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CUTWORM (TURNIP ) UPDATE – 3 November 2015

Moth captures in pheromone traps at Wellesbourne in 2015 (2 traps per species). Since egg-laying by the first generation was over some time ago (few male being captured), the risk of damage due to third instar larvae will now be over.

Date Turnip moths caught Date Turnip moths caught Date Turnip moths caught

26 May 0 10 July 4 25 August 0

29 May 3 13 July 1 28 August 1

1 June 3 17 July 0 1 September 0

5 June 2 20 July 1 4 September 0

8 June 1 24 July 0 7 September 0

12 June 10 27 July 0 11 September 0

15 June 7 31 July 0 15 September 0

19 June 12 3 August 1 18 September 0

22 June 10 7 August 0 21 September 0

26 June 4 11 August 0 25 September 1

29 June 2 14 August 2 29 September 0

3 July 12 18 August 3 2 October 0

6 July 6 21 August 0 6 October 0

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The 'Cutworm Risk Model'

Since egg-laying by the first generation was over some time ago, the risk of damage due to third instar larvae will now be over.

The cutworm model is a computer program that uses weather data to predict the rate of development of turnip moth eggs and caterpillars. It also predicts the level of rain-induced mortality among the early- instar caterpillars. The cutworm model published by Bowden et al (1983) has been programmed into the MORPH decision-support software. This version will be used to produce cutworm forecasts in 2015, with the weather data used to produce the cabbage root fly and carrot fly forecasts.

The model was run from 15th May – based on the capture of a small number of turnip moths in Eastern England. Once eggs start to hatch then rainfall becomes important for the forecast. The cutworm forecast indicates that eggs laid on 15th May will have hatched between 5th and 17th June at the sites used to produce the forecasts (Table 1 below). Rainfall events of 10 mm or more have major effects on the survival of young cutworms (from when they hatch until they reach the third instar (third larval stage – achieved through moults)) and this forms the basis of the forecast i.e. if a heavy rainfall event occurs when a particular cohort of young cutworms is present then it is assumed that they will be killed. Table 1 also shows the predicted durations of the first and second instars for eggs laid on 15th May.

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Table 1

Predicted date when eggs laid Predicted date when eggs laid

Region on 15 May will hatch on 15 May will reach the third instar

Cornwall (Newquay) 11 June 7 July Kent (Sittingbourne) 5 June 28 June Suffolk (Woodbridge) 5 June 29 June Wellesbourne, Warwick 7 June 2 July Norfolk (Norwich) 9 June 1 July South Lincolnshire (Boston) 6 June 2 July Nottingham 8 June 3 July Lancashire (Ormskirk) 12 June 3 July York (Market Weighton) 10 June 4 July Scotland (Blairgowrie) 17 June 12 July

Table 2 below shows predicted survival (as a percentage) of the caterpillars likely to enter the third instar on each date (the first date shown for each site is the date on which eggs laid on 15 May were predicted to enter the third instar (Table 1)). Since egg-laying by the first generation was over some time ago, the risk of damage due to third instar larvae will now be over.

Table 2 (below on 2 pages) Predicted survival (as a percentage) of the caterpillars likely to enter the third instar on each date (the first date shown for each site is the date on which eggs laid on 15 May were predicted to enter the third instar).

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Date by which will reach 3rd instar 3rd willreach which by Date Cornwall (Newquay) Cornwall (Sittingbourne) Kent (Woodbridge) Suffolk Warwick Wellesbourne, (Norwich) Norfolk (Boston) Lincolnshire South Nottingham (Ormskirk) Lancashire Weighton) (Market York (Blairgowrie) Scotland 26-Jun-15 27-Jun-15 28-Jun-15 16 29-Jun-15 16 57 30-Jun-15 18 62 01-Jul-15 17 80 9 02-Jul-15 17 75 46 20 20 03-Jul-15 21 75 46 22 20 30 11 04-Jul-15 9 30 61 17 12 2 4 0 05-Jul-15 0 8 61 19 14 2 3 0 06-Jul-15 0 8 61 46 28 2 1 0 07-Jul-15 0 0 8 61 26 24 4 1 0 08-Jul-15 0 0 5 60 26 23 0 1 0 09-Jul-15 0 0 5 60 5 24 0 1 0 10-Jul-15 0 0 5 73 5 24 0 1 0 11-Jul-15 0 0 6 61 5 30 0 1 0 12-Jul-15 0 0 2 54 5 17 0 0 0 0 13-Jul-15 0 0 2 38 4 16 0 0 0 0 14-Jul-15 0 0 2 38 4 16 0 0 0 0 15-Jul-15 0 0 1 38 4 13 0 0 0 0 16-Jul-15 0 0 1 38 3 13 0 0 0 0 17-Jul-15 0 0 1 50 3 13 0 0 0 0 18-Jul-15 0 62 3 50 0 13 0 0 0 0 19-Jul-15 0 62 13 50 0 30 0 0 0 0 20-Jul-15 0 62 13 50 0 11 0 0 0 0 21-Jul-15 0 62 13 41 0 11 0 0 4 0

