What is the yield loss from direct feeding damage?
Aphid feeding can cause direct damage, in the absence of the plant virus; barley yellow dwarf virus (BYDV), by reducing yields by up to 10% and by reducing seed size.
Damaging populations may develop in potentially high-yielding crops (2.5 tonnes per hectare (t/ha) or more).
Direct feeding damage occurs when colonies of aphids develop on stems, leaves and heads, from the seedling stage through to head filling.
The degree of damage depends particularly on the percentage of tillers infested, the number of aphids per tiller and the duration of the infestation.
What are the crop symptoms?
There may be no obvious symptoms while aphids are feeding and causing direct damage. Heavily infested plants may turn yellow and may be covered in a sugary honeydew produced by the aphids and on which black sooty moulds may develop.
Much larger yield and quality losses can be sustained when crops are also infected with yellow dwarf virus. Refer to 'See also' section for further information on this virus.
Parasitic wasps, ladybirds, lacewing and hoverfly larvae can provide useful biological control of aphid feeding damage at low aphid densities.
When aphids are in moderate to high densities, these predators and parasites are usually unable to control an increasing population, although given the right conditions certain fungi may kill a large proportion of the population over a short period of time.
If predators are present use 'soft' insecticides such as pirimicarb that selectively kill aphids and leave predators intact.
When and how to monitor for aphids?
For decisions on whether or not to spray insecticides to prevent aphid feeding damage, crops should be checked from late tillering onwards for oat aphids on stems, undersides of leaves and ears and for corn aphids in the furled growing tips.
Walk through the crop in a 'W' pattern, noting the numbers of aphids per tiller at every few paces.
What is the threshold?
Spraying with an approved insecticide is worthwhile if 50% of cereal tillers have 15 or more aphids.
Crops sprayed before Zadok's growth stage 30 (start of stem elongation) should be checked again 3-4 weeks after spraying as aphids may re-establish and build up again to threshold levels.