Potato aphids

Monitoring good hygiene and a control programme that takes aphicide resistance into account are essential for a healthy seed potato crop. Central Science Laboratory's Phil Northing assesses the risks growers face, while Scottish Agricultural College's Andy Evans summarises best practice control.

What threat do aphids pose to British potato crops?

The main damage caused by aphids is their role as vectors for damaging viruses, such as potato virus Y (PVY).

This is the principal virus disease of potatoes, with estimated annual losses reaching £18m in epidemic years. Symptoms of PVY range from a mild mosaic to severe foliar necrosis.

While significant losses are rare in ware crops, seed growers need to practise carefully-planned control strategies, since viruses can be carried into the daughter crop.

Seed production has increased in recent years in virus-prone lowland areas. Virus transmission is a particular problem where seed crops are grown near large areas of ware crops, or other crops that host aphids.

The threat for high-altitude seed growers is lower, as conditions are less favourable to aphids. The high health status of Scottish potatoes is a major selling point for both the domestic and export markets, and preserving this is again down to carefully-planned control strategies.

Which viruses cause concern?

There are two groups - persistent viruses, such as potato leaf roll virus (PLRV), and non-persistent viruses, which include PVY, potato virus A (PVA) and potato virus V (PVV).

PLRV resides in the phloem cells of infected plants and is acquired by aphids ingesting the plant's sap. The virus has to pass through the digestive system of the aphid and enter its saliva before transmission can occur, causing a delay of several hours. But once the virus has been acquired, an aphid remains infective for life.

By contrast, PVY, PVA and PVV are rapidly acquired when an aphid probes an infected plant. The viruses reside in the epidermal cells of plants, and are carried on the aphid mouthparts, so can be passed on to another plant within a few seconds' probing.

Colonising aphids that settle in the crop are therefore largely responsible for spreading the persistent PLRV. Winged, transitory aphids can acquire non-persistent viruses easily with their brief, superficial probing to tell whether plants are suitable hosts. They then move on to another plant, so have the potential to spread these viruses quickly within the crop.

How can crops be kept free of viruses?

Keep all sources of aphid-borne viruses to a minimum. This means sourcing high-quality, virus-free seed - using untested home-saved seed to grow ware can exacerbate the risk. Also rogue any virus-infected plants and groundkeepers and prevent haulm growth on outgrade piles.

Varieties have differing levels of susceptibility to PVA and PVY, while infection by PVV appears to be largely confined to Estima. More information on varietal susceptibility can be found on the Potato Variety Database.

It typically takes 10-14 days for the virus to spread from the original infection into the tubers. Growers can take advantage of this and desiccate crops where a late-season increase in aphid numbers has occurred.

How can virus risk be monitored?

Monitoring risk is a key component of an effective control strategy, best carried out using the aphid-monitoring service run by aphid-monitoring service.

Available only to Potato Council levypayers, this is a development of levy-funded research that has taken place over the past 10 years. Data on aphid flights are based on catches in 100 Potato Council-funded traps placed around the main seed-growing regions in Britain.

Growers receive SMS and email alerts and a standard weekly email service that gives results detailing regional averages. Maps on the website allow users to quickly find local trap-count data. This shows the type of aphid found, while a score, weighted according to virus vector, indicates overall threat. This is displayed on graphs offering growers a quick check on local migrations.

Rothamsted Research and Scottish Agricultural Science Agency (SASA) run a network of suction traps to monitor aphids at 16 sites. Daily samples are taken for most of the year, which give an indication of aphid behaviour and potential threat.

The main thing to look for is a sudden increase in aphid numbers. Very high vector pressure at the beginning of the season is of greatest concern. This is because crops are at their most vulnerable within the first four weeks from emergence. "Mature plant resistance" is then thought to build up gradually, making it more difficult to transmit virus within a crop. So a sudden increase in vector pressure at the end of the season would be of less concern.

How have aphid burden and virus risk changed in recent years?

These days the relatively mild winters and warm springs lead to more aphid survival and a swifter build up of populations at over-wintering sites. This in turn can lead to an earlier appearance of the winged aphids in crops than in the past.

There is also the threat of PVA. This virus is becoming more common, although that could be down to increased awareness. PVA appears to be transmitted early in the season, before the appearance of peach-potato and other potato-colonising aphids. Consequently, aphicide use is recommended from crop emergence to limit the transmission of PVA by aphids in susceptible varieties.

What is the best control strategy?

Local vector pressure should be assessed using data from aphid-monitoring services and control programmes started once the numbers of these aphids start to rise.

Growers should consider the potential threat from aphicide-resistant peach-potato aphids. Tank mixtures of two different aphicide chemical groups are recommended to minimise the risk of resistance.

The key component of a tank mix is a rapid knock-down component (lambda-cyhalothrin) which also acts as a deterrent to aphids probing treated leaves.

The second component of the tank mix is a translaminar active ingredient. Choose from pirimicarb, pymetrozine, flonicamid, thiacloprid, thiamethoxam or acetamiprid. This is taken up by leaves, so kills aphids that feed on them. It is essential that the translaminar products are alternated during the spray programme, as this will reduce the risk of aphicide resistance arising.

Aphids can have resistance to pirimicarb (so-called MACE resistance), so if using pirimicarb follow it with one of the other translaminar active ingredients. However, no more than two applications of a neonicotinoid product (thiamethoxam, thiacloprid or acetamiprid) should be applied over a season to an individual crop.

Treatments should be maintained at 7-10 day intervals.

Three golden rules for effective virus control

1. Monitor - use the Potato Council-funded CSL aphid-monitoring service and reports from suction traps to assess local vector pressure and decide appropriate control strategy.
2. Hygiene - keep the area around seed crops virus-free through controlling groundkeepers, volunteers, growth on outgrade piles.
3. Resistance - don't let aphicide resistance build up. Use tank mixes of different chemical groups and alternate translaminar products.