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Crop Crack 7.04.10
By Wendy Fearon
I am amazed at the extent of damage in both winter cereal crops and grassland from leatherjackets. Leatherjackets are very resilient to cold and frost and they will just reduce their feeding until the soil warms up.
Leatherjacket numbers are much higher this spring than they have been for a few years. Farms across Northern Ireland were struggling for a few weeks but a combination of damp weather and slurry application has certainly helped green things up. Where fields are still looking poor and yellow or bare patches have appeared it is essential to inspect for leatherjackets.
Damage is already becoming apparent in some established grass swards. With this evidence of high leatherjacket populations, spring cereals are also certain to be at risk especially those following a grass ley. As crops are most susceptible to damage at the seedling stage it is most important to monitor leatherjacket numbers from emergence onwards. In newly sown cereals the need for treatment is assessed by scratching along 30cm (12``) drill lengths to a depth of 5cm and searching for leatherjackets.
The following grub numbers are the threshhold figures at which damage is likely to occur: -
For drills spaced at 17.5cm (7``) 15 or more grubs from 10 30cm (12``) lengths of row.
For drills spaced at 12cm (5``) 10 or more grubs from 10 30cm (12``) lengths of row.
These numbers are equivalent to 0.3million larvae per hectare and would pose a threat to spring cereals.
To limit damage leatherjackets need to be detected as soon as possible. A good early indicator of high populations is the presence of birds, in particular rooks, crows and starlings, in fields as they search for the grubs.
Symptoms of leatherjacket damage are yellow dead plants or withered patches of grass that have been cut off from their roots just below the ground. On closer examination leatherjackets can be found by digging in the top 2-3 cm of the soil. At this time of year they are typically 1-2 cm in length grey in colour with a tough leathery coat, hence their name, and resemble a large maggot.
A Leather Jacket in the Soil
If numbers meet the threshold chemical treatment should be applied using a product containing chlopyrifos such as DURSBAN WG or CYREN. As the leatherjackets live in the soil the spray must reach the soil therefore use a high water volume (500l/ha) and a coarse spray quality to ensure good penetration into the soil surface. Light rain after the spray application will also help.
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