BBRO Weekly Advisory Bulletin 28 June 2013

Silver Y moth
For the first two weeks in June the winds blew for several days from a south, south-westerly direction, creating the ideal migration conditions for Lepidoptera.  Large numbers of Silver Y moth (Autographa gamma) migrated to our shores on these winds, and have not taken long to move inland to lay their eggs.  Moths can be found in crops from Essex through to North Lincolnshire although the BBRO moth traps indicate that there are slightly larger populations in the south.

Small round eggs are laid singly or in small groups on the underside of the beet plants leaves. Caterpillars emerge 1-2 weeks later and feed for up to four weeks. As the caterpillars mature they become increasingly voracious and eat through the leaves whilst leaving the veins, leaving the canopy with a skeletal appearance. If a field becomes severely attacked considerable loss of yield can occur.

Monitor crops for signs of silver Y moth damage over the coming weeks.  The current spray threshold for treatment is 5 caterpillars per beet plant. Be aware that treatments are likely to adversely affect the predators that we rely on to control black aphids and red spider mites.

We have had a number of queries as to whether irrigation is worthwhile at the moment. In areas that have had near normal rainfall in May and early June, it is unlikely that irrigation now will provide a cost-effective return. If it remains dry for the next week or two however, irrigation could provide a useful boost to yield. 

Often we turn our thoughts to irrigating beet when they are showing signs of wilting, by which stage yield potential has been hit and irrigators are already committed in other crops.  The best approach is to apply water before wilting occurs just to help keep the crop growing through a dry spell and 25mm of irrigation can return around 2.5t/ha in yield where soil moisture deficits exist.

Aphid numbers and seed treatments
Aphid numbers remain low this year apart from one site in Cambridgeshire which demonstrates how aphid populations can build up quickly under favourable conditions.  Ibt is important that we do not become complacent, as aphids remain a potentially serious threat to future sugar beet crops through their transmission of virus yellows.  Around 4-5% of the UK crop is still grown without an insecticide seed treatment, and this is mostly where a granular nematicide is being used to overcome docking disorder caused by free living nematodes.

Common practice has been to monitor these crops for aphids and to apply a foliar insecticide as required. However, the prevalence of aphids carrying MACE and kdr resistance in the UK has left us with no effective foliar aphicides and as a result, crops grown without an insecticide seed treatment are now at significant risk from virus yellows. We therefore recommend that ALL seed for 2014 is ordered with an insecticide seed treatment.

Last edited on: 28:06:2013

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