Arable Cropping & the Environment - a guide

Pest Thresholds

Integrated pest management involves using thresholds intelligently as the basis for control strategies.

Crop and pest 

Threshold Treatment strategy


Autumn aphids

High risk of aphids and BYDV forecast.
Aphids in crop in late Oct/early Nov.
Delay drilling until mid-Oct.
Destroy previous crop residues to remove ‘green bridge’.
Apply spray treatments 5-6 weeks after crop emergence. If mean daily temperatures < 3ºC and remain low sprays may not be required.

Summer aphids

> 50% stems infested and increasing at stem extension to flag leaf emergence.
66% of ears/stems infested and increasing at flowering to watery ripe.
Use half-rate selective insecticide if threshold is exceeded, parasitoid activity is low and < 2% aphids are mummified.
Do not treat if > 5% are mummified.

Wheat bulb fly

Millions of eggs/ha < 1 – no action.
1-2.5 – damage possible on late-sown crops.
2.5-5 – economic damage very likely.
> 5 – damage inevitable.

Do not cultivate fields due for wheat from late July to end of wheat harvest.
Increase seed-rate and choose variety more suited to high-risk situations. Use seed treatment or egg hatch spray if necessary.
Consolidate soils after earlier cultivation to avoid rough tilth.

Orange wheat blossom midge

Cocoons/kg soil 11-40 – moderate risk.
> 40 – high risk.

Midge numbers/ears
1 midge/3 ears (feed wheat).
1 midge/6 ears (milling or seed wheat).

Monitor at ear emergence if risk is high - soil is wet and > 13ºC in May.
Consider an insecticide at ear emergence if threshold is exceeded.

Frit fly

> 10% of plants attacked at 1-2 leaf stage (wheat after grass).

Destroy grass or grass weed infested stubbles at least 6 weeks before drilling.
Only apply insecticide at 1-2 leaf stage if threshold is exceeded.


5 leatherjackets/m of row – highest risk with spring cereals but only when drilled after ploughed out grass.

Monitor susceptible crops sown after grass for first month after crop emergence.

Oilseed rape

Cabbage stem flea beetle

5 larvae/plant Nov/Dec or 50% petioles scarred.
Damage by adults is most severe on slow-emerging or poorly established crops.

Only apply treatment if adult damage is very severe at the cotyledon stage, and/or threshold is exceeded later in autumn.

Pollen beetle

Beetles/plant 15 at green/yellow bud stage on winter crops.
5 on slow-growing, backward crops, restored hybrids and varietal associations.
3 on spring crops.

Apply pyrethroid insecticide if threshold is exceeded.

Cabbage seed weevil

2 - during flowering on winter crops.
1 - in northern England and Scotland.
0.5 - in the north if a fungicide required or significant pod midge damage likely.
2 - at any time from green bud to late flowering on spring crops.
Apply a pyrethroid at the correct timing if threshold is exceeded.

Source: Dr Bill Parker, ADAS Wolverhampton.

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