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Damaged oat and grass seedlings show dead hearts, where the plant is green but only the central leaf turns yellow and dies. It is most noticeable when the plant is at the four-leaf stage. If the plant is dissected the larva will be found in the centre of the shoot below the dead leaf. The growing point of the plant is killed and new shoots (tillers) are developed laterally. Each plant may therefore have several shoots, so that the plants are at different stages of development. The dead hearts in overwintering oats and rye are seen from November to mid-February in England. In the larger shoots of maize there are often several larvae and there is stunting and distortion of plants, with ragged leaves and small holes where larvae have burrowed across rolled, unemerged leaves. In later generations on oats, the seeds are damaged resulting in lowered yield, but the seeds must be opened and examined carefully to detect attack.
Control of cereal pests is heavily dependant on insecticides and little work has been done on IPM schemes for this pest. An exception is the determination of thresholds for commencement of spraying and in general it seems that unless at least 10% of young shoots are affected it is uneconomic to apply insecticides.
Cultural Control and Sanitary Methods
In oats, plants which have developed beyond the four-leaf stage are very resistant to attack and thus early sowing will minimize infestation. Autumn-sown cereals may be severely infested if the land was previously used for grasses. The danger is minimized if 4 weeks are allowed between ploughing and seeding, to allow existing larvae to die. In sweetcorn and maize the plants are resistant to attack after the five- to six-leaf stage. Where grasslands are seeded they should be ploughed 4 weeks before seeding to allow larvae to die, and seedings in the spring are less vulnerable to attack as they can grow away from damage. It is considered that cutting or grazing grasslands increases the populations of O. frit because it increases the numbers of tillers for infestation.
There is evidence that some cereal varieties are more resistant to O. frit than others (Jonasson, 1975), but some resistance may be due to the speed of growth, so that the plants are beyond the most susceptible stage when fly numbers are highest; other varieties are less attractive to the pest. Varieties of rye [Secale cereale] also show variable resistance and the perennial ryegrasses [Lolium spp.] are less susceptible.
Although there are many recorded natural enemies of O. frit, biological control has not been used to any great extent and given the high population levels of the species in natural grasslands the chances of success seem limited. In Sweden, parasitism may reach 50% in the tiller generation (Nordlander, 1978).
Chemical control of O. frit has been used for many years. Commonly used insecticides include dimethoate and permethrin. It is now only used in severe infestations or where there is a high chance of infestation from a previous crop. For winter cereals an insecticide spray should be used if the crop has more than 10% of shoots infested. This will not save the infested shoots but will prevent further damage. For wheat and maize granular insecticides may be applied at drilling or seed pelleted with insecticides may be planted.
Early Warning Systems
Some research has shown that heat sums will predict the emergence of O. frit in temperate regions (Umoru et al., 1990) and high populations of O. frit in autumn indicate attack the following season.
Field Monitoring/Economic Threshold Levels
Sweep-netting crops will provide an assessment of whether populations have reached an economic threshold for spraying (Volkmar et al., 1990). For cereal crops a sample of shoots should be taken and if more than 10% are infested spraying should be commenced.
In oats economic damage varies considerably from area to area and year to year, but O. frit can cause serious loss of young plants, requiring replanting and reduced yield of grain (up to 90% loss) from the second generation. In maize O. frit is one of the main pests and may prevent establishment of seedlings and cause up to 50% loss of yield in heavily damaged plants. In grasslands the effect on permanent pasture is to reduce yield by a small amount, an effect that is probably more common than realized but not economically worth controlling, but it is more important in preventing or reducing the establishment of new sowings of ryegrass [Lolium spp.] after planting in ground infested by larvae.