THE PEAR-MIDGE.

New Zealand Journal of Agriculture, Volume XXX, Issue 4, 20 April 1925, Page 220

THE PEAR-MIDGE.

FURTHER OBSERVATIONS AND CONTROL WITH CALCIUM CYANIDE.

DAVID MILLER,

, Entomologist, Biological Laboratory, Wellington.

The original account of the pear-midge (Perrisia. pyri) in New Zealandpublished in . this- Journal for August, 1921-dealt with the life and seasonal histories of the insect, and results of control experiments carried , out at that time. In the following pages the position is reviewed from the first appearance of the midge in the Dominion to the present time, and the results of control experiments with calcium cyanide are discussed. . ' A feature in the midge-infested areas near Auckland, where the pear-trees have been subjected to severe infestation for the past four or five years, is that the trees are stunted, and the leaves which do develop are dwarfed, while, their green colour has assumed a fairly yellowish tint. SPREAD OF THE MIDGE. ~ Auckland : The initial outbreak of pear-midge in New Zealand occurred in the spring of 1916 at Avondale, near Auckland (area 1 on map). From there it rapidly spread during the succeeding years to adjoining districts, and by the 1920-21 season had. become established over Eden County and southern part of Waitemata County (area 2, shaded), as well as in the Waikato (area 6). In the following season it appeared northward at Huapai (area 3) and Komiti (area 4), and southward at Te Kauwhata (area 5). Hawke’s Bay : At Hastings (area 7) the midge appeared during the spring of 1920, infesting a block of orchards eastwards of the railway. Nelson: The midge made its first appearance in the South Island in an .orchard at Tasman (area 8) during ..the spring of 1921, and spread from there to adjoining areas, as well as attacking orchards separated from the originally infested one by low ridges. Shortly after this the midge was reported westward at Riwaka (area 9), while not until last spring (1924) did it appear to the west in the orchards in the Redwood’s Valley and Stoke districts (area 10). A consideration of these midge-infested districts shows that they lie in at least five naturally isolated areas : Waiternata and Eden Counties (1, 2, and 3), Komiti (4), Waikato (5 and 6), Hawke’s Bay (7), and Nelson (8, 9, and 10). The natural barriers are distance, air-currents, sea, or mountain-ranges. The absence of pear-orchards over given areas is also an isolating factor. <• ,. Two factors have been responsible for the spread of the midge from its initial point of establishment at Avondale to the other districts recorded above'. These factors are (1) natural dispersion by the flight of - the insect aided in many cases by wind, and (2) artificial dispersion in the larval and pupal stages in soil surrounding roots of nursery stock. .‘'V?

Natural dispersion was no doubt the chief means by which the areas in the vicinity of Auckland City became infested, though the movement of nursery stock played some part, particularly in the infestation of an area such as at Huapai, which is partially isolated by low hill country. The establishment of the midge in the other districts enumerated can be accounted for only by artificial dispersion in the first instance, though natural agencies came into play in these districts

after the initial establishment.' It seems, therefore, that attention has not been given to the recommendation made in the former article (page 89, Journal, August, .1921) that the greatest care should be taken in exporting from a midge area not only pear-trees but all nursery stock, since it requires but very little soil infested by midgelarvae to spread this pest, broadcast over a wide area.”

INFLUENCE OF CLIMATE.

Though no detailed observations have been made on the extent to which the pear-midge is influenced by climate, there are certain features worthy of mention. It is well known that climate is a limiting factor in the establishment and dispersal of insects, and variations in the intensity of midge infestation and in the period of first spring emergence in the different districts seem, in part at least, to be. due to differences in. moisture. The areas where infestation is most severe are in the Waitemata and Eden Counties, and at Tasman and Riwaka in the Nelson District, but particularly the former ; while in Hawke's Bay infestation is comparatively light. Further, the emergence of the first spring brood of midges commences during late September in Waitemata and Eden Counties and also at Tasman and Riwaka, but during the second week of October in Hawke’s Bay. The following figures (kindly supplied by the Dominion Meteorological Office) show the total mean rainfall in the three main midge areas for the months April to October, when the hibernating midge-larvae are in the ground : Auckland (area 2), 29-32 in. ; Motueka (area 9), 32-32 in. ; Hastings (vicinity of area 7), 22-62 in. These figures show that the rainfall at Auckland and Motueka is much higher than at Hastings. This feature, when correlated with the earlier spring emergence of midge in late September in the vicinity of Auckland City and in the two Nelson districts; and the later emergence in October in the Hawke’s Bay area, shows that . moisture has apparently a direct influence upon the emergence of the overwintering stage of the. midge. There are at present no temperature statistics from actual midge-infested areas available for comparison, though figures from the meteorological stations at Auckland, Nelson, and Napier show that Amckland has the highest temperature and Nelson the lowest —a feature seemingly bearing out the opinion that moisture is the main climatic factor influencing midge emergence. CONTROL ’WITH .CALCIUM CYANIDE. Owing to the pear-midge larvae when on the trees being protected by the rolled-up leaves, none of the sprays tested have been sufficiently effective. However, the . habit of the larvae entering the ground to hibernate and pupate presents an opportunity for control. Though a number of insecticides have been experimented with in soil-treatment (Journal, August, 1921), no results of a practical value were obtained until the present season, when calcium cyanide was used. This is a preparation manufactured by the American Cyanamid Company, New York, and . at the time of these experiments was prepared in three —granules, flakes, and dust. Owing to'later improvements in the manufacturing processes, however, the granules can now be made at. less cost and placed on the market at the same rate as formerly, charged for the flakes, which have been withdrawn. On the cyanide being exposed to the atmosphere, hydrocyanic-acid gas is generated, and. its value as a soil-fumigant is at once apparent. The opinion has been put forward that too great a danger to life is involved by the use of this material, but that is by- no means so if ordinary common-sense is used in its handling. Certainly the danger is comparatively small when the cyanide is used out of doors.

