbers of predatory mites or that the released mites were able to disperse into control plots. Alternatively/in addition to this, application of sulphur sprays may have adversely affected predatory mites, evening out numbers across all control plots. However, this would require further investigation.
• If the presence of similar numbers of predatory mites in the control plots and treated plots is due to dispersal, the cause of dispersal would be wind rather than insect movement.
• The grower-led trials had similar findings as the replicated small plot trials. However, there was some suggestion on at least four of the six farms that predatory numbers were more abundant on the hop leaves where they had been released, although this did not result in a significant reduction in spider mite numbers
RECOMMENDATIONS:
• Future, long term trials should be dedicated to the releases of biological controls with minimum pesticide use in order to build up predator numbers in the cropping area
• It may be more cost effective to spot treat areas known to be hotspots of spider mite infestation and hop-poles where spider mites are known to overwinter
• Work is needed to test habitats (e.g. different cover crops) which may offer alternative prey for A.andersoni during winter
Group Coordinator
Jonathan Blackman
Hutchinsons
Jonathan is a graduate of Wye College, University of London and studied for his DPhil at the University of Sussex. After nearly 10 years working for ADAS as a Soil Scientist and as a research scientist at Rosemaund in Herefordshire, he has since 1999, worked in the West Midlands as a Fruit and Hops Agronomist, currently with Hutchinsons where he is also their Horticultural Technical Manager. He is Secretary of the Herefordshire Hop Discussion Group, Steward of the Commercial Apple and Pear classes at the Malvern Autumn Show and is a member of the Marden Fruit Show Society General Committee. In 2017 he was appointed as a member of the Expert Committee on Pesticide Residues in Food (PRiF).