Much of the UK has experienced an extended cold spell in recent weeks, but how will this have affected aphid populations?
Invertebrate pests are cold blooded and so as temperatures drop, they become less active.
Studies suggest that the bird cherry-oat aphid, an important vector of barley yellow dwarf virus (BYDV) in cereals, is frost susceptible with the lethal temperature to kill 50 per cent of aphids (LT50) being -0.5degC, says Dr Steve Ellis, senior research consultant (entomology) at ADAS.
“However, the grain aphid is more cold-hardy with an LT50 of -8degC. Overnight temperatures this winter have certainly been below -0.5degC on a number of occasions but may not have been lower than -8degC.”
As well as being killed by a one-off cold shock, aphids can also accumulate a fatal cold dose where there has been an extended spell of sub-zero temperatures, even if the LT50 is not exceeded, Dr Ellis says.
“It is possible that this may have happened during the recent cold spell. This is in contrast to 2019 when it was extremely wet, and conditions probably limited the spread of BYDV.
“Needless to say, we cannot be certain that the recent cold weather will have killed all aphids in crops, but it will certainly have had some impact on their survival.
"The only way to check if any hardy individuals remain is to get on your hands and knees and examine crops but this is not a particularly popular exercise in the current conditions. However, when the weather warms up it would be worth checking to see if any aphids remain.”
How each insect species spends its winter is also an important factor, Dr Ellis says.
“If it overwinters as an aphid it will be much more susceptible to the impact of cold than if it overwinters as an egg. Therefore, black bean aphids which usually overwinter as eggs on spindle will probably have been little affected by the recent cold. Insects can certainly prepare for winter by becoming acclimatised.”
For sugar beet, Prof Mark Stevens, head of science at the British Beet Research Organisation says: “We are closely watching the weather and anything that will help to reduce aphid populations and delay aphid migration to later in the season when plants are bigger would be an advantage.
“The lethal temperature to kill 50 per cent of [peach potato] aphids is -7degC but anything that drives the temperature down and below freezing starts to have an impact. We have had cold weather in the last two weeks [Jan 6] and it looks like it will remain cold in the next two weeks.”