Monday 26 September 2016

Pesticides still effective despite rising resistance levels in aphids

Published 24/06/2015 | 02:30

Farmers can still control aphids.
Farmers can still control aphids.

Despite the discovery of aphid resistance to the most commonly used pesticides in Munster in 2013, farmers can still effectively control the pest, provided they know what they are looking for, writes Teagasc horticultural researcher Michael Gaffney.

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While the Knock Down Resistance (KDR) gene was first discovered in Cork and Tipperary two years ago, only grain aphids with one copy of the resistance gene have been located here since.

This suggests that this is not a 'full' resistance, but instead a loss in sensitivity to pyrethroid sprays.

We don't know the extent of the resistance gene in the Irish population yet, but with reports of significant Barley Yellow Dwarf Virus (BYDV) in early sown autumn crops, it seems that pyrethroid sprays may be set to be less effective.

If a grower suspects that resistance is an issue, they should initially establish the species of aphid present.

Application

If a wide range of aphid species are present, it's likely that there was an issue with the application of the pyrethroid. If only grain aphids are present, then you should contact your advisor and apply a non-pyrethroid insecticide if available and required. Advice on how to identify common cereal aphids will be available during the Crops Open Day.

Other issues in relation to aphid control for farmers is the reservoirs of infection.

The Grain Aphid is the most abundant in cereal stubbles, proving that the remnants of past cereal crops is an important source of infection for new crops.

Coupled with mild winters, this 'green bridge' allows aphid numbers within the crop to establish more quickly.

Teagasc research shows that aphid numbers can usually be managed by a combination of sowing dates and pyrethroid application.

Early sown autumn crops, reaching two to three leaf stage by mid-October should be sprayed at that stage, followed by a further spray in the first week of November.

Crops sown from the last week of September require a single aphicide during the first week of November. There is no benefit from additional aphicide applications beyond this, even with high aphid numbers and BYDV occurrence.

If crops emerge after the end of November they do not require spraying, except in mild winters when aphids are plentiful and active.

In early spring sown crops BYDV is not generally a problem. However, crops sown after mid-April can be affected. Depending on numbers, aphids should be sprayed with a single aphicide at the four-leaf stage.

Earlier sown crops tend to avoid BYDV by reaching growth stage (GS) 31 before aphid migration - infection after this stage causes little yield loss. However, apart from transmitting BYDV, aphids can also cause damage by direct feeding on plants. If there are more than five aphids per ear at GS65-85 an aphicide may be beneficial.

However, pyrethroids will be ineffective and an alternative active ingredient should be used.

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