Attack On Grass Grubs

Press, Volume CV, Issue 30983, 12 February 1966, Page 10

Attack On Grass Grubs

The introduction of thynnid wasps from Patagonia in South America was only one phase of an intensive and extensive campaign against the New Zealand grass grub. Costelytra zealandica, Mr B. B. Given, of the Entomology Division of the Department of Scientific and Industrial Research at Nelson, said this week in response to an inquiry from “The Press.” Mr Given is in charge of biological control for the division.

Other phases of this maj or project were ecological study (the study of the insect in relation to its environment). chemical control in which acceptable chemicals were constantly being tried and resistance of the grubs to DOT etc checked, and the use of parasites and predators other than thynnid wasps—for instance. parasite and insect disease trials against grass grub larvae in France. Mr Given said that It was anticipated that during the next six months some 80(H) to 10.000 live grubs would be airfreighted to France and of these some 60 per cent

were expected to survive the journey so that they

could be subjected to parasite and disease in-

This was not the first time! that thynnid wasps had been) introduced. Mr Given said.] Between 1946 and 1952 some!

14.000 pairs of these wasps had been introduced from Australia. Unfortunately none had managed to permanently establish in the new environment.

The species being introduced from South America were somewhat less highly specialised than their Australian relatives and consequently might be more tolerant of a change in environment including a new host. Since the New Zealand grass grub was a native species and did not occur elsewhere, no introduced parasite or predator would receive it as a natural and normal host. However any parasite introduced would accept only grass grub or close allies if establishment was to be achieved, since the relationship between parasite and host was sufficiently specific to ensure an extremely restricted range of relationships. Mr Given said that the appearance of thynnid wasps was unusual in that whereas males were winged

and similar to many other wasps and bees, the females were totally wingless and similar to large ants. They were solitary in habit and the females depended on males for transport when it was necessary to cover considerable distances. The current introductions were of females only, these having been mated before departure from Patagonia. This was considered an advisable course at this stage as the absence of males limited the initial spread which could disperse the population to such an extent as to make unlikely the meeting of males and females in the first field generation. Initially it was an advantage to obtain a dense population build-up within a limited area rather than a sparse population over a wide area.

Criticism might be levelled at the official policy regarding liberation sites. The argument that the wasps were not being liberated where grass grub was most severe certainly required an answer. As bad already been indicated the wasps w-hich were being introduced were being asked to attack a host foreign to them in their native habitat. The adult wasps also required food during their life span and this had to be in the form of nectar or honeydew. As these parasites were being introduced into what was an entirely new habitat to them and as the immediate aim was to establish them, the objective was to select areas for liberation which would provide (a) a wide range of host material for selection and (b) a wide range of honeydew and nectar sources for adult wasp feeding.

Once establishment had been achieved it would be possible to encourage the wasps to extend their activities to other areas of greater modification —that was areas which man had modified to suit his ends but which were not “natural” in composition.

“We must realise that in most cases it is man’s interference with natural life communities which has produced pest problems,” said Mr Given. In the areas which had been selected for wasp liberation Mr Given said that there was (a) at least three host species to select from, including the common grass grub; (b) a range of nectar sources (manuka, matagouri, various weeds etc) and honey-dew sources (scale and mealybug infestations) and (c) climatic and floral components similar to those of Patagonia. “From the point of view of biological control this is a ‘long shot’,” commented Mr Given, “but one which is well worth attempting. We may be successful and if we are it will mean increased prosperity to our country.”