PARASITIZING THE WOOLLY APHIS.

New Zealand Journal of Agriculture, Volume XXV, Issue 1, 20 July 1922, Page 31

PARASITIZING THE WOOLLY APHIS.

PROGRESS OF THE WORK OF BREEDING AND DISTRIBUTION OF APHELINUS MALI IN NEW ZEALAND.

R. T. TILLY

M.A. Sc.D. (Cantab.), D.Sc. (Sydney), C.M.Z.S., F.L.S., F.E.S., Entomologist and Chief of the Biological Department, Cawthron Institute of Scientific Research, Nelson.

In a previous article in this Journal (July, 1921) I gave an account of the introduction into New Zealand of Aphelinus mali, a valuable parasite of the woolly aphis, from North America. In that article I showed how, from three males and two females bred from the parasitized material received from America in January, 1921, a total of 127 adult Aphelinus had been bred and placed out in various orchards up to the 3rd May, 1921, when the cold weather stopped all further emergences. The purpose of the present article is to take up the account of the work where the' previous article left off, and to show the position at the present time. ' With a view to the more intensive breeding of this parasite, the Cawthron Institute insectarium, built in November last, was designed with one room suitable for work with minute parasites. This room was closed in with phosphor-bronze wire-gauze measuring sixty meshes to the inch. The width of the adult male Aphelinus being about onefiftieth of an inch, and the female a little stouter, it was necessary to secure gauze -with a mesh as fine as this in order to confine the adults within the insectarium. Four young apple-trees were planted in large tubs and placed in the Aphelinus room, ready for the emergence of the parasite in the spring. Mr. A. Philpott, Assistant Entomologist of the Institute, took charge of this work during November and December, while I was away in Australia. ' He also effectively established colonies of woolly aphis on all four trees in the' usual manner, by making cuts of the bark , with a knife and placing small clumps of aphis around the fresh cuts. By the time the first, Aphelinus emerged from tree No 5 in the Institute gardens (this being the tree on which they were originally placed early in 1921) the four trees in the insectarium showed a vigorous growth of aphis, due to the sheltered conditions under which they • were being grown and the care taken to spread the aphis in every possible way. Aphelinus mali began to emerge on tree No. 5 on 10th November, 1921, and continued until the 19th, only six specimens being secured altogether. All these were taken and placed on tree No. 3, one of the special trees inside the insectarium. By

the end of December a numerous progeny of these six Aphelinus had appeared on tree No. 3. During January numbers of these were taken and placed upon the other three trees in the insectarium, while not a few migrated thither themselves by flying. Aphelinus continued to emerge throughout January, and by the end of the month were sufficiently abundant to allow of the collection of large consignments. every two or three days, without the risk of destroying the supply entirely. In order to get the best results with the parasite two plans were put into operation, as follows :— (1.) It was decided to put out consignments in gardens in Nelson itself where there were apple-trees infested with woolly aphis which

did not get the systematic treatment in spraying that trees in commercial orchards undergo. Such trees as these form one of the chief sources of reinfection of orchards from year to year, as they yield immense numbers of winged aphids during April, and these winged forms, drifting far and wide over the whole district, reinfect the clean orchards, however carefully the latter may have been sprayed during the year. (2.) It was also decided, in dealing with commercial orchards, not to let the batches of Aphelinus go free at random, but only to liberate them on specially prepared trees. Such trees are intended to become the secondary distributing-centres of the parasite during the next summer. It will readily be recognized that it is beyond the power of two men, however hard they work, to cover New

