Tillage management: Aphid numbers fluctuating
Nearly 22 aphids/m² were counted in the weekly Teagasc figures on January 30. 50% of these aphids were grain aphids and none were winged, which limits the spread of the virus.
Aphid numbers have been fluctuating over the winter. These figures are relatively low and the bursts of cold weather should help to keep populations under control, as rain has not left land in good condition for spraying.
Lael Walsh spoke about resistance to pyrethroids at last week’s Teagasc National Tillage Conference. She stated that 30 different types of insects have developed resistance to pyrethroids.
The current investigation into aphicide resistance in Teagasc involves the collection of aphids from different areas of the country and testing these samples for levels of resistance.
LC50 is the lethal concentration that it takes to control 50% of the aphid population. Walsh stated that the LC50 was 7.5g/ha in some regions and moved to as high as 25.94g/ha in other regions.
Second resistance mechanism
The large variation in resistance levels suggested that there was a second resistance mechanism at play in aphid populations.
This second resistance mechanism was shown to be a metabolic mechanism, where enzymes in the aphids break down the pyrethroid compounds or detoxify pyrethroids.
Cultural control methods
As control options are limited, cultural control methods can help to reduce pressure on insecticides by altering sowing date. This may involve sowing later in the winter and earlier in the spring time or choosing a less-susceptible variety.