EXPERIMENTS ON THE MODE OF ACTION OF DIELDRIN SEED-DRESSINGS AGAINST SEEDLING ATTACK BY THE LARVAL FRIT FLY, OSCINELLA FRIT L.

Experiments with dieldrin seed-dressings were done to see whether the conditions favouring their action against wheat bulb fly larvae apply also to frit fly. Experiments with oats and wheat in boxes and a small field trial with oats showed that fewer newly hatched larvae were killed when dieldrin-treated seed was sown deeply than when sown 1/4-1/2 in. deep; shallow sowing probably puts the insecticide where the larvae are likely to meet it before they attack the plant. Kill was never complete even with dieldrin applied at almost forty times more than the highest rate normally used. This is partly because some larvae hatch from eggs on the plant and these, unlike those from soil eggs, can enter the shoot without touching the seed dressing. Even in the most favourable circumstances (large doses of dieldrin applied to the shallow-sown seed and the plant at the one-leaf stage at the time of attack) there was little evidence that frit fly larvae are killed inside the shoot by systemic action of dieldrin translocated from the seed. This is not because the pales which normally surround the oat seed impair the uptake of insecticide, or because of any other factor specific to oats. Frit fly larvae also survived in wheat from seed dressed with doses of dieldrin that can kill almost all wheat bulb fly larvae by systemic action. Possible reasons for this difference are discussed.