Populations of stem-boring Diptera in leys and in subsequent winter wheat and the effects of aldrin and phorate

SUMMARY In 1957-61 stem-boring larvae were removed for identification from grasses shortly before ploughing in October and from the subsequent crop of wheat. At Hurley populations of up to 700 larvae per sq.yd. including up to 400 Meromyxa spp., 270 Oscinella vastator and 250 O. frit agg. were found in the leys and up to forty larvae per sq.yd. of all species were found in the wheat. Migration of the larvae from the buried grasses into the wheat was associated with soil temperatures above 7·2°C. at 4 in. depth. The larvae occurred in the wheat mainly in February and March. A survey of twenty-nine leys in the southern half of England gave an average of eighty larvae, including twenty-three larvae of O. frit agg., per sq.yd. In wheat grown after the leys there was an average of only two larvae per sq.yd. Attempts were made to kill the migrating larvae by applying aldrin and phorate dusts to the grassland before ploughing and by applying aldrin granules to the soil surface after establishing the wheat. To ensure different levels of infestation the larval population was doubled on some plots by ploughing in a double layer of turf. The aldrin dust treatment of the grassland killed 70% of the migrating larvae. The aldrin granules and the phorate were less, if at all, effective.