Newly-sown pastures are more vulnerable to pests than well-established grassland and can be badly affected by specific problems.
Grassland Pests such as Frit fly, slugs, leatherjackets and chafers are the most important pests of ryegrass, while pest damage to white clover is primarily from slugs and sitona weevils. In addition to the main pasture-land pests, aphids and nemotodes can also cause significant problems from time to time.
Frit fly larvae, from eggs laid on or near seedlings or mature grass plants, bore into the base of seedlings and tillers, causing death or greatly reduced vigour. Populations in new sowings can reach several thousand per square metre and exceed the number of seedlings present. Italian ryegrasses are more susceptible than perennials and mature crops of all ryegrasses are less affected than new seeds
Frit fly populations are greater in grassland than under arable cropping, with larvae migrating from ploughed-up swards to infest new seeds. Direct-drilled fields are most at risk. Chlorpyrifos is the recommended chemical treatment, although it will also reduce populations of carabid beetles valuable in keeping on top of many insect pests. Sowing outside the main periods of egg-laying and allowing a gap of at least four weeks between sward destruction and re-seeding can give valuable control. Selecting varieties of Italian ryegrass reported to be more resistant to attack may also be useful.
Slugs prefer white clover to ryegrass, rasping away the leaf tissue in strips between the veins. Seedlings are more susceptible than mature plants but feeding on the leaf buds in spring can cause damage and reduce output. White clover slot-seeded into grass swards is particularly vulnerable. Molluscides based on metaldehyde and methiocarb are the most effective treatments. White clover varieties showing greater resistance to slugs should be chosen for conditions under which large infestations are most likely, in heavy land and where soils have been loosely tilled without adequate compaction.
The soil-dwelling larvae of craneflies, leather-jackets feed on many grasses and legumes, causing the greatest damage in the spring. Although they mainly attack root tissue, they will consume leaves where accessible. Plants are severed just below ground level, resulting in patches of yellowing plants which invariably die.
Sitona weevils cause characteristic notching at the leaf margins of clovers and lucerne. Weevils are abundant in warm, dry weather, with April and August sowings particularly vulnerable. The adult weevils attack the first leaves and cotyledons of seedlings while their larvae can damage root nodules, reducing the plant’s ability to transfer N. Larval feeding can also pre-dispose the plant to damage by crown or root-rotting fungi. No insecticide is specifically approved for weevils but treatment for frit fly or leather-jackets is likely to reduce the adult population.
The larvae of several species of chafer beetle can also cause damage to grassland in various parts of the UK. The adults are 8-10 mm long with a green head and thorax and reddish brown wing cases: The grubs are white and about 18-20 mm long when fully grown. The feeding of the larvae produces patches of poorly grown grass that may turn very brown in dry weather. Damage is most likely to be seen in September–October. Substantial bird activity may indicate infestation, as they actively search out the grubs. Once infested, pastures tend to be re-infested in subsequent seasons unless they are treated with an appropriate agrochemical.