THE PEAR-MIDGE PEST.

New Zealand Journal of Agriculture, Volume XXX, Issue 4, 20 April 1925, Page 224

THE PEAR-MIDGE PEST.

SPRAYING EXPERIMENTS AT HENDERSON

R. H. MAKGILL,

, Auckland.

The damage caused by the pear-midge in those parts of New Zealand which this pest has reached is so great, and the control is so difficult, that the writer ventures to record even the small measure of success he- has obtained by repeated spraying in his orchard at Henderson, near Auckland.

In the annual report for 1919 of the Bristol University Horticultural Research Station Mr. A. H. Lees, Research Entomologist, describes a nicotine - paraffin • insecticide spray having the' following composition : Soft soap, 15 lb.; paraffin, 2 gallons; nicotine, | lb. ; water, 100 gallons. He advocates the use of this spray in summer-time against woolly aphis. In conversation with the writer Mr. Lees suggested that it might be found of service against pear-midge, not with a hope of destroying the well-protected larvae, but in order to deter the adult female from laying her eggs. Trials were accordingly made in 1922 and again in 1924, but on both occasions in place of paraffin and soft soap given in the formula a miscible oil (" Olene ”) was used in a strength of two parts per cent. As the paraffin is used chiefly as a carrier and spreader of the nicotine, it was thought that this change was of little consequence, and it simplified the making of the spray. Certainly the modified mixture acted well when used against woolly aphis, and its application for that purpose would have been continued had not the introduction of A'p’helinus malt made its use unnecessary. If the mixture were carefully made no scorching of leaves resulted. However, it was found that if the nicotine (Black Leaf 40 being used) were put into the oil emulsion without previous dilution a certain amount of the oil was thrown out of suspension and. floated as a scum. If this scum was sprayed on the leaves, especially of P. Barry, a certain amount of scorching resulted.

The ; - trees selected for experiment were eighteen young ■ pears of mixed- ! j varieties, planted, in 1922 in a block of land newly broken in from pasture that year. Fifteen P. Barrys planted in 1921 among other older pears were also used. • ■ SEASON' 1922-23.. In 1922 the midge was detected in the tent traps on 14th September, but no female midges were found till 1st October. The Black Leaf and oil spray was first applied on 3rd October to the newly planted block of pears and to nine of the P. Barry trees. . . • On 10th October the first evidences of infestation were detected on some Beurre Diel trees next to the P. Barrys, and five days later it was seen on the P. Barrys which had not been sprayed. . None of the sprayed trees showed any signs of.-the midge.. 5 On 25th October the ground immediately around all the young pear-trees was sprayed' with pure kerosene, with a view to attacking the pupae which by that .time would be due to hatch out from the first infestation. . ' ., On 1st November the P. Barry trees were sprayed with a mixture composed of molasses, 6 lb., to 100 gallons of water, and Black Leaf 40, one part in 1,000. This spray seemed to act well, for, despite daily rain showers,- by 8th November the newly formed shoots showed no signs of midge-infestation, and the odour of the Black Leaf could still be detected on the sprayed leaves. This was not due to the kerosene spraying of the ground, ■ as other trees so treated but which had not received the Black Leaf and molasses mixture showed infestation. It was decided then to continue to treat the P. Barry trees with this spray, and use the Black Leaf-oil spray only on the newly planted pears. The next application of the ' Black Leaf and molasses mixture to the P. Barrys was on 21st November, three weeks after the first. It was found, however, that this interval was too long, because, although the Barrys were nearly free from infested leaves at the time the second spray was applied, by 27th November . a fairly wide infestation was manifest, and as the leaves were by then beginning to blacken it seemed probable that the eggs were layed just prior to the second' spray. The constant rains during the first fortnight of November no doubt contributed to the failure of the remedy. The spray was repeated on 6th December, but with little, effect, and for the rest of the year the Barrys were' markedly, infested with midge. . ' . Turning again to the block of newly planted pears which had been sprayed on 3rd October with the Black Leaf and oil mixture, in common with other pears the ground round these trees' was treated with kerosene on 25th October, but the trees were not again sprayed , with the mixture till 10th November. At this, time scarcely one infested leaf could be found on the eighteen trees, which had made satisfactory growth and had acquired a good crop of leaves. On 21st November a female midge was observed' laying eggs on one of these trees, showing that the spray had lost its effect during the eleven days which had elapsed since its application. The spray

