ABSTRACT

Eucalyptus grandis and Acacia mearnsii are among the South African forest industry’s most planted commercial species. Whitegrubs are high-status pests affecting plantation forestry; they are root-feeding insects and cause poor seedling growth and elevated mortality. The aim of this study was to elucidate the impact of whitegrub damage on the growth of E. grandis and A. mearnsii trees during the first year of the establishment stage in sites previously planted with A. mearnsii in the KwaZulu-Natal Midlands of South Africa. The hypothesis was that the prophylactic application of insecticides against whitegrubs would increase tree height at six or 12 months after planting. Seven insecticides were assessed in 12 treatments over three trials in Bloemendal, KwaZulu-Natal. Statistical analyses were performed using PRIMER and SAS. Whitegrubs were the predominant mortality factor (17.1%), followed by pathogens (3.3%) and 12 other factors that cumulatively caused the remaining (7.0%) mortality. Pegylis sommeri, Schizonycha affinis and two unknown Maladera species were the most important, typical and prevalent whitegrub species, but their ranking differed between A. mearnsii and E. grandis, suggesting a host preference. The prophylactic application of insecticides against whitegrubs significantly increased tree height for E. grandis. Acacia mearnsii tree height, however, did not consistently show a significant effect with insecticide control, presumably because of additional mortality by pathogens. The crop-rotation scenario may have potentially benefited E. grandis over A. mearnsii and warrants further investigation. Whitegrubs can kill seedlings and reduce tree height when left uncontrolled.

摘要

Eucalyptus grandis和Acacia mearnsii是南非林业种植最多的商业物种。白蛴螬是影响人工林的高级害虫；它们是食根昆虫，会导致幼苗生长不良和死亡率升高。本研究的目的是阐明在以前种植过A. mearnsii的地点的建立阶段的第一年，whitegrub 损害对E. grandis和A. mearnsii树木生长的影响在南非的夸祖鲁-纳塔尔中部地区。假设是，在种植后 6 个月或 12 个月预防性使用杀虫剂对抗白蛴螬会增加树高。在 Bloemendal、KwaZulu-Natal 的三个试验中，对 12 个处理中的 7 种杀虫剂进行了评估。使用 PRIMER 和 SAS 进行统计分析。白蛴螬是主要的死亡率因素（17.1%），其次是病原体（3.3%）和其他 12 个累积导致其余（7.0%）死亡率的因素。Pegylis sommeri、Schizonycha affinis和两个未知的Maladera物种是最重要、最典型和最普遍的白蛴螬物种，但它们在A. mearnsii和E. grandis之间的排名不同，建议主机偏好。预防性应用杀虫剂对抗白蛴螬显着增加了E. grandis 的树高。然而， Acacia mearnsii树高并未始终显示出对杀虫剂控制的显着效果，这可能是由于病原体造成的额外死亡率。作物轮作情景可能使E. grandis比A. mearnsii受益，值得进一步研究。当不受控制时，白蛴螬可以杀死幼苗并降低树高。