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Aphids can alter the pattern of seed softening in burr medic

C.K. Revell

WA Department of Agriculture, Dryland Research Institute, PO Box 432, Merredin WA 6415

The presence of aphids in annual medic pastures during late winter and spring can lead to reductions in both dry matter and seed production (1). The loss in seed production results from a reduction in the number of sites of seed production, fewer seeds per burr and smaller seed size (Revell, unpublished data). The aim of this study was to determine whether the presence of aphids during seed development could also influence the subsequent pattern of softening of hard (impermeable) seeds.


Santiago burr medic, Medicago polymorpha L., was grown on a neutral red clay loam soil at Merredin (WA) in 1990. Cowpea aphid, Aphis craccivora, and blue-green aph id, A cyrthosiphon kondoi, were both present from mid-August. Half the experiment was sprayed regularly for aphid control. Seed yields were measured in November. Burr samples were collected on 19 November 1990 and 12 April 1991 when seed from groups of 20 burrs were separated by hand for germination tests. The experiment was ungrazed over summer.

Results and discussion

The results show a substantial decline in seed production when aphids were not controlled. Part of this decline was attributable to a reduction in seed size. Initial levels of soft seeds in November were low for both treatments. The percentage of soft seeds present at the end of summer, however, was substantially higher where aphids had not been controlled in the preceding spring and 3% of the soft seeds were not viable. It appears that the presence of aphids during seed development reduces the longevity of seed hardness, presumably through some restriction on the development of the seed coat. While the higher proportion of soft seed will partly compensate for reduced seed production, the medic content of the pasture would still be expected to decline with time where aphids are present. In addition, reduced seed size may be reflected in lower seedling vigour of plants germinating in the following year.


This work was funded by the State Wheat Industry Research Committee of Western Australia.


Lodge, G.M. and Greenup, L.R. 1980. Aust. J. Exp. Agric. Anim. Husb. 20, 457-462.

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