Aphid control in potatoes
In Northern Ireland Potato Virus Y (PVY) is the most damaging of the insect transmitted viruses. It is non-persistent virus that is transmitted rapidly when infected aphids probe plants to identify food sources.
The dry weather over the past six weeks will result in a high number of aphids this season. Seed growers must be prepared to spray crops with aphicides to prevent the spread of virus and maintain the health of high grade stocks.
Potato crops are most susceptible to virus infection at emergence and it is essential that seed crops are sprayed with insecticides during the early part of the season, if it’s warm, dry weather. Do not wait until aphids are seen on leaves. Action must be taken before aphids are found colonising the leaves. In Northern Ireland most virus is spread by aphids migrating across crops and much damage can have occurred by the time aphids are found on the leaves.
Varieties with high resistance to PVY, , for example Navan are unlikely to benefit from applications of insecticide. For varieties with low resistance to PVY , for example, Premiere, Maris Piper, Cultra, and Kerr’s Pink (refer to Potatoes, Varieties for Northern Ireland 2006), or to viruses PVV and PVA, , for example Desiree, the following aphicide programme is suggested;
- Apply the first aphicide at 80 percent crop emergence, when plants are most susceptible.
- Repeat at 14-day intervals up to early August or three weeks prior to desiccation, whichever is earliest. Desiccation of seed crops as early as possible helps to limit virus spread.
To prevent insecticide resistance developing in the aphid population it is critical that an anti-resistance strategy is adopted. Tank mixtures of two different aphicide groups are recommended to reduce the risk of resistance and to minimise the transmission of virus.
There are two main groups of aphicides:
- Rapid knock-down chemicals usually formulated around the pyrethroid lambda-cyhalothrin are recommended to rapidly kill aphids. They also act as a deterrent to aphids feeding on the leaves, which is the main means whereby they can restrict spread of viruses Y, V and A.
- Examples include Dovetail and Hallmark Zeon.
- Systemic and translaminar chemicals are taken up by the leaf and kill aphids after they feed on the leaf sap. This method of activity effectively controls spread of leaf-roll virus, but is ineffective against spread of viruses Y, V and A.
- These products include Actara (thiamethoxan), Aphox (pirimicarb), Plenum WG (pymetrozine), Teppeki (flonicamid), Biscaya (thiacloprid). Note Aphox and Plenum WG should not be used during flowering.
It is recommended to tank mix a pyrethroid such as Dovetail or Hallmark Zeon with one of the translaminar products.
In ware crops the virus transmitting activity of aphids is of little concern. Application of insecticides should only be considered when high infestations of aphids are likely to reduce yield through sap feeding causing the leaves to curl in response to drought stress. A threshold of five aphids per leaf will justify an aphicide on ware crops. Ideally aphid populations should be monitored weekly to determine when numbers start to rise, as this is the ideal time to target them. A good method for detecting aphids is to hold a white sheet of card under the leaves and gently shake the foliage. Aphids can be clearly seen as they fall on to the white card.
Plenum, Teppeki, Biscaya, Hallmark Zeon, Dovetail, Aphox and Actara are recommended for ware crops.
Always check product label for details of maximum number of applications, spray and harvest interval.