Experiments on the mode of action of insecticidal seed-dressings, especially against leptohylemyia coarctata fall., muscidae, the wheat bulb fly

Certain chlorinated hydrocarbon insecticides applied as seed-dressings to wheat can kill wheat bulb fly larvae either by contact or by systemic action. Newly hatched larvae may be killed by contact action when they pass close to the treated seed on their way to attack the shoot. Such kill depends on the behaviour of the larvae and on the depth the seed is sown. Larvae move upwards after hatching and probably reach the surface soil where they search for the host plant, entering the shoot at a depth of between 1/4 and I in. This behaviour explains results of experiments in which about half of the larvae were killed by contact action of dieldrin on seeds sown 1/4 in. deep and few or none by similarly treated seeds sown at 3 in. It is not known whether the larvae are killed directly by contact action or whether they die of starvation after being deterred. Larvae not killed by contact may be killed by systemic action. The systemic action of dieldrin and aldrin, unlike their contact action, does not protect the plant from damage by wheat bulb fly larvae, which tunnel within the shoot and destroy the growing point before they are killed. gamma-BHC applied as a seed-dressing apparently remains systemically active for less time than dieldrin or aldrin, but while active usually kills, or perhaps deters, the larva before it significantly damages the plant. The uptake of dieldrin and aldrin by the wheat seedling depends on contact between the insecticide and the seed or roots, close contact with the seed seeming especially important. Systemic action is also favoured by late sowing, by sowing the seed very shallowly and by increasing the dose of insecticide adhering to the seed. Dieldrin applied as a seed-dressing at 4.5 mg./seed was not phytotoxic to Cappelle wheat sown within 10 days of treatment, whereas gamma-BHC, even at 0.022 mg./seed, delayed the germination of some seeds. Preliminary experiments with dieldrin seed-dressings against onion fly larvae showed that newly hatched larvae mostly enter the young onion plant at the base of the bulb close to the position of the treated seed and thus are likely to be killed by contact with the insecticide on the seed. There was no evidence that dieldrin seed-dressings act systemically against onion fly larvae.