Influence of soil moisture and organic matter on scarab damage to grasses and clover.
The following is based largely on the author's summary. In a pot experiment in New South Wales in 1963, second-instar larvae of the Melolonthid Rhopaea morbillasa Blkb. and a Rutelid of the genus Anoplognathus were found to damage the roots of temperate grasses (Dactylis glomerata, Lolium perenne and Phalaris tuberosa) and white clover (Trifolium repens) growing separately in proportion to the root density. The plant species all suffered equal root damage, which was significantly affected by soil moisture (partly because of the depth of feeding induced by moisture gradients) and by the addition of cow manure (partly through the mechanical effect on intake by the soil-ingesting larvae). These unexpected responses made the assessment of the relation between root damage and plant vigour impossible. For the grasses, the ratio of weight of roots to weight of foliage (root/top ratio) averaged 1.8 in the absence and 0.9 in the presence of the larvae at all levels of soil fertility, indicating that 50% of the roots were redundant within the experimental conditions. On this basis, the calculated depression in yield of foliage agreed with the actual damage, which varied between 85% at 10% moisture without manure and 16% at 25% moisture with manure. In clover, the critical root/top ratio in the presence of larvae was about 0.4, whereas without them it varied between 3 at low and 0.3 at high soil fertility. A disproportionate reduction in the yield of clover foliage is explained by the varying proportion of redundant roots in the different treatments.