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Absolute symptoms vary depending on the crop, but there are some common features. When seedlings are attacked by larvae, typical damage is stem severance. In cereals, young plants may remain in the ground but isolated from roots. This can be superficially similar to 'frost lift' in northern climes but manifests itself as patchy damage where groups of larvae have been feeding rather than more widespread plant deaths associated with freezing soils.
In older cereal plants, leaves in contact with the ground may be severed, leaving a ragged edge.
Damage to grassland is less obvious but starts in early autumn. In the spring and early summer, extensive damage may manifest itself as dead or dying patches. Occasionally, entire swards are destroyed. The majority of damage, however, fails to yield these signs and frequently passes unnoticed.
Early Warning Systems
Some countries, notably the UK and the Netherlands, have operated regional forecasting schemes for a number of years. These involve collecting samples from a number of fields in the winter, extracting larvae and predicting whether there will be a high or low risk year for damage. In practice, these forecasts contain little usable information for individual farmers. The oviposition behaviour of gravid females means that each field/crop still has to be sampled to determine if populations exceed the threshold. The relatively high labour requirements to achieve this means it is rarely done and so the favoured approach to this pest is reactive. There is a need to develop in-field forecasting systems that are cheap and easy to operate.
A consensus between researchers has evolved to suggest that the economic threshold for T. paludosa larvae in grassland is about 1m/ha. There is, however, substantial evidence that populations lower than this may be worth controlling (Blackshaw, 1985) and that timing of control is important in that early insecticide applications will result in greater spring herbage yields (Newbold, 1981; Blackshaw, 1984). An added complication in calculating economic thresholds is that grass itself rarely has a value until it has been converted into an animal product so that different livestock systems result in different values.
The threshold in spring cereals is usually presented as 15 larvae from 10 x 30 cm drill lengths at 18 cm drill spacing. This equates to about 0.25 m/ha.
Thresholds do not exist for other crops but lower thresholds can be anticipated with reduced plant densities.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Damage occurs in both reseeds and established grassland. However, autumn reseeds with cultivations coinciding with the T. paludosa flight and oviposition period are far less vulnerable than spring reseeds, especially where the latter follows a ley. In surveys of grass reseeds in the UK (Bentley, 1984; Ellis et al., 1984; Clements et al., 1985; Bentley and Clements, 1989a, b) leatherjackets were not implicated as damaging pests in England and Wales. The majority of damage from leatherjackets is to long term leys and pastures. Blackshaw (1985) estimated that leatherjackets were responsible for in excess of £15m worth of damage in grassland in Northern Ireland alone each year. The majority of this was attributed to insidious feeding in populations below the economic threshold.
Leatherjackets have historically been associated with spring cereals and farming experience has linked extensive damage with those crops that are sown into ploughed up grass swards. Despite the long association between leatherjackets and damage to spring barley there have been few published data on yield losses. Golightly (1967) and Rayner (1969) reported experimental studies, although the similarity in reported rates of loss suggests that they may have been commenting, at least in part, on the same data.
Such rotations help determine where T. paludosa is a cereal pest. Over much of its range, the economic impact is on herbage production in grassland systems.
Given the inability of most workers to correctly identify T. paludosa larvae and the restricted dispersal ability of gravid females, doubt must be cast on many of the records of damage to other crops in the absence of corroborating evidence. The closely related species, T. oleracea, is more opportunist and may provide an alternative explanation.