Influence of Soil Moisture and Organic Matter on Scarab Damage to Grasses and Clover

Abstract

On the northern tablelands of New South Wales in Australia, where native and improved pastures have been examined for scarab larvae over the past few years, the predominant species belong to the melolonthine genera Rhopaea and Sericesthis, and the ruteline genus Anoplognathus. It has become clear, however, that dense aggregations of larvae are not always associated with visible damage to pastures. In this region damage to pastures by scarabs is most commonly'encountered between May and September, especially in dry weather. On the north coast of New South Wales damage is said to be most severe in autumn and winter, after active growth of grass has ceased (Braithwaite 1959). Field observations on Sericesthis geminata Boisd. (= S. pruinosa Dalman) in Canberra suggest that damage is most severe where soil moisture is deficient (Carne & Chinnick 1957). In the United States it has been shown that keeping soil moisture and nutrients at adequate levels enables vegetation to withstand scarab feeding (Graber, Fluke & Dexter 1931). It is clearly desirable to elucidate the relationship between scarab feeding on roots and the growth of pastures under a variety of soil and climatic conditions. In the simplest form of association between a fixed density of the poikilothermic scarabaeid larvae and a pure stand of one species of pasture plant, the metabolic rate of each component varies independently in response to daily and seasonal environmental fluctuations. Furthermore both plant and animal may respond to moisture, temperature or other gradients. The resultant pasture yield is the outcome of an interaction between these responses. This is usually expressed as an effect of scarab feeding on pasture growth, by an absolute or relative measure of damage to pastures. The present paper deals with the effect of soil moisture and the addition of organic matter in quantities sufficient to increase plant growth, and to provide alternative or additional nutrition for the scarab larvae.