EXPERIMENTS ON THE MODE OF ACTION OF INSECTICIDAL SEED‐DRESSINGS, ESPECIALLY AGAINST LEPTOHYLEMYIA COARCTATA FALL., MUSCIDAE, THE WHEAT BULB FLY

Abstract

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 1 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. γ‐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 γ‐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.

Further Seed Dressing Trials against Wheat Bulb Fly (Leptohylemyia coarctata (Fall.)) in East Anglia, 1966–68

“…If this assumption is correct, then the larvae found dead in the shoots must have picked up a lethal dose of insecticide when moving towards the plant through the "zone of protection" afforded by these chemicals. This emphasizes the importance of shallow drilling (Way, 1959).…”

Persistence and distribution of insecticidal seed‐dressings for control of the wheat bulb fly, Leptohylemyia coarctata

“…If larvae move in the upper layers of soil before burrowing down to enter the bulb of the plant, as suggested by Way (1959) and by Long (1958), larvae could enter plants without coming into contact with insecticide, and Way (1959) showed that with deep sowing y-BHC and dieldrin seed-dressings were less effective against wheat bulb fly either as contact or as systemic poisons.…”

“…Dieldrin and y-BHC seed-dressings have been used for more than 10 years to control wheat bulb fly. Results of many trials have shown that y-BHC acts mainly by preventing larvae entering the plants and kills few larvae that succeed in entering, whereas dieldrin acts mainly by killing larvae after they enter (Gough & Woods, 1954;Bardner, 1958;Way, 1959;Maskell & Gair, 1961 ;Gough, Woods, Maskell & Towler, 1961). Both materials are less effective in organic soils than in mineral soils (Gough et al, 1961.…”

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

“…The examination for pupae made on 15 August 1958 showed that the dieldrin treatments did not kill frit larvae during their development in the plant. Similar dieldrin treatments would almost certainly have killed nearly all wheat bulb fly larvae by systemic action (Way, 1959). The species of host plant therefore seems not to be responsible for the dieldrin seed-dressings failing to kill frit fly larvae by systemic action.…”

“…A dieldrin seed-dressing will effectively control attacks on seedling crops by some Diptera Muscidae. For example, it prevents damage to ionions by the onion fly, Delia antiqua Meig., probably by contact action on the young larva before it enters the plant (Way, 1959). The action of dieldrin against wheat bulb fly Lepto-hyZemyia coarctata Fall.…”