Zygogramma bicolorata biology and its impact on the alien invasive weed Parthenium hysterophorus in South Africa

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, In partial fulfilment of the requirements for the degree of Master of Science, 2018Parthenium hysterophorus L. (Asteraceae: Heliantheae) is an annual herbaceous plant native to the tropical regions of the Americas, which has become invasive in about 40 countries including India, Australia and South Africa. The weed causes negative health effects on people and animals, reduces crop and animal productions and reduces biodiversity conservation. Both chemical and mechanical control have some negative aspects, making biological control an attractive option. Based on the biological control program in Australia, the leaf-feeding beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) was first released in 2013 in South Africa. However; the beetle failed to establish at many release sites around South Africa, despite laboratory based research indicating that the climate of South Africa is suitable for the beetle. The level of control of the beetle on P. hysterophorus is not well understood at many release sites where populations have established. Additionally, it was not known if any biological factors were restricting establishment of the beetle. Furthermore, an integrated control program using a combination of biological control together with fire is of interest. The likelihood of the beetles surviving any burning regime was considered based on the results of this study. The hypothesis that Z. bicolorata has the potential to control P. hysterophorus in South Africa was tested. A field study at selected sites indicated that when Zygogramma bicolorata successfully established, it reduced P. hysterophorus density (from 50 plants/m2 to zero plants in some cases), and canopy volume (from 20 cm3 to 2 cm3 at best). However, the beetle did not affect flower production relative to plant size or the chlorophyll content of individual leaves on the plant. 5 The hypothesis that Z. bicolorata is affected by soil type and soil moisture due to larval pupation and adult diapause occurring in the soil was also tested. This study showed that Z. bicolorata struggles to establish at sites which have perennially dry soils i.e. below a volumetric soil water content of 10%, or at sites with soil saturated at 100% field capacity. Additionally soils which have high organic content (grey in colour) restrict the beetle. The hypothesis that Z. bicolorata establishment is limited due to predation and parasitism of the eggs was tested. This study found that the eggs of Z. bicolorata experienced high predation rates (between 37.2% and 100%) at a field site. However, it is not known which predators attack the eggs of the beetle. No parasitoids emerged from the eggs. The hypothesis that Z. bicolorata will be affected by the use of fire as a management tool was investigated by studying the diapause depth of the beetle in the soil. The shallow diapause depth of the beetle (less than 2 cm below the soil surface, with the majority of beetles diapausing at less than 1 cm below the soil surface) indicated that the beetles would likely be vulnerable to a burning regime. However, further study would be needed to determine whether lethal temperatures for the beetle are reached at 1 cm and 2 cm below the soil surface. This study suggests that Z. bicolorata has the potential to control P. hysterophorus in South Africa if sites which match the biological needs of the beetle are targeted for releases. Sites with the correct soil moisture and soil texture should be selected. Additionally, release methods should be improved in order to lessen the impact of indigenous predators by releasing high numbers of beetles to saturate predator populations.XL201