Infestation Density, Damage Severity and Potential Biocontrol of Eucalyptus Gall Wasps, Leptocybe invasa (Fisher & La Salle) and Ophelimus maskelli (Ashmead), in Tanzania

Abstract

In Tanzania, eucalypts are a significant fast-growing hardwood tree used to produce wood, paper, timbers, poles, bioenergy, and numerous other minor goods and services. Tanzania introduced eucalypts in the 1890s intending to supplement wood supplies from natural forests. Eucalyptus spp. is the world's most widely planted hardwood timber species, native to Australia, Indonesia, the Philippines, Timor, and New Guinea. Distribution is attributable to the ability exceptionally adapt to diverse ecological conditions and the substantially arraying products and services they offer. Leptocybe invasa and Ophelimus maskelli, two common Australian wasps (Hymenoptera: Eulophidae), have caused significant damage to eucalypt plantations in Tanzania since 2005 by inducing galls on petioles, young stems, and leaves that inhibit vascular bundle conductivity and photosynthesis. Although biological control is seen as the greatest option for conventional control measures from an ecological and financial standpoint so far, no eucalyptus gall wasp parasitoids have been documented or reported. Similarly, the number of parasitoid species and their parasitic capacity is unknown. Due to limited knowledge and literature. Therefore this study was set out to examine the current infestation, damage severity, and to identify potential natural enemies for controlling this pest in Tanzania. The research was conducted from October 2021 to August 2022 in three government-owned plantation forests, namely the Korogwe, Morogoro and North Ruvu forests that were planted with several Eucalypt spp. Samplings were done using systematic approach and field assessments involved examination of infested crown parts, highly infested species, and invasive capacity of gall inducers by counting and measuring gall length and diameter. Few selected trees were used to assess damage severity. Galls from infested eucalypt germplasms were collected and reared in the laboratory until the wasp emerges. The emerged wasps were primarily sorted and identified using morphological characters (Keys) under a stereomicroscope at the Sokoine University of Agriculture, Tanzania, and the Insect Biosystematics Laboratory at IPB University, Indonesia. Data were analyzed using Ms Excel, Minitab 19, and R statistical software. The analysis included computations of percentages of infestation and parasitoid parasitization, averages of galls per tree, and the relationship between stand altitudes and the infestation magnitude, damage index, and gall inducers. The results were summarized in tables and graphs and included in this final thesis. The results indicated that clonal hybrids were highly infested than E. tereticornis this may be due to poor adaptation less tolerance and resistance traits of clones compared to E.tereticornis raised from seeds. The middle and lower tree crown parts were highly infested compared to upper canopy. L. invasa had a higher invasive capacity in all locations and both host plants than O. maskelli. Damage severity generally ranked as severe in all eucalyptus germplasms assessed. Two species of parasitoids of eucalyptus gall-inducers were identified as Megastigmus sp. and Quadrastichus mendeli. Parasitization rates (collectively) in Korogwe, North Ruvu, and Morogoro forest plantations were 29.7%, 64%, and 33.3%, respectively. A higher parasitization rate was recorded from L. invasa galls on E. grandis x E. camaldulensis clones. Ecological factors such as temperature, wind, and rainfall influenced gall wasp infestations. Control efforts should focus on biological control agent candidates including the two parasitoids; Megastigmus sp. and Q. mendeli reported in this study findings. However further studies are required to determine their distributions, host species range and specificity, interaction, and efficacy in controlling the population of eucalyptus gall wasps. Breeding of resistant genotypes, and greenhouse-gall screening. Nevertheless, mass trapping using a yellow sticky trap and cattle stable can be used to monitor the pest population immediately. Furthermore, regular assessments of other insect pests and diseases and their management options are also necessary for Tanzania, as the infestation of some other insects and diseases were observed during field assessments.