was accordingly repeated on the same day, but by 27th November a fairly widespread infestation of the new leaves was apparent, the eggs probably having been deposited prior to 21st November. The spray was repeated on 5th and 22nd December, 14th January, and 3rd February, but with very little result during December and January, and during these months there was little new growth or formation of serviceable leave-s. After the final spraying on 3rd February, however, some midge-free leaves developed during the ensuing week, but no further observations were made that season. During the same season a number of adult Bon Chretien and Beurre . Bose pear-trees were sprayed with the Black Leaf and molasses mixture, the dates being 1st and 21st November and 6th December. The spraying on the first of these dates was productive of some result, as the new growth was fairly free from midge infestation till 10th November. Thereafter the midge appeared to gain ground, and the two last sprays had little, if any, effect. season 1923-24. During the season 1923-24 only the ■ Black Leaf and molasses mixture was usedapplied about every third week. As before, the young trees were kept almost free from midge infestation till the last week in November, after which the spray showed little effect, and was not repeated after December. ’ ' SEASON 1924-25. It was thought that possibly a better result might be obtained with more frequent sprayings, so during the spring of 1924 the new block of pear-trees was . treated with the Black Leaf and oil mixture once a . week. The dates of the sprayings were 6th, 13th, 21st, and 27th October, 1st, 10th, 17th,. and 24th November, 1st, 7th, 15th, 22nd, and 29th December, and 5th January. This season the midge infestation was noticed first on the Beurre Diel leaves about 13th October, the leaves beginning to blacken about the 18th. By 27th October the young pears sprayed with the mixture were quite free from midge, except a small tree, planted this season, which, stood apart and was overlooked in the spraying on 13th October. During the first three weeks of November this freedom continued, only one or two leaves- — perhaps had not been well sprayed — infestation. The trees by now had made a reasonable growth and had a good crop of leaves. Some continued wet weather about the middle of November may have weakened the spray deposits on the twigs, for on 26th November slight infestation, was found on all the trees.

|It may be doubted, however, whether rain alone was the cause of the spray losing its effect, for on 4th December, only three days after the last spraying, a female midge was found depositing eggs on a bud. The weather had not been wet, and the question arises whether the increasing temperature may not have been responsible for the failure, nicotine being a volatile substance. However this may be, from December onwards the spraying failed to control the midge, thus repeating the experience of the two previous years.

During this season a material sold as Vistolene was tried in a strength of i part in 100 parts of water on the P. Barry trees. The dates of the sprayings were 21st October, 10th November, and 1st and 7th December, while one tree was sprayed with the Black Leaf and oil mixture each week on the same dates as were the block of younger trees. The first two sprayings with Vistolene had some effect, the treated trees showing considerably. less infestation than neighbouring unsprayed trees, though not nearly so free as the tree which received the weekly treatment. The Vistolene had little effect after the middle of November, however. . conclusions. . The results suggest that by the use of nicotine the pear-midge can be kept in check during October and the first two weeks of November sufficiently to permit of the early twig-growth becoming established and to enable the trees to set a fair covering of leaves. This is certainly an advantage, especially when, dealing with young growing trees. Some of the treated trees, planted in 1922, are now over 6 ft. in height, contrasting favourably with untreated trees of like varieties, . which have made little growth in the same time. Insufficient experiments were made to demonstrate whether with adult trees the destruction'of buds by the midge could be delayed sufficiently to enable the fruit to set, and to allow of the establishment of the leaves necessary to the nutrition of the fruit-bud. The Keiffer variety appears to be attacked by the midge just as much as other varieties, but, being of early habit, the buds have set and the leaf crop is established before the midge has become prevalent. After the middle of October the new growth on the Keiffer is infested Growth is checked, but not the fruit crop. With later - blossoming varieties, such as Bon Chretien and Beurre Bose, the difficulty arises that measures taken to combat the midge must also tend to discourage the visits of bees, and thus pollination may be interfered with. Spraying should be suspended while the blossom is fully open. From the middle of November till late in February nicotine doses its effect. During the seasons under consideration wet weather may have been responsible for this failure—at least in November ; but the effect of the increasing temperature on " a volatile substance like nicotine has to be considered. Again, it has been shown ■ by Miller that, after the hatching-out of the second infestation between 1st and 10th November, the midge remained on the wing for the remainder of the month. This must greatly, increase the difficulty of protection. This raises the question of the frequency of spraying. During October the results when the spray was applied weekly were little better than when applied at intervals of three weeks, but in the latter part of November it appeared that a weekly spraying was insufficient. It is proposed next spring to apply fortnightly sprays during October, weekly in the first half of November, and thereafter bi-weekly sprays, by way of experiment. From the commercial point of view bi-weekly spraying can hardly be regarded as practical. In regard to the composition of the spray, the nicotine salt is- the active agent, and the most suitable “ carrier ” has not been determined. On the whole the miscible oil appeared to give the most lasting results. If ordinary-oil be used it is advisable to use a strength

not greater than ij per cent., owing to the tendency to scorch. Molasses proved satisfactory, and is easily dissolved. Possibly a simple 'soap solution may be found equal to - the agents tested. - r - The experiments with Vistolene were not sufficiently extensive - to warrant a definite opinion, but it did not seem as effective as nicotine. The results of the various trials viewed in general were disappointing, but suggest that in the use of a deterrent spray the solution of the problem may be found.