Seedling survival, yield and seed production of three species of annual medics exposed to lucerne aphids

Abstract

The effects of spotted alfalfa aphids and blue-green aphids on the seedling survival, yield and seed production of Medicago truncatula var. truncatula cultivars Cyprus and Jemalong, M. polymorpha var. vulgaris and M. minima var. minima were investigated in field plots to which three insecticide treatments were applied: sprayed continuously to the post-flowering and haying-off stage; sprayed only until flowering ; and unsprayed. In the unsprayed plots all four medics were suitable hosts for both aphid species. The highest number of blue-green aphids developed on M. truncatula cv. Cyprus and the highest number of spotted alfalfa aphids on M. polymorpha var. vulgaris. Aphid numbers were relatively low from emergence to establishment and had no effect on the seedling mortalities of the medics in the unsprayed plots. The relatively low aphid numbers through winter also had no effect on dry matter production, but as numbers of both species increased in the spring the mean yields from all the unsprayed plots at successive harvests in September, October and November were reduced by 50, 54 and 55%, respectively. Flowering commenced in M. truncatula cv. Cyprus, M, polymorpha and M. minima before aphid numbers were high enough to reduce herbage yields in the unsprayed plots. However, M. truncatula cv. Jemalong flowered 10-1 4 days after the other medics, and its seed yields from the unsprayed plots were significantly lower than those of M. truncatula cv. Cyprus. The germination percentage of seed produced in the unsprayed plots of M, polymorpha was significantly lower than that of the other unsprayed medics.