The timing of sprays in relation to the stage of development of wheat plants and larvae of the wheat-bulb fly, Leptohylemyia coarctata (Pall.), was studied in the laboratory and field. The main growth of young wheat plants was of the centre shoot and such plants died when the centre shoot meristems were destroyed. Older plants survived by growth of lateral shoots. Sprays of dimethoate, trichloronate and thionazin applied before the larvae had emerged did not kill many larvae in the soil, and only insecticide that entered the plants was effective. Severely attacked two-leaf plants yielded little, whether sprayed or not, whereas three-leaf plants sprayed in early March gave a worth-while increase in yield: older plants had enough shoots to give moderate yields even when not sprayed.

In a large field trial, plants from two sowing dates, 22nd October and 1st November, were both at the late two-leaf stage of growth in late February. Wheat-bulb fly larvae had entered nearly all the shoots but plants from both sowings had buds or small lateral growths hidden beneath the outer leaves. The yields of October-sown plots sprayed on 22nd February, 2nd March, 9th March or 16th March were 28·2, 28·8, 25·5 and 23·2 cwt. grain per acre, respectively, (yield of unsprayed plots = 18.9 cwt./acre) and of November-sown plots were 31·0, 27·5, 17·9 and 14·9 cwt. per acre, respectively, (yield of unsprayed plots = 116 cwt. per acre). Sprays had most effect when applied soon after larvae had entered the plants. Spraying severely-attacked late two-leaf to three-leaf plants gave the greatest benefits because these plants recovered by growth of lateral buds or small lateral shoots.