In the following experiments against one of the summer broods of midge, carried out at Mr. F. G. Platts’s orchard at Henderson, the granular cyanide was used. Six dosages were laid out, each involving three plots of infested soil under as many trees, making a total of eighteen trees treated. Over the treated area under each tree two emergence tents were pitched, so that six observations were made for each dosage. As a check twelve tents were pitched under untreated trees.. Owing to lack of sufficient cyanide the experiments were not made more extensive. The dosages of cyanide to each area of 200 sq. ft. were as follows, the material being spread on the ground and worked, not turned, in with a spade: (1) 2 1b., (2) if lb., (3) i|lb., (4) 1 lb., (5) fib., (6) lb. An examination of the emergence tents at the time when the midges were due to emerge showed that the efficiency of the cyanide divided the dosages into two groups, one comprising dosages (1), (2), and (3), and the other the dosages (4), (5), and (6). In the first group there was 100 per cent, control, and in all of the second group but little control, if any. Certainly the weakest dosages, (5) and (6), cannot be claimed to have had any effect, since the numbers of midges emerging into the tents over these plots could not be said to be any less than those in the check tents. Owing to results between the two groups of dosages(1), (2), (3) and (4), (5), (6) —being so decidedly positive and negative, *a later attempt was made with dosages of strengths each ' descending by 1 oz. from 11 lb. to 1 lb. to every 200 sq. ft. of infested ground. ' In this experiment, carried out to determine if there was a weaker effective dosage between (3) and (4), thirty-six observations were made, exclusive of checks. Nine dosages, each involving infested ground under two trees, were applied. On each of these eighteen plots an emergence tent was pitched, together with an emergence box. This latter, turned mouth downwards, measured 10 in. (high) by 14 in. by 20 in. (inside measurements), and . was lined with black paper. On the top five holes were bored, in each of which was inserted a glass tube 1 in. in diameter and open at both ends except for a muslin cap over the outer one. A cork fitted with a narrow tube was inserted into the opposite end within the box, in order to prevent any midges from leaving the tubes once they had entered. It was hoped by this means to make counts of the midges emerging from each dosage, but the moisture which collected in the bottom of each tube interfered with this. The results of these experiments, however, as gauged from the tents, showed that the dosage of i| lb. to 200 sq. ft. was the weakest effective strength that could be used for 100. per cent, control. Acting on this basis it is intended to extend the work on a larger scale, treating whole orchards under commercial conditions in order to test the efficiency of the cyanide against the hibernating midge-larva?. A point of interest in the experiments was that though the pear-midge was controlled by the stronger dosages., the latter had apparently no effect on certain other underground insects, since there was a general emergence in all the tents of such insects as the cicada (Melampsalta cingulata), and several .ichneumon flies and muscid flies.

The quantity of cyanide required to treat a midge-infested orchard will vary with the spread of the trees. In mature orchards where trees

overlap, the full acreage must be treated at a maximum cost, but with upright mature varieties or younger trees less cyanide will be required, until a minimum is reached in a newly planted orchard or in a nursery. It is not known at present just what the exact cost of treatment with calcium cyanide would be, but in. any case the results here recorded, together with the reduced price of the granules, show that at least young orchards (or even some mature ones) and nurseries, where isolated, could be economically treated at present. That ordinary winter cultivation,, if carefully carried out, will reduce the numbers of midges emerging in the spring has been proved (Journal, August, 1921), but no concerted attempt has been made to follow this line of treatment, which 1 requires that orchards be well kept throughout the year, so that the thorough turning-in of the midgeinfested surface soil is made possible during the insect’s hibernatingperiod. Not only'is the pear-midge so reduced in numbers, but also other insects hibernating underground. . '