The effect of temperature on the feeding and development of Neochetina eichhorniae weevil larvae on water hyacinth (Eichhornia crassipes)

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. Johannesburg, March 2017.Water hyacinth (Eichhornia crassipes) is an example of an alien invasive plant, threatening ecosystems around the world. Taking into consideration all the negative impacts caused by the proliferation of water hyacinth, many control measures have been implemented to manage this weed. These include mechanical control, chemical control and biological control. But in recent years, the focus has shifted towards natural enemies for control (biological control) of water hyacinth because they have many advantages for environmental safety. However, in South Africa, success is limited in most parts of the country. Factors affecting the success of biological control of water hyacinth in South Africa have been attributed in part to weather conditions. Therefore, further studies and other strategies are needed to reinforce the biological control of water hyacinth. This is why this research investigated the influences of temperature on Neochetina eichhorniae larvae feeding and development on water hyacinth, to contribute to the future management of water hyacinth biological control. To measure the effect of temperature on the feeding and development of Neochetina eichhorniae larva, experiments were conducted at four fixed temperatures (15 °C, 20 °C, 25 °C, and 30 °C). Larval feeding is expected to be greater and the development faster at warm temperatures compared with cold temperatures. The results of this study confirmed that, both feeding and larval development increased with an increase in temperature. The highest larval feeding damage was recorded at 30°C and the lowest at 15°C. Similar to the feeding of larvae, warmer temperatures were more favourable to the growth of larvae than cold temperatures; all larvae found at 30°C were in their third instars (with 0.72 mm being the biggest headwidth found) while all larvae at 15°C were in the first instars (with 0.28 mm being the smallest headwidth found). Even if the optimal temperature for larval performance took place at 30 °C (with a high amount of biomass removal), it was at 25 °C that the larvae had a larger effect on the growth of water hyacinth. Larval feeding damage reduced the growth of water hyacinth by 30 % at 25 °C and only affected 24 % of water hyacinth growth at iv 30 °C. Therefore, for the biocontrol of water hyacinth, the ideal temperature to slow the progression of the weed will be recommended at between 25- 30 °C.LG201