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PJ Phelan: Our dependence on chemical control of aphids is a high-risk strategy

Several parts of the country have high levels of resistant insects and further use of chemicals is increasing the problem

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Risk: Continuous use of insecticides with the same mode of action results in susceptible insects being eliminated and replaced by resistant insects.Photo: Roger Jones

Risk: Continuous use of insecticides with the same mode of action results in susceptible insects being eliminated and replaced by resistant insects.Photo: Roger Jones

Risk: Continuous use of insecticides with the same mode of action results in susceptible insects being eliminated and replaced by resistant insects.Photo: Roger Jones

THE advice this autumn to be cautious about sowing too much land early to minimise BYDV risk was unnecessary as the rainfall since the end of September determined sowing days.

Free-draining soils have fared well, with one good day enabling sowing the following day; heavier soils need more time. The frequent rainfall should provide adequate control of aphids and prevent Barley Yellow Dwarf Virus (BYDV) transmission on the early sown crops.

Chemical control of aphids is a high-risk strategy as it kills a wide range of other insects, including those that feed on aphids. When the natural predators are killed, the next wave of aphids that land on the crop are free to feed on it without any risk from other insects.

Every population has diversity and that diversity presents a challenge to chemical control. Some aphid populations have developed resistance to insecticides.

The more frequently an insecticide is used, insects with the resistant gene survive and pass on the resistant gene.

Continuous use of insecticides with the same mode of action results in susceptible insects being eliminated and replaced by resistant insects.

In Ireland, several areas have high levels of resistant insects, and further use of chemicals is increasing the problem.

Other areas have low to negligible levels of resistant aphids. Insecticides should also be avoided in those areas to maintain diversity, allow natural predators to do their work and avoid causing adverse changes in aphid populations

Resistance management of pesticides consists of using cultural control and rotation of pesticide usage by using products with different modes of action. The withdrawal of pesticide approvals has now left us with only one mode of action for control of the aphids which transmit BYDV.

The only approved chemical prevention for BYDV is with pyrethroid insecticides – Decis, Karate, Sparviero, Sumi-Alpha etc. Each have the same mode of action so there is no point is swapping from one to another.

Cultural control

Having only have one mode of action means we have to base our resistance strategy on cultural control.

Inspect crops carefully in sheltered areas, on calm evening for aphids. Use of sticky traps placed 5m from field boundaries will give guidance on the presence of aphids.

Such traps should be inspected frequently as a large range of flying insects will be trapped and it is almost impossible to determine what you are looking at if it is left unobserved for any length of time.

Base your decision on insecticide usage on the presence of aphids, previous severity of BYDV in the area, weather forecasts and the risk of building resistance wiping out the natural predators – your last line of defence.

If you decide that you must apply an insecticide, delay application until early November and do not plan a two-spray programme.

When applying insecticides use the full recommended rate. Reduced rates will allow some aphids to survive and may do very little for the survival of predators.

This will be aggravated by the fact that the slower multiplication rate of natural predators will not be adequate to enable aphid control.

An insecticide rate greater than the recommended rate will do even more damage. It will kill every natural predator and every insect apart from the most resistant.

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Given suitable soil conditions, sow crops a little deeper than normal as the later the crop emergence, the less the risk of BYDV. Crops emerging after mid-November are at a significantly lower risk of BYDV and should not be sprayed.

Looking ahead, plant breeders are working on developing BYDV-tolerant varieties. Seedtech have a BYDV-tolerant variety, KWS Joyau, which is a conventional six-row variety.

It did very well in 2019 with a yield of 121 but was back to a more modest 100 in 2020. RAGT have produced a BYDV-resistant wheat, Wolverine, from a 20-year-old breeding programme.

Counties outside Europe are using genetic engineering to short-circuit the time for development of such characteristics. Without a dramatic change in EU policy we will not have access to those advances but may well end up selling our grain into Irish and EU markets where third country produce is also being sold.

PJ Phelan is a tillage adviser based in Tipperary and is a member of the ACA and ITCA


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