The pest status and chemical control of whitegrubs and cutworms in forestry in the Natal Midlands.

Abstract

The limited availability of land to forestry and the ensuing emphasis on intensive silviculture,

developed a renewed interest in soil pests in the establishment of plantations. Ten field trials

were planted over three seasons to determine the mortality factors influencing the

establishment of commercial eucalypt and black wattle plantations in the Natal Midlands, and

simultaneously, to investigate the chemical control of the soil pest component. A complex

of indigenous soil pests contribute to an average 22,9 % failure of Acacia mearnsii and

Eucalyptus grandis seedlings from reaching full establishment. This pest complex, which

includes termites, whitegrubs, cutworms, tipulid larvae, wireworms, millipedes and

nematodes, was responsible for an average 12,3 % of the failure of the plantings to establish.

In the absence of termites, in shallow humic soils, whitegrubs followed by cutworms were

the most frequent and economically important pests. Eucalypts are more susceptible than

wattle seedlings to whitegrub damage when planted in marginal sites. Seedlings in the

summer rainfall region were most susceptible to whitegrub damage from December to April;

and to cutworm damage during the first two months after planting. An average of 398

hectares was annually damaged by whitegrubs and cutworms. The total annual loss in

planting costs and the additional costs of blanking over the three year study period were 1,22

and 2,65 million rands respectively.

Existing non-chemical control applicable to woodlot forestry is reported. Chemical control

as one of the options in the management of whitegrubs and cutworms was evaluated. The

controlled release formulations of carbosulfan 10% and chlorpyrifos 10% at 1,0 g active

ingredient/tree (a.i./tree), gamma BRC 0,6% dust at 0,06 g a.i./tree and the synthetic

pyrethroid deltamethrin 5 % SC at 0,025 g a.i./tree were persistent and effective in

controlling whitegrubs, even when applied early in the planting season. Deltamethrin 5 % SC

at 0,025 g a.i./tree was also successful in controlling cutworms.