The effect of pesticides and biopesticides on natural pest controls was one of the themes at this year's Soft Fruit Day, hosted by Kent's East Malling Research (EMR) with AHDB Horticulture on 25 November.
Highlighting the importance of such controls to UK production horticulture, EMR entomologist Dr David Buss pointed out that since 2012 more than two-thirds of strawberry crops have been treated with predatory mites - a 20-fold increase since 2001.
"Ideally growers need to be using crop protection that does not kill these mites," he said. But he added that the growing threat of pests such as spotted-wing drosophila (SWD) means that "we do require predatory mites that can survive new spray regimes ".
This prompted a project at EMR to select strains of predatory mites that are able to survive applications of typical controls for SWD. Buss established populations of the predatory mite Amblyseius andersoni, which is recommended for the control of spider mites, and Neoseiulus cucumeris, used against tarsonemid mites, then tested the effect on them of the insecticides Spinosad and lambda cyhalothrin.
The latter product, even at one-100th of the recommended field dose rate, resulted in a mortality rate of 76 per cent, while the Spinosad test, which used the recommended field dose rate of 0.15ml per litre, resulted in a smaller mortality rate of 28 per cent.
Buss's efforts to then breed mites more tolerant of Spinosad were hampered by low population numbers, a problem only partly rectified by using a commercial propagation company. The work has highlighted the difficulty of working with such microscopic yet vital creatures, he added.
A recent related project at EMR has examined the effect of spraying against SWD on predatory insects on raspberries, EMR programme leader Professor Jerry Cross explained. He posed the question: "When you spray a full programme of insecticide on raspberries, what happens to predatory mites and their ability to regulate two-spotted spider mite (TSSM)?"
In the trial, raspberry crops in nine polytunnels were repeatedly treated with an insecticide programme, with numbers of TSSM per plot assessed weekly from mid July to mid September, along with numbers of predatory Phytoseiidae mites. "Without any doubt the SWD spray programme was having a detrimental affect on controlling the spider mite populations, but it wasn't so bad that it was causing a catastrophe," Cross concluded.
Even identifying the cause of some fruit damage remains a challenge. Redberry disease in blackberries causes serious losses in commercial blackberry plantations worldwide but the pathogen involved has yet to be identified, according to James Hutton Institute (JHI) researcher Dr Stuart MacFarlane.
Though the problem is generally thought to be caused by the blackberry mite, recent research has shown that controlling the mite does not lead to a lower incidence of redberry, MacFarlane explained. He and his team investigated whether the damage is caused by a virus, and with the help of cutting-edge gene sequencing the researchers identified several viruses in blackberry samples, including a previously unknown luteovirus as well as black raspberry necrosis virus (BRNV) and raspberry leaf mottle virus (RLMV).
"Luteoviruses, BRNV and RLMV are all aphid-transmitted, so this may be evidence that the blackberry mite is not necessarily involved in redberry disease," said MacFarlane. "Perhaps aphids or another pest are acting as a vector."
Meanwhile, his JHI colleague Dr Julie Graham is investigating how the genetic marker Rub118b helps raspberry plants resist Phytophthora root rot. "The marker works but we have no idea why," she said. The variety Latham, for example, "always stands up to raspberry root rot" while Moy succumbs to it. In an ongoing project, Graham and her team are examining the gene expression differences between the two when they are infected with the disease. Graham is also applying genomics to gain an understanding of why there is such great yield variation in identical blueberry varieties grown in different parts of the UK.