THE GRASS GRUB

Press, Volume LXXI, Issue 21387, 1 February 1935, Page 10

THE GRASS GRUB

♦ Control By Use of Parasite WORK AT CAWTHRON INSTITUTE LFrom Our Own Reporter.] _ NELSON, January 31. The latest of the problems of the farming community to be tackled by the entomological department of the Cawthron Institute is the control of the grass grub, which constantly plays havoc with pastures, and in its adult stage, as the brown beetle, does great damage to turnip crops. Experiments are beginning with small burrowing wasps, a consignment of which arrived from Australia to-day, as well as with others from Chile, and with tachinid flies from Japan. All of these are parasites preying on the group of beetles to which the grass grub belongs. A new line of experiment is also beginning for the control of ragwort, as the cinnabar moths have not done all that was expected of them in preventing its spread. For this purpose a fly whose maggot is hatched in the seed heads of ragwort is being experimented with. An attempt to prevent the undue spread of gorse by introducing a weevil which lives on the seed pods is also being made. This weevil has been found to confine itself to gorse, and not to attack broom. Other work concerns the control through parasites of the mealy bug, which attacks apples, pears, grapes, and the shoots of potatoes, and of the horntail borer, which attacks pine wood.

Nature of Attack Of the parasites to be used in the work on the grass grub the wasps—from Australia and Chile—lay their eggs m it when it is in the grub stage and the fly—from Japan—in the adult stage. If it is found that these insects do what is expected of them and can be acclimatised and induced to breed in New Zealand in sufficient numbers, there will be a double attack on the grass grub, which is an outstanding example of an indigenous insect which is a serious pest. The thynnid wasps, of which the female is wingless, burrow into the ground and lay their eggs in the grubs of the beetle. They feed, like others of their family, on the nectar of flowers, and not on vegetation. Several earlier consignments of the wasps have been sent from Australia, but the work has been delayed because most of the insects have died on the way. The present shipment seems to have 'arrived in good condition. The consignment of Chilean burrowing wasps has been sent over in the pupa stage, and will be hatched out at the institute. The Japanese tachinid fly also takes its food from honey-dew, plant secretions and the nectar of flowers. It lays its eggs in the beetles while these are mating, and lives the whole of the maggot stage of its life in the beetle. It either emerges from the beetle to pupate in the ground, or passes into the pupa stage inside the beetle, which is also of a burrowing habit. In either case the beetle dies. Success in Other Countries All of these parasitic insects are known in their own countries to attack beetles which are related to the New Zealand grass grub. In particular something Is known about the Japanese tachinid fly because its beetle host appeared in the eastern states of the United States, and did a great deal of damage there. The fly was one of the things introduced as a parasite to attack the beetle there, and the steps taken, including the introduction of_other pests to attack the beetle at "other stages of its life, were very successful.

There is considered to be no danger that these insects, if found successful and liberated, will prove harmful in other directions. They cannot turn round and change their habit from feeding on the nectar of . flowers, honey-dew and plant secretions, to feeding on vegetation. In this respect experiments with parasitic insects—feeding on other insects—differ from experiments with insects intended to control a plant pest, for these last have already the habit of feeding on vegetation. The question whether parasitic insects may not do harm when they have killed out their hosts is considered to be irrelevant, because no parasite ever entirely kills out its host, and if it did would itself die out. What happens is that the population of both host and the parasite falls. , The Safeguards Observed The three insects now being experimented with represent groups which do not exist in New Zealand. There is a gap in the fauna, which lacks parasites predaceous on the group of chafer beetles to which the grass grub belongs. Consequently the grass grub has hitherto gone unchecked. All the work with the parasites is being done with the safeguards customary at the institute. The cages in which the insects arrive are onened in a special laboratory totally enclosed in a fine-meshed wire gauze. The insects are then placed in other cages in the open air, the cages including soil in which are I some of the grass grubs in the stage at which they serve as hosts for the insects. The parasites have to be observed carefully—one of the things to be looked for being to see that they do not bring with them | some secondary parasite. There is often a large mortality m insects brought in from overseas, while often they refuse to mate under the new conditions. Another difficulty is the difference in the seasons so that some of them have to be kept at the pupa stage in cold storage until the right season arrives, so that their hosts will be ready to receive then,!. In addition they are never liberated until their feeding habits have been thoroughly observed. If the experimental work proves successful either an attempt will be made to breed the parasitic insects at the institute or supplies will be obtained from the country of origin. One of the main difficulties of the institute is its lack of adequate funds. The ideal way of doing such work would be to send a man to the country of origin of the parasite to observe its habits in its natural environment before any experimental work was done in New Zealand, but this cannot be done with i the money at present available.