Effect of temperature and humidity on the rate of predation of Tasmanian lacewing, Micromus tasmaniae walker (Neuroptera : Hemerobiidae) on cabbage aphid, Brevicoryne brassicae (Hemiptera : Aphididae) on cabbage plants : A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Applied Science

Abstract

The Tasmanian lacewing (Micromus tasmaniae Walker) is a common aphid predator and is known from Australia and New Zealand. It is a cold-adapted insect and there is no diapause in any life cycle stage. A review of the literature on Tasmanian lacewing and cabbage aphid, Brevicoryne brassicae (L.) reveals that there is a need to determine the rate of predation by lacewing larvae under different combinations of physical environmental factors, e.g., temperature, humidity and photoperiod before this predator can be exploited as a biocontrol agent to control cabbage aphids in cabbage crops. The rate of predation, (e.g., prey consumption per larva I day) and the total number of prey consumed per larva during the period of larval development was determined under different constant temperatures, fluctuating daily temperatures and different constant relative humidities. The influence of temperature and humidity at constant photoperiod (16L : SD) on larval mortality, duration of larval development and physiological time required to complete larval development has been determined in this study. In addition the total mean number of prey consumed by Tasmanian lacewing larvae during the period of larval development using a range of arenas with different internal surface areas was also investigated. The main findings of this study were that the rate of predation and the rate of larval development was linear in the temperature range 10°C - 25°C. The rate of predation under fluctuating daily temperature was higher than that of the corresponding mean temperature. Humidity also influenced the larval development period and the physiological time required to complete development. Larvae took the least time to develop at 60% RH but this increased with increasing relative humidity from 60% RH to 90% RH. A rise and fall of predation rate was noticed with increasing humidity. Arenas with different internal surface areas showed an impact on the rate of predation of cabbage aphids by Tasmanian lacewing larvae. The mean total predation increased with increasing surface area up to a certain level (e.g., 300 ±25 cm² ) and thereafter remained unchanged. Discussion is provided on the number of Tasmanian lacewing larvae required to be released in the field to achieve successful biological control of cabbage aphids as well as future research required commercialise this biological control agent.