Early season aphid activity in 2019
Prolonged exposure to low winter temperatures are known to have lethal and sub-lethal effects on populations of M. persicae which, in Scotland, overwinters as larvae or apterous adults. Poor over-winter survival delays the build-up of populations of this species in the forthcoming growing season. Prior to 2011, SASA has used the mean temperatures for January and February to predict when M. persicae will become active in the summer. In 2011, following an extremely cold December 2010, SASA predicted early season aphid activity has been based on the mean temperatures during the three-month period of December-February. We intend to continue to use the model based on the three month period.
During winter 2018-19, the mean temperatures were well above the mean over the last 50 years: 5.1°C at SASA (Edinburgh; mean = 4.0°C) and 4.1°C at JHI (Dundee; mean = 3.7°C). These winter temperatures rank the 9th warmest from the last 51 years at Edinburgh and the 21st warmest from the last 53 years at Dundee. Based on these figures, the predictions for the first flight of M. persicae is 1 June at Edinburgh (average date of first catch is 14 June) and 9 June at Dundee (average date of first catch is 13 June). Therefore, M. persicae activity in 2019 is expected to commence 1 to 2 weeks earlier than in an average summer.
In 2018, the first M. persicae at Edinburgh was recorded on 6 July, 21 days later than the average date of first catch and 15 days later than predicted. The first M. persicae at Dundee was recorded on 15 June, 1 day earlier than the average date and 8 days earlier than predicted. First arrival dates for both sites were well within the 75% confidence limits of predictions.
The predictions for the Potato aphid Macrosiphum euphorbiae, Rose-Grain aphid Metopolophium dirhodum and the Grain aphid Sitobion avenae are provided by Rothamsted Research and are based on mean temperatures over January and February 2019 (Table 1).
|2019 Prediction||75% Confidence Limits||2018 Obs'n||
|Myzus persicae||1 June||8 May - 19 June||6 July||28 May|
|Macrosiphum euphorbiae||20 April - 3 June||25 May||24 April|
|Metopolophium dirhodum||22 April - 10 June||31 May||8 April|
|Sitobion avenae||29 April - 9 June||30 May||7 May|
|Myzus persicae||9 June||14 May - 5 July||15 June||30 April|
|Macrosiphum euphorbiae||2 May - 20 June||24 May||1 May|
|Metopolophium dirhodum||7 May - 23 June||9 May||21 April|
|Sitobion avenae||13 May - 20 June||28 May||23 May|
The moderately warm temperatures over 2018-19 winter, particularly at Edinburgh, indicate that the first flights of M. persicae should be 1 to 2 weeks earlier than on average, and at least 1 week earlier than in 2018. Consequently, population levels have the potential to develop to levels that could significantly threaten the virus health of seed crops. The prediction for the total of M. persicae caught by 31 July is 24 at Dundee and 33 at Edinburgh. The 75% confidence intervals for these predictions are 7 at the lower end for Dundee and 9 at the lower end for Edinburgh, and 81 at the upper for Dundee and 119 at Edinburgh. Therefore, populations of M. persicae could be relatively high during the growing season for potatoes in 2019. Therefore, given the increase in inoculum observed during 2018, there is a significant risk of further leaf roll transmission in potato crops in 2019, presenting a risk for the 2020 crop exhibiting significantly higher levels of leaf roll than the average of 5.5% predicted for 2019.
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