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Understanding the impacts of foliar insect pests on eucalyptus species growth and the development of appropriate management strategies

Nelson Mandela University

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  • Title:
    Understanding the impacts of foliar insect pests on eucalyptus species growth and the development of appropriate management strategies
  • Author: Dlamini, Lindokuhle Sithembiso
  • Subjects: Eucalyptus -- South Africa ; Trees -- Breeding Wood-pulp industry -- South Africa
  • Description: Within South Africa (SA), eucalypts contribute significantly to the commercial production of pulpwood, treated poles, wood chips and saw timber. Due to their rapid growth and desirable timber properties, 47% of the afforested area within SA is planted with various eucalypts and their hybrid combinations. Biotic and abiotic factors pose a risk to the productivity of eucalypts, with the most significant risk currently associated with the increased impacts of foliar insect pests on tree productivity, for example Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae); Gonipterus scutellatus Gyllenhal (Coleoptera; Curculionidae); Thaumastocoris peregrinus Carpintero and Dellape (Hemiptera: Thaumastocoridae); Glycaspis brimblecombei Moore (Hemiptera: Psyllidae) etc. Past and current research has focused more on the individual components of biological, cultural and chemical control, with limited research conducted on the practical integration of these management strategies. Although the combination of biological and cultural control methods for long-term management would be preferred, the use of environmentally acceptable insecticides can contribute to the short-term management of insect pests where necessary. Environmental constraints (associated with forest certification) around the use of insecticides, means that there are currently few insecticides available for controlling foliar insect pests of eucalypts within the SA forest industry. Due to the recent increased introduction of insect pests, there is also a lack of data related to their impacts on tree growth, as well as the success of any management methods used. To improve our understanding regarding these components, three groups of trials (five in total) were implemented in the summer rainfall region of SA to quantify the impacts of insect pests on eucalypt growth, test integrated management practices (chemical and cultural management), and test environmentally acceptable insecticides. In 2010, two insect exclusion trials were established at Palm Ridge and Teza plantations in KwaZulu-Natal to quantify the impact of insect pests on two Eucalyptus hybrids. The trial locations were selected adjacent to compartments for which the pests Gonipterus spp. and Thaumastocoris peregrinus were present. Each trial had an insecticide treatment with an active ingredient of alpha-cypermethrin at a rate of 100 g l-1 and a control treatment where trees were not sprayed. Insect pests were collected after every spraying event. A greater number of insect pests were collected from the Palm Ridge site. Gonipterus spp. were the predominant insect pests collected. No significant growth differences were recorded between treated and untreated trees at Teza. Measurements taken at Palm Ridge indicated that insecticide-treated trees had significantly greater tree growth than untreated trees. Final measurements showed a v 25% reduction in volume for the control treatment compared to the insecticide treatment. Results of this study indicate that timing and duration of insecticide application could hold larger and additive benefits for insect exclusion. Due to the lack of non-hazardous registered insecticides, and the susceptibility of many of the eucalypts grown within SA to L. invasa, a trial was initiated in 2011 in Zululand (KwaZulu-Natal). A 3 x 4 x 4 factorial combination of 48 treatments were replicated 3 trials and arranged in a split-split-plot design. The factors included insecticide type as the whole plot (fenvalerate, azadirachtin and imidacloprid), timing of insecticide application as a sub-plot (0, 3, 6 and 9 months) and type of hybrid as the sub-sub-plot (a susceptible and tolerant E. grandis x E. camaldulensis hybrid (GC_s and GC_t) and a susceptible and tolerant E. grandis x E. urophylla hybrid (GU_s and GU_t)). To determine insecticide efficacy, plots of untreated clones were included as control treatments within each replicate. The incidence and magnitude of L. invasa damage and tree performance were assessed for up to 5.5 years of age. L. invasa damage recorded over the trial period indicated that irrespective of treatment, damage was lower in the GU clones than the GC clones resulting in a 63.9 m3 ha-1 volume difference at 5.5 years. In addition, the difference between the GU_s and GU_t clones (3.2 m3 ha-1) was marginal when compared to the GC_s and GC_t clones (37.2 m3 ha-1). A decrease in GC stocking correlated with the period when there was a high damage incidence recorded for L. invasa. Although the three insecticides tested delayed the onset of L. invasa damage, they did not result in improved tree performance at 5.5 years. Results indicate that the planting of a tolerant species (hybrid in this trial) was the most viable option for the management of L. invasa. Two trials were implemented in the KwaZulu-Natal Midlands in October 2016 (at Ingwe and Balgowan plantations), to test the efficacy of eight different insecticides (synthetic and organic) for the management of foliar insect pests. The Ingwe trial was planted with Eucalyptus dunnii and the Balgowan trial was planted with Eucalyptus badjensis. The insecticides tested were azadirachtin, deltamethrin, imidacloprid (liquid and tablet), Beauveria bassiana, Bacillus thuringiensis, potassium silicate and a combination of plant oil extracts (garlic oil, soybean oil and capsicum oleoresin extract). The imidacloprid tablet, deltamethrin and an untreated control were implemented as additional treatments. The insecticides were tested at the recommended rate (single rate) and at double the recommended rate (double rate), except for the insecticides in the additional treatments which were only tested at the single rate. These insecticides were applied three times over a period of four months, and tree growth measurements (Gld/Dbh and Ht) were taken four times over a period of seven months. Gonipterus spp. vi was the only insect pest found in both these trials, thus the insecticides were tested for their efficacy in managing this specific insect pest. A higher population and damage of Gonipterus spp. was found in the E. dunnii trial and very little to no damage was observed in the E. badjensis trial. All insecticides tested were not effective in managing Gonipterus spp. in these specific sites. A low population of insect pests and incorrect timing and method of application of some of the insecticides could offer a plausible explanation of results. Results from these trials indicate that where foliar insect pests occur in high abundance, significant tree-growth reduction occurs. Although insecticides (deltamethrin - for G. scutellatus; imidacloprid for L. invasa) can be used for the short-term control of these insects, their commercial benefits may be reduced due to the number of repeat applications required and/or with high levels of infestation. Of the eight environmentally acceptable insecticides tested for foliar insect control, the low abundance of insect pests present in the trials meant that their efficacy could not be quantified. If an alternative insecticide were to be found, its application would need to be linked to the environmentally stimulated outbreaks of specific insects. Of the treatments implemented, the selection and planting of eucalypts with known resistance to specific insect pests currently remains the most viable management strategy. Breeding for increased tolerance within resistant eucalypts may also provide additional benefits. Although the above research showed the negative impacts of foliar insect pests on tree growth, it also indicated that no single control method is viable for all insect pests. Future research should look at the integration of different management methods while incorporating more insect pests and different seasons in the study.
  • Publisher: Nelson Mandela University
  • Creation Date: 2018
  • Format: 73 leaves
  • Language: English
  • Source: South East Academic Libraries System Digital collections of Nelson Mandela University
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