New Approach To Pasture Pest Problems

Press, Volume CIV, Issue 30769, 5 June 1965, Page 8

New Approach To Pasture Pest Problems

Long-term studies of insect pests of New Zealand grasslands—notably the grass grub, porina caterpillar, and Argentine stem weevil —which are beginning at Lincoln College with the aid of a £4OOO grant for the first year from the Nuffield Foundation should eventually lead to further understanding of these pests by indicating the points in their life cycles where they are most vulnerable to attack.

“We hope that this work will augment the understanding of pasture pests in New Zealand because we will be doing our research on the most recently developed lines in population ecology,” Dr. R. A. Harrisson, reader in agricultural zoology at the college, said this week. Taking a prominent part in these studies will be Dr. R. P. Pottinger, lecturer in agricultural zoology, who recently returned to New Zealand after spending three years at the McGill University at Montreal under a Commonwealth scholarship. Dr. Harrison said that Dr. Pottinger had been specifically trained in the most modern principles in the field of population dynamics and he would be applying these

principles to the problem of pasture pests in New Zealand. Discussing the reasons for embarking on this project, Dr. Pottinger said that there was evidence that resistance to D.D.T. was developing in grass grubs and this tended to suggest that conventional methods of treating pastures for this pest might have to be adjusted later. At the same i time new regulations might i result in an insecticide in use • becoming obsolete for control of different pasture pests. If there was a better understanding of the natural factors regulating trends in

the populations of such species of insects they would be in a better position to determine future control methods The basis of the work envisaged at Lincoln is that if long-term control programmes are to be successfully applied to insect pests it is essential to determine the “key” or significant factors that regulate natural populations of these pests from generation to generation, and then to proceed by experimentation to understand how the regulatory mechanisms of these factors operate. This will enable more precise use of control measures at a time when populations are most vulnerable. Conceivably the use of an insecticide to depress a population when it is at a vulnerable level may lengthen the recovery period to epidemic levels and therefore reduce the amount and regularity of insecticide application.

It will ultimately allow comparisons to be made between plots treated with insecticide and untreated plots to assess whether insecticides do play a key regulatory role bn pasture pest populations. It will also permit an understanding of what happens where an insect moves from an indigenous habitat to an exotic one and why many insect pests are of greater economic significance in introduced grasslands. This knowledge could be of considerable importance in achieving a cheaper and more efficient control of insect pests in hill country and tussock grasslands where the present use of insecticides is often prohibitive or restricted by the low net return per acre.

These studies should also enable predictions to be made as to when epidemic populations will develop. The formulation of sound integrated control programmes involving the use of insecticides in conjunction with biological control methods should also be possible.

The first step in the direction of these objectives involves the formulation of the most statistically sound and accurate sampling plans

and methods for pasture insects.

The second phase involves the construction of life tables so that the effect of survival within each stage —egg, larva, etc.—on the trends of each population can be determined. A life table is a summary of certain vital statistics of a population beginning with the eggs or individuals starting life together. For every stage of the life cycle it shows the number of deaths, the survivors remaining, the rate of mortality and the expectation of further life. The preparation of these tables is the result of regular sampling of the insects within each stage with measurement of all mortalities for each generation and their respective causes. In order to obtain conclusive results, Dr. Pottinger said, it would probably be necessary to continue these observations for 15 years to take into account varibility of climatic and pasture conditions.

The third phase is an analysis of the key factors to obtain an understanding of their regulatory mechanisms on the trends in populations. One Or Two Dr. Pottinger said that work done by other scientists and himself had indicated that only one or two mortality factors usually regulate insect populations and often these were not the most obvious ones that people assume to be responsible. Initially populations will be studied in tussock grasslands and improved lowland pasture. Later plots under insecticidal treatments will be compared with plots under natural conditions. With this research going on for a number of years at the college, Dr. Harrison said, that the agricultural

zoology department would be training insect Ecologists who could go from the college to organisations like the Department of Scientific and Industrial Research and Department of Agriculture, where the study of the basic biology of New Zealand pests could be done.