Applied ecology of the Tasmanian lacewing Micromus tasmaniae Walker (Neuroptera : Hemerodiidae)

Abstract

The Tasmanian lacewing (Micromus tasmaniae Walker) is one of the most common aphid predators occurring in lucerne crops in New Zealand. A comparison of sampling techniques, and the output from a simulation model, suggest that the abundance of this lacewing may have been significantly underestimated in the past. Although the occurrence of aphid predators was erratic M. tasmaniae occurred more often and in far greater numbers (up to 100 m⁻²) than any other predator species. A simulation model for lacewing development in the field indicated that the large adult populations which occurred could be accounted for on the basis of reproductive recruitment. Independent evidence that immigration was not involved in the occurrence of these large populations was gathered using directional flight traps around the field perimeter. The major factors influencing lacewing population dynamics were the availability of aphid prey and, in the autumn, parasitism. Otherwise, survival of all life-histoty stages was high with no evidence of egg or larval cannibalism. Several instances of high lacewing mortality were identified by the model and the lack of any obvious cause for these highlights inadequacies in the understanding of lacewing bionomics. The model, which used a linear relationship (day-degrees) between development and temperature, was incapable of accurately predicting lacewing emergence under field temperatures which fluctuated outside the linear region of the development rate curve. Temperature thresholds and thermal requirements estimated under fluctuating temperatures similar to those in the field produced almost identical model output to those estimated under constant temperatures in the laboratory. Prey species was capable of influencing the rate of lacewing development. M. tasmaniae has the attributes necessary to produce large populations in the short time available between lucerne harvests. The asymptote of the functional response curve is low but the efficiency at converting aphids to eggs is high. Therefore, the lacewing is able to attain maximun reproductive output at low prey densities. A low temperature threshold for development (4-5° C), rapid development and short preoviposition period results in a short generation time (49 days at 15° C). Long adult life, high fecundity and the absence of any form of estivation or diapause, results in complete overlap of generations and multiple generations per year. M. tasmaniae's role as an aphid predator is restricted by its low appetite for prey and by the lucerne management regime currently practiced in New Zealand. Because it consumes relatively few aphids per day the lacewing's ability to destroy large aphid populations is limited. However, this may be offset by its ability to attack aphids early in the aphid population growth phase, and by the large numbers of lacewings which may occur. Under the present lucerne management schemes the large lacewing populations which do occur are forced out of the fields, or die, following harvest. A number of management options for increasing the lacewings impact as an aphid predator are briefly discussed.