Effect of aphids and mites on herbage and seed production of subterranean clover (cv. Daliak) in response to superphosphate and potash

Abstract

The dry matter production (DM) and seed yield of subterranean clover (Trifolium subterraneum L. cv. Daliak) were reduced by infestations of redlegged earth mite (Halotydeus destructor Tucker) and blue-green aphid (Acyrthosiphon kondoi Shinji) during spring growth, flowering and burr burial. The dominance of these pests varied with season. The effects of spraying with insecticides on the DM and seed yield responses to superphosphate and potassium chloride fertilisers were measured. Responses to superphosphate were described by Mitscherlich functions for each of 3 levels of potassium chloride, except for seed yields with pest sprays. At optimum levels of superphosphate and potassium chloride, controlling pests increased DM by up to 150% (from 4.37 to 6.52 t/ha). For all levels of superphosphate, spraying to control pests where no potassium chloride was applied significantly increased DM over that on unsprayed plots that were fertilised with potassium chloride. The maximum DM response to superphosphate application was achieved at 15-20 kg P/ha. With optimum superphosphate, the value for DM depended on the combination of spraying for pests and amount of potassium chloride applied, generating a series of Mitscherlich response curves for superphosphate application with differing maximum yields. With optimum superphosphate applied, the least DM recorded within a season was 3.47 t/ha (pests not sprayed, nil potassium chloride), and the most was 6.52 t/ha (pests sprayed, 120 kg potassium chloride/ha), an increase of about 180%. At optimum levels of superphosphate and potassium chloride, controlling pests increased seed yield by up to 380% (from 290 to 1100 kg/ha). With optimum superphosphate, seed yield within a season ranged from 210 (pests not sprayed, nil potassium chloride) to 1100 kg/ha (pests sprayed, 120 kg potassium chloride/ha), an increase of 524%. With pests sprayed, seed yield declined with superphosphate applications >20 kg P/ha; the relationship was best described by a quadratic function. With pests not sprayed, seed yield did not decline with increasing amounts of superphosphate, and the relationship fitted a Mitscherlich function.