THE EFFECT OF ROOT DAMAGE CAUSED BY SIMULATED WHITE GRUB ATTACK ON THE GROWTH, YIELD AND WATER-USE OF GROUNDNUT PLANTS

White grubs (larval scarabaeids) are now recognized as being important pests of groundnut (peanut) in many parts of the world because of the ir ability to damage roots. A method of simulating white grub damage t o groundnut plants was developed to obtain an indication of how the fe eding activity of these insects influences plant growth. The effect of root cutting and drought stress on water uptake and biomass productio n was evaluated, with roots being cut at three depths in the late vege tative and early podfilling stages. As groundnut plants are often grow n under conditions of drought stress, the effects of which would be ac centuated by root damage, this factor was introduced into the experime nt. Plant water-use measurements indicated that the root systems of pl ants cut 30 days after emergence (DAE) and watered twice a week became fully functional again after 40 days. Pod and total biomass productio n were however significantly less than those of the uncut control plan ts, with drought stress reducing yields below the well-watered control s, particularly when cut at 10 cm below the soil surface. The root sys tems of plants cut 51 DAE did not regrow to any appreciable extent, an d rates of plant water-use remained less than half of the uncut contro l plants. Over all treatments, there was a strong positive correlation between total (and pod) biomass and plant water-use. It was concluded that the phenological stage of the plant at which root damage occurre d had a profound influence on the subsequent recovery in root growth a nd function, and ultimately on pod yield.