The Effect of Liming on Whitegrub Numbers in the Midlands Area of the South African Sugar Industry

It has been suggested that raising soil pH through liming could reduce the impact of whitegrub on crop yield in the treated soil. To test this hypothesis, whitegrub incidence was monitored in a trial designed to examine the effects of various lime treatments on crop yields of varieties N16 and N12. Lime was applied at two rates (5 and 10 t/ha) and in combination with gypsum at 5 t/ha each. Results showed that, for N16, crop yield increased from 62 tons cane/ha (control) to 93 tons cane/ha (10 tons lime/ha treatment), and ERC increased from 6.4 to 10.8 tons sucrose/ha. Values for N12 were 85 tons cane/ha (control) to 87 tons cane/ha (10 tons lime/ha treatment). ERC increased from 10.3 to 11.5 tons sucrose/ha. The aluminium saturation index (a measure of the impact of liming on soil pH) declined from 80% (control) to 20% (10 tons lime/ha treatment). However, no significant decrease or increase in white grubs numbers was recorded from any of the treatments. In the control plots, mean counts of 21.2 grubs per sample were recorded, while counts of 18.8 grubs were recorded in the highest lime treatment (LSD 8.9). These results show that, although crop yields responded positively to the lime treatments, there was no clear association between the lime treatments and the number of whitegrub recovered. It is concluded that liming has no significant effect on the incidence of whitegrub. Introduction It has been asserted that the use of very high levels of lime eliminates whitegrub, and represents a logical control measure which has the additional advantage of correcting the balance of soil nutrients. However, in an industry-wide survey Way (1999) could find no clear correlation between whitegrub numbers and soil pH. In 2001 there was an outbreak of whitegrub in the New Hanover, Seven Oaks and Blinkwater areas of the KwaZulu-Natal Midlands. This presented an opportunity for a lime calibration trial, situated in the middle of this area, to be used to examine the relationship between aluminium saturation index (ASI), soil pH and whitegrub numbers. Methods and materials Whitegrub counts were carried out on a lime calibration trial at Dalton, previously reported by Schumann et al. (1999) and Nixon et al. (2003). The design was a split plot, five replicate latin square (4 x 5) with varieties N12 and N16 balanced in columns. The main treatments applied at planting were control (no lime), 5 and 10 tons dolomitic lime/ha, and a combined treatment of 5 tons dolomitic lime and 5 tons gypsum/ha.