Aphid offensive
18 March 2000
Aphid offensive
A new addition to the aphicide armoury should help potato growers in the battle against aphids. Tom Allen-Stevens explores the aphid risk this season
VIRUSES, aphicide resistance and the role of beneficial insects all have to be taken into account when deciding aphicide strategy. But the first step is to gauge how big the problem is likely to be.
There are a number of factors that have a bearing on the scale of potential aphid damage to potato crops, according to IACR Rothamsteds Dr Richard Harrington: "Winter weather has a major impact on whether aphid problems in any particular year. The crucial period is January and February."
Milder conditions mean aphids continue multiplying, and fly from their winter host plants earlier and in larger numbers. With few beneficial insects around, populations begin to explode which can be damaging to a young crop. Colder conditions will keep the aphids in check for a little longer. Beneficial insects will also have a chance to complete their winter dormancy period and emerge at a time when there are more food sources about.
"Planting date also has a bearing on the vulnerability of the crop. If the crop is established late, the plants will be relatively small at a time when aphid numbers are likely to be on the increase," continues Dr Harrington.
This can also have an impact on the amount of damage a virus can do. "If the plants are young when flights are greatest, there is the greatest danger of severe damage from viruses. As plants mature, its more difficult to get a virus in."
Potato leaf roll virus (PLRV) and potato virus Y (PVY) are the most damaging viruses in potato crops. "PLRV is a persistent virus. It resides in the phloem of the plant, so for the vector aphid to pick it up and become infected takes a matter of hours – plenty of time for a pesticide to control the aphid. But PVY is harder to control, being a non-persistent type which resides in sub-epidermal cells. The aphid only has to probe under the surface to pick it up," explains Dr Harrington.
There are some 500 species of aphid in the UK, of which only four will actually feed on potatoes. If a population of non-potato aphids passes through the crop, they are likely to test a number of plants, looking for one that they can feed on. This is one way the virus can spread through a crop very quickly.
Cleanliness
"With seed crops you dont want to be harvesting infected potatoes for planting the next year, so they must be kept clean. In ware crops, depending on the natural resistance of the variety and the planting date, you might tolerate up to five percent infection because neighbouring plants can compensate for the loss," Dr Harrington advises.
This season sees the launch of new aphicide Plenum, from Novartis. The company claims that it could offer solutions for some of growers key concerns. The active ingredient, pymetrozine, works by disabling an aphids stylet – its feeding probe. It will survive for up to four days and then starve to death.
The new molecule tackles all the key aphid types, but is claimed to be much safer to use and relatively harmless to beneficial insects. "Pymetrozine only targets sucking pests. Natural predators remain in the crop unharmed, reducing the risk of aphid population rebound later in the season," states Novartis field crops campaign manager, Michael Tait.
Research has shown that if beneficial insects are removed with the aphids, aphid numbers will recover quickly and, with no predator control, can exceed previous populations.
It takes about an hour for the chemical to have an effect on a feeding aphid, not enough time for it to pick up PLRV. This is enough for the vector to pick up PVY, so Plenum cannot effectively control it. "However, studies have shown that subsequent transmission of PVY by Plenum-treated aphids is reduced, because the aphid cannot feed," points out Rod Burke, insecticides project leader with Novartis.
But its with aphicide resistance that Plenum claims the biggest advantage. There are two types of resistance: esterase resistance, where aphids overproduce an esterase enzyme, which gives them a varying, but broad-spectrum resistance to most aphicides. And then theres MACE resistance (modified acetyl choline esterase), which is less common, but more total.
Until now theres been little in the pesticide armoury that will tackle aphids when the two types of resistance occur together. Novartis claims there is no cross-resistance to Plenum, however: "It should form part of an anti-resistant strategy. Start the season with two applications of Plenum and then switch to other forms of control. If necessary, you can come back to Plenum to control buckthorn aphid," advises Mr Burke.
Plenum in brief
• Controls all aphids commonly found on potatoes
• Disables feeding mechanism
• Lasts up to 14 days
• Safe to humans and beneficial insects
• Can help stem spread of PVY
• One application costs £15-22.50/ha depending on rate
Aphid alert
THE warm winter has increased the aphid risk – but spring weather will be the deciding factor. IACR Rothamsteds weather watch results show the January and February average temperature was 4.9íC, higher than the 30-year average of 3.5íC and the eighth warmest in the same period.
Rothamsteds Dr Richard Harrington expects the first peach potato aphid to fly on around 28 April, three weeks earlier than average. For potato aphids, migration is likely to start nearer 8 May, which is 10 days before the average date.
"Although this is earlier than usual, its about right for the last 10 years. Were expecting numbers caught up to 1 July to be about average. If theres a warm, early spring, theres potential for aphid numbers to be a real problem, but at the moment theres not enough to set any alarm bells ringing," says Dr Harrington.