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Date by which will reach 3rd instar 3rd willreach which by Date Cornwall (Newquay) Cornwall (Sittingbourne) Kent (Woodbridge) Suffolk Warwick Wellesbourne, (Norwich) Norfolk (Boston) Lincolnshire South Nottingham (Ormskirk) Lancashire Weighton) (Market York (Blairgowrie) Scotland 23-Jul-15 0 65 19 0 0 10 0 0 9 0 24-Jul-15 0 0 19 0 0 10 0 0 9 0 25-Jul-15 0 0 0 0 0 10 0 0 9 0 26-Jul-15 0 0 0 0 0 0 0 1 9 0 27-Jul-15 0 0 0 0 0 0 12 0 0 0 28-Jul-15 0 0 0 0 0 0 12 0 0 0 29-Jul-15 0 0 0 0 0 0 12 0 0 0 30-Jul-15 0 0 0 0 0 0 12 0 0 0 31-Jul-15 0 0 0 0 0 0 14 0 0 0 01-Aug-15 0 0 0 0 0 0 14 0 0 0 02-Aug-15 0 0 0 0 0 0 14 0 0 0 03-Aug-15 0 0 0 0 0 0 14 0 0 0 04-Aug-15 0 0 0 0 0 0 14 0 0 0 05-Aug-15 0 0 0 0 0 0 14 0 0 0 06-Aug-15 0 0 0 0 0 0 14 0 0 0 07-Aug-15 0 0 0 0 0 0 18 0 0 0 08-Aug-15 0 0 0 0 0 0 18 0 0 0 09-Aug-15 0 0 0 0 0 0 18 0 0 0 10-Aug-15 0 26 56 96 0 1 18 0 0 0 11-Aug-15 0 58 64 96 0 42 41 0 0 0 12-Aug-15 0 58 75 0 0 55 41 0 0 0 13-Aug-15 0 34 75 0 5 53 41 0 0 0 14-Aug-15 0 38 58 0 13 0 0 0 0 0 15-Aug-15 0 38 58 0 46 0 0 0 0 0 16-Aug-15 0 38 58 0 46 0 0 0 0 0 17-Aug-15 0 38 58 0 46 0 0 0 0 0 18-Aug-15 0 38 12 0 46 0 0 0 0 0 19-Aug-15 0 37 12 0 17 0 0 0 0 0 20-Aug-15 0 37 12 0 17 0 0 0 0 0 21-Aug-15 0 37 12 0 18 0 0 0 0 1 22-Aug-15 0 37 12 0 18 0 0 0 0 1 23-Aug-15 0 37 12 0 18 0 0 0 0 3 24-Aug-15 0 37 12 0 18 0 0 0 0 22 25-Aug-15 0 43 12 0 18 0 0 0 0 23

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Reference: Bowden, J., Cochrane, J., Emmett, B. J., Minall, T. E. & Sherlock, P. L. (1983). A survey of cutworm attacks in England and Wales, and a descriptive population model for segetum (: ). Annals of Applied Biology 102 29-47.

Background

“Cutworm” is the name given to caterpillars of certain Noctuid moths, in particular those of the turnip moth Agrotis segetum. The name derives from the habit of the older caterpillars of feeding underground, damaging plant roots and stems (including the storage organs that we use for food), sometimes so badly that the plant topples.

The adult moths lay eggs on plants or on pieces of litter and debris in the soil, usually from the end of May or early June. These hatch in around 8-24 days, depending on temperature. The young caterpillars seek out and feed on the aerial parts of plants. In a further 10-20 days, again depending on temperature, the caterpillars go through their second moult, becoming “third instar” caterpillars. It is at this point that they adopt the cutworm habit, becoming subterranean and feeding on roots etc.

Unhatched turnip moth eggs and the older, subterranean cutworms are largely invulnerable to the effects of the weather and insecticides. The two early caterpillar instars differ, however. If there is substantial rainfall (defined as 10 mm or more of rain falling in showers of moderate intensity over a 24- hour period) whilst these caterpillars are feeding above ground then this causes high mortality among them. They are also vulnerable to insecticides and irrigation whilst feeding on the foliage.

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Crop Susceptibility

Crops differ in their susceptibility to cutworm damage. The most vulnerable are lettuce, leek and red beet. Young lettuce and leek plants are easily bitten through by cutworms, and though beet plants may survive an attack the bulbous root is rendered unmarketable by cutworm feeding. Moderately- susceptible crops include , carrot, , parsnip and sugar beet. The least susceptible of those vulnerable to damage are onion, potato, swede and turnip.

Day-degree sum to start of flight activity

Several years ago, data on trap captures in 2005-7 were used to estimate a day-degree (Do) sum for the start of flight activity at Wellesbourne. This was 340 Do above a base of 7oC from 1 January. The graph below shows accumulated day-degrees for 2015. The sum of 340 Do was accumulated by 5th June 2015.

400 Accumulated day-degrees at Wellesbourne above a base of 7°C from 1 January 2015 360

320

280

240

200

160

120

80

40

0 1-Jan 22-Jan 12-Feb 5-Mar 26-Mar 16-Apr 7-May 28-May

See the HDC pest blog for monitoring updates http://blogs.warwick.ac.uk/rosemarycollier/.