Some aspects of the biology, population dynamics and economic status of Wiseana cervinata (Walker) (Hepialidae: Lepidoptera)

Abstract

Entomology, like many other biological disciplines has developed into a quantitative science in nearly all aspects of the subject. Insect ecology is no exception, and in fact, has probably received more attention recently with the rapid development of insect population dynamics, the emergence of synecological studies, and the challenge such studies offer to statisticians and mathematicians. There is also a growing awareness of the significance and limitations of the environment in which insects play an important role. Searching questions are now being asked concerning the effect of insects and their control on man's existence (see Report of Environmental Pollution by the U.S.A. President's Science Advisory Committee 1965). The only universally meaningful way to attempt to answer these questions and to co-ordinate the work involved is, firstly, to quantify or measure the environmental parameters involved and, secondly, by their rational manipulation towards intelligent goals. The former is aptly described by Chant (1964) in relation to insect control, and the latter by Geier (1966). With the current specific emphasis on costs of food production in New Zealand, insect ecological studies with an economic basis should be directed towards the provision of urgently required knowledge to help answer the following questions.

1. How much harm or benefit is brought about by both the direct and indirect actions of insects? 2. What are the various courses of action available to minimise the benefit of insects? 3. What are the costs involved in pursuing these aims?

The theme of this thesis is based on this ecological economic cost benefit philosophy. Not only have various quantitative entomological tools been used, but economic principles have been included in a conceptual framework to study the ecology and economic status of the Wiseana cervinata (Walk.) complex in New Zealand. This pest and Costelytra zealandica (Wh.) are considered the two most important insect pests in New Zealand. As pastoral farming has been, and will remain, the basis of this country's economic structure for some time, damage caused by these pests is of national and farm significance. This conclusion, together with recent political stimulus, prompted the demand for wider and urgent research on the control of porina and the associated costs. Studies in porina population dynamics had been initiated as early as 1966 (Pottinger, 1967) but an epidemic population in 1967, coupled with the phasing out of D.D.T., increased the need for alternative control measures and greater understanding of the insect's behaviour. Because of the urgency to obtain a practical and cheap method of porina control, and the preliminary nature of this study, a broad based quantitative approach was used to develop meaningful but limited life tables. It was decided that such an approach would be best for a univoltine species and in view of the limited aims of the thesis it was not considered essential to develop complete and detailed life tables or to explore in depth all behavioural aspects of the insect. Instead, effort was concentrated into the study of those aspects which had some direct bearing on the practical and economic evaluation of the pest. It was thought that porina was amenable to ecological studies using life table methods. Except for the egg and juvenile larval stages it has reasonably large larval and adult stages and is relatively easy to sample. Areas for study are not difficult to find, but the insect does have the disadvantage (for life table work) of being univoltine. This could be advantageous in that there are no overlapping populations. The project briefly outlined below was designed to investigate the effect of porina populations on farm production and to study some aspects of the insect's ecology.

1. Development of sampling techniques for use in life table and other ecological studies.

2. (a) The study of the life cycle and natural history of the pest. (b) The development of life tables. (c) Economic assessment of the pest.

3. (a) Determination of the 'key mortality factors' regulating porina populations. (b) The practical testing of 'key mortality factors' as a means of control.

4. The development of mathematical models leading to predictive equations and simulation studies.