Aphids and virus transmission Q&A
What are the most important aphids? Does it vary between England and Scotland?
Over 500 aphid species have been reported in GB. Various researchers have trapped and tested winged aphids in GB and the Netherlands for their ability to transmit one or more strains of PVY. When these species lists are combined, there are around 30 species that have been found to transmit PVY. The aphids vary in the efficiency with which they transmit viruses. This is defined as the relative transmission efficiency, or relative efficiency factor (REF). It is a measure of how often an aphid species will transmit the virus relative to the 'best' vector, the Peach-potato aphid, M. persicae. It is given a value of 1.0 and the values for the other species are proportional to this. For example, the Black bean aphid has a relative transmission efficiency of 0.1, meaning that in the same conditions, this species transmits the virus only 10% of the time compared to M. persicae.
AHDB funds an Aphid Monitoring Scheme and the results are reported as a vector pressure index. It is designed to give the user an assessment of the risk of PVY spread. It is calculated using the catches of aphid species and their respective REF values.
Researchers at SASA have studied the relationship between the numbers of different aphids and PVY transmission. They concluded that the cereal aphids (Rose-grain aphid, Metopolophium dirhodum, Grain aphid Sitobion avenae and Bird cherry-oat aphid, Rhopalosiphum padi) were the key vectors. In other countries, other aphid species are considered to be more important, for example, Black bean aphid, Aphis fabae, in Finland and Leaf curling-plum aphid Brachycaudus helichrysi in Switzerland.
How important are Willow-carrot aphids in GB?
In laboratory testing the Willow-carrot aphid, Cavariella aegopodii, was a moderately efficient transmitter of PVY and PVA, however in the field situation the relative importance of this aphid by comparison to Peach-potato aphid or cereal aphids remains to be demonstrated.
Does the timing of aphid flights matter (early vs late season)?
Recent mathematical modelling work from the US suggests that an early-season peak in the numbers of non-colonizing aphids resulted in the highest number of PVY-infected plants in the end of the season, while mid- and late-season peaks caused relatively little virus spread. We don’t know if this work is directly transferable to GB. Epidemiological modelling using Scottish aphid and seed certification data indicates that the inclusion of aphid data up to the end of July is critical in Scotland.
What do suction traps vs in field traps tell us?
Suction trap information is summarised in AHDB’s Aphid News. The yellow water trap information is available by an open access website. Suction traps give a stable, long- term, area wide sample of the population of aphids in an area. Field traps, like the yellow water traps, tell you what is moving on a local scale.
Where is the virus coming from?
Aphids are not believed to carry PVY over long distances and therefore the most likely sources of virus are the input seed and/or neighbouring potato crops and groundkeepers that are already infected with virus.
How important is winter temperature in determining aphid numbers? How reliable is the forecasting?
Using the suction trap information researchers at Rothamsted Research have shown strong relationships between winter temperature and the time that M. persicae are first caught in traps and their abundance. Relationships for other species are less strong. Compared to 50 years ago, many aphids are flying a month or more earlier.
Can aphids be tested to see if they are carrying PVY?
Not reliably due to the non-persistent nature of virus transmission and the short period of contact with the aphid’s mouthparts. Virus particles are believed to dissociate from the mouthparts once aphids are held in collecting fluids, e.g., as used suction and yellow water traps.