Zealand with . sufficient Aphelinus to make a definite mark against the enormous numbers of woolly aphis which occur in every orchard. These trees were to be prepared by enclosing them in a large cage covered with bird-proof wire netting on the sides and top, with the addition of scrim on the roof and around the windward side of the cage. Orchardists willing to lend trees for this purpose were also asked to keep these trees unsprayed, so that enough aphis might develop on them to allow of a plentiful supply of Aphdinus being bred out the following summer. Thus each protected tree would form a sort of local insectarium from which local distribution could be carried out, and also restocking made from the Cawthron insectarium when necessary. It is pleasing to be able to record that a number of fruitgrowers’ associations took up this work with keenness. Covered-in trees were prepared at Tasmati (i), Mapua'(4), and Redwood’s Valley (1), in the Nelson District, and also at Hastings (3) and Christchurch- (3). In this connection, we desire to thank Mr. J. H. Thorp, Orchard Instructor at Mapua; Mr. W. H. Rice, Orchard Instructor at Hastings; Mr. F. W. Cone, president of the Canterbury Fruitgrowers’ Association; . and all those gentlemen. who set apart and covered in trees in their orchards, for the valuable assistance given in. this work. The following table gives the dates and numbers of individuals' of Aphdinus in each consignment sent out from the Cawthron insectarium during the past summer. All consignments placed out in Nelson itself come under the heading of treatment of unsprayed trees not covered in ; all the rest of the consignments were placed out on specially prepared trees.

It will readily be seen that the position of Aphelinus mali in New Zealand at the present time is a great advance on that of the previous year, nearly three thousand individuals having been reared and placed out ■ under specially favourable circumstances. The work of rearing and distributing this parasite will be undertaken again next year in the same manner. It is hoped that every fruitgrowers’ association in the Dominion will avail itself of the opportunity of obtaining supplies from Nelson, so that Aphelinus may become completely established throughout the fruitgrowing areas of the Dominion. It is now established that these tiny insects can be sent by post safely to any destination in New Zealand without appreciable loss in transit. Consignments will be sent out from the. Cawthron insectarium to any association or orchardist who notifies us that he has a tree prepared in the manner indicated in this article, it being understood that each locality will, as far as possible, undertake the further distribution of the parasite in the orchards surrounding the prepared tree. Further trees will be placed inside the Cawthron insectarium during the winter, so that a larger supply of Aphelinus can be reared from them. As a guide to the. manner of covering in the trees in the cheapest possible way to be effective, a photograph is here given .of the tree covered in by Mr. Maisey at Redwood’s Valley. A crude doorway can be made at one . side, if thought necessary, for entrance and exit; or it will suffice if the netting is simply hooked on, so that it can be unhooked and turned back when the tree is to be examined.

Date. Number sent out. ’ . Locality. Consignee. Jan. 28 ... - 80 Nelson . . Cawthron orchard (Annesbrook) Feb. 14 4° Nelson Mi's Pool. Feb. ■14 200 Nelson Colonel Burton. • Feb. 16 .'. 80 Nelson Mr. Duncan. Feb. 18 .. 140 Nelson Miss Davidson. Feb. 20 100 Nelson Mr. Oldham. Feb. 24 240 ■ Tasman . . Captain Murray. Feb. .25 140 Nelson Mr. Simpson. Feb. 27 160 Hastings Mr. W. H. Rice. Mar. 2 200 Redwood’s Valley Mr. Maisey. Mar. 6 160 Christchurch Mr. F. W. Cone. Mar. 7 140 Mapua . . Mr. Coutts. Mar. 9 220 Hastings Mr. W. H. Rice. < Mar. 13 240 Christchurch Mr. F. W. Cone. Mar. 20 . . 200 Mapua . . Mr. Thorp. • Mar. 27 140 . Christchurch Mr. F. W. Cone. Mar. 28 . . 100 . Mapua Mr. Thorp. Mar. 30 100 Hastings Mr. W. H. Rice.' April 1 80 Stoke .. Mr. Pickup. Mar. 4 ' . . 100 Mapua Mr. Thorp. . Total distributed, 2,860 (Nelson Province, 1,840; Hawke’s Bay, 480; Canterbury, 540).

Distribution of Aphelinus mali in New Zealand during the Summer Of 1922.