Dealing with pests and diseases

Frit fly, leatherjackets and slugs are some of the pests to consider when reseeding grassland. Our information will help you manage the risks of pest burdens and control methods.

Back to: Managing grassland reseeds

Frit fly

Frit fly populations are found across most of the UK and can pose a threat to summer and autumn reseeds. Frit fly lay their eggs on grass during July to October.

The larvae, which are yellowish-white, grow to 5 mm in length. These larvae move from one ryegrass seedling to the next, feeding on the central shoot and leaving patches of stunted or dying seedlings. Populations in new sowings can reach several thousand per square metre and exceed the number of seedlings present.

Risks of frit fly attack

  • Italian ryegrass is more susceptible than perennial ryegrass or clover. Timothy and cocksfoot are not affected
  • Grass-to-grass reseeds
  • Autumn reseeding is particularly prone
  • Direct-drilled swards are most at risk

Control of frit fly burden

  • Introducing a summer break crop such as brassicas
  • Reseed outside the main egg-laying period (July–October)

Frit fly. Copyright Bayer CropScience.

Frit fly image courtesy and copyright of Bayer CropScience Ltd. 


Leatherjackets are the larvae of crane flies (daddy long-legs). They are up to 50 mm long, dull green in colour and have tough, rubbery skin.

Crane flies lay their eggs between July and September in both grass and cereal crops. The larvae live in the soil, eating the roots and stems of grass plants, at or below the ground surface. This causes significant damage to the sward base.

The most severe damage occurs when leatherjackets are almost fully grown and active in April and May. Crane flies tend to lay their eggs close to where they emerge, creating a persistent problem for the field.

Risks of leatherjackets

  • Grass-to-grass reseeds
  • Highly productive grasses
  • Mild, wet autumns favour survival of eggs and larvae

Control of leatherjackets

  • Using a break crop or a minimum of a two-week break from grass-to-grass leys
  • Rolling to consolidate soil can minimise short-term damage
  • Ploughing grassland in July can reduce leatherjacket populations by 50%

Leather Jackets. Copyright Oliver seeds.

Leatherjacket image courtesty and copyright of Oliver Seeds. 


Slugs graze grass leaves, rasping away the leaf tissue in strips between the veins. The grey field slug is the most common type of slug that breeds throughout the year when conditions are correct. Wet summers encourage significant population growth.

Risks of slugs

  • White clover and grass/clover reseeds are at greater risk of slug attack
  • Slot seeding can increase slug damage
  • Wet land or cloddy, unconsolidated seedbeds provide a habitat

Control of slugs

  • Ploughing can reduce the risk of slug damage
  • Creating a fine, firm seedbed with no cracks allows slugs to be exposed
  • Slug pellets are an effective method of control. However, for products containing metaldehyde, extra care must be taken to prevent chemicals reaching waterways

Grey field slugs. Copyright ADAS.

Grey slug image is courtesy and copyright of ADAS

Disease risks


Plant symptoms  


Barley yellow dwarf virus 

Bright yellow upper leaves, gradually spreads throughout the sward, leading to severely stunted plants  

Apply insecticides*  


Spread by aphids - use aphid monitoring information to time applications  


Sow tolerant varieties 

Ryegrass mosaic virus 

Light green streaking of leaves, which eventually turn brown and die 

Autumn rather than spring sowing will delay the ingress of the mite  


Use resistant ryegrass varieties 

Crown rust 

Strikes mainly in late summer and early autumn and results in distinctive yellow-orange pustules on the leaves 

Ensure grass receives sufficient nutrients to fight off the disease  


Cutting or topping the grass to remove the leaf ‘food supply’ for the fungus  


Choose resistant varieties 

*Seek advice from a qualified professional and ensure sprayer operators are fully qualified. 

Useful links

Learn more about integrated pest management (IPM)

For more information on dealing with pests, consult a BASIS adviser.