A metabolic resistance mechanism is contributing to the partial resistance of Irish grain aphid populations to pyrethroids. According to research presented by Lael Walsh at this year’s National Tillage Conference, at least two resistance mechanisms are now confirmed in Irish aphid populations. This may explain the varying levels of aphid control which growers are experiencing throughout the country.

Target site resistance to lambda-cyhalothrin (a synthetic pyrethroid which forms the main active ingredient in most aphicides), otherwise known as knockdown resistance (KDR), has been well documented among Irish grain aphid populations.

However, tests carried out on samples of grain aphids collected in Irish fields found varying levels of resistance between populations. This led to the suggestion that there may be a second form of resistance such as a metabolic resistance mechanism within the sample population.

Metabolic resistance

A metabolic resistance mechanism means that there are enzymes present in the gut of the aphid which detoxifies or breaks down the pyrethroid compound. The aphid can then survive exposure to the chemical.

Tests carried out under lab conditions achieved a 42% mortality rate of the sample population of aphids when using a pyrethroid only.

In a subsequent test, an enzyme blocking synergist known as piperonyl butoxide (PBO) was applied to the sample population before the application of a pyrethroid. The mortality rate increased to 77%.

The results show that when enzyme activity is blocked, higher mortality rates could be achieved in the sample group. This confirms the presence of a metabolic resistant mechanism in addition to KDR target site mutation which is helping increase levels of survival in the Irish grain aphid population.

This presents a challenge for growers as current grain aphid control options are quiet limited. The use of delayed sowing dates and choosing varieties with resistant traits are just two of a number of cultural control options which growers must consider.

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