Why Using Biological Control Approaches Against Sugar Beet Aphid Pests Will Not Control Virus Yellows – a Post-script

As an advocate of natural pest control wherever possible, the author has no quarrel with Professor Loxdale?s appeal for research funders to consider enhancing the resources already available in the natural environment. In an ideal world, this would be the most acceptable method for producing healthy crops for our benefit, not to mention encouraging biodiversity within the landscape. However, in the real world, a more pragmatic approach is sometimes required to avoid catastrophic yield losses, such as those which occurred in sugar beet in the mid 1970s, and most recently in 2020, when it was estimated that 25% of sugar beet yields were lost to the aphid-transmitted yellowing viruses that infected 38% of the national sugar beet crop. Controlling up to 50 % of aphids by parasitoids or other natural enemies, whilst commendable in a situation where aphids might cause significant yield loss by direct-feeding, is certainly not sufficient to prevent widespread virus infection within crops, simply because it does not take many aphids to spread viruses. The threshold for aphid control using insecticide sprays in sugar beet in the UK is, and has been for decades, one green wingless aphid per four plants. This level of infection is actually quite difficult for a human to detect to make the judgement to use a spray or not, and it is a population level that does not attract many parasitoids or predators, even though they are better at finding them than we humans. But that threshold is necessary to help reduce the spread of the yield-reducing viruses and protect the yield of sugar beet.