Transactions of RHASS Volume 1940 - Page 061
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| Year | 1940 |
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11 p .. . n 0 , INSLCI AND OTHER PLSLS OF 1939' INSECT AND OTHER Pics'rs OF 1939. 111 of the plants. The females are said to live for two or three Zveeks, and may lay about 100 eggs, which are inserted into tfie tissue of the lower surface of the leaf. In four to six days sugsgalggot: Emerge and at once begin feeding on the green Control—In cases of slight infestations mined leaves should realm Jet cvwvceii'the upper and lower surfaces, with the be removed and burnt. In commercial glass 1101189S the leaf W?) “£11;de gallenes develop slowly .311 through the plants should be sprayed regularly at intervals of two weeks . ltll u 1 grown the maggot stops feeding and changes ‘ with a nicotine-soap WaShv “are being taken that the waSh out-Of—doors it is confined to summer and autumn, the last generation hibernating as pupae in the fallen leaves. to a pupa inside its last skin, which persists unmoulted. A. b Fig. 8.—Two leaves of Chrysanthemum mined b I h ' y the maggots Q] the Chrysanthemum Leaf Mmer, Phytomyza atricorms. A, Lower surface of leaf showing galleries with maggots and pupa-cases in situ ' B ‘ ' coalescence of galleries. ’ , Upper smface 0f leaf Showmg From nature. About natural size. Instead, it contracts, hardens, and gradually changes from yellow to dark-browu. Both the maggots and the pupa-cases can be seen in their mines, Where they form small bulgings on the lower side of the leaves (Fig. 8A). By holding the leaves to the light the inhabitants of the mines are the more readily discernible. The pupal stage lasts for one to three weeks, when the flies push off the front part of the pupa-case and escape. In glass houses breedmg proceeds continuously during the whole year ; is applied to both surfaces of the leaves. This frequency of treatment is neCessary because the life-cycle of the miner is brief and there is.continual risk of infestation from flies bred on neighbouring chrysanthemums or on wild plants. Flies in glass houses may be destroyed by light fumigations of hydrocyanic acid gas or nicotine. SWEDE AND CABBAGE APHTDES (Breoicorg/ne brass-latte and M yzus persicae). In the ‘Transactions’ of 1936 reference was made to an outbreak of B. brassicae and M. pcrsicae, both of which caused extensive damage to cruciferous crops in the Lothians in the summer of 1935. Although the two are frequently associated on the same host plant they can be readily distinguished. B. brassicae, the Mealy Cabbage Aphis, is greyish-green with a mealy waxen coat. M. persicae, or the Potato Aphis, on the other hand, is green, yellow, or rose in colour, and is devoid of a waxen coat. Both have the habit of overwintering on Brassicas, but, whereas B. bmssicac is confined to cruciferous plants throughout the year, M. persicae migrates to the potato crop in summer. Therefore, the growing of large fields of Brassicas and swedes in close proximity to large acreages of potatoes, as is done in the Lothians, becomes an important factor in the maintenance and spread of virus diseases of potatoes, of which M. persicae is the chief carrier. Last year (1939) the Lothians again experienced a severe aphid outbreak, which involved not only the large market- garden area around Musselburgh, Tranent and Prestonpans, but extended also to Dunbar. The aphids which were chiefly concerned were the Mealy Cabbage Aphis and the Potato Aphis, but that conditions were generally favourable to aphids was shown by the abundance of the Grain Aphis (M acrosiphum granarium) on cereals and the Black Bean Aphis (Aphis fabae) on beets. At one market garden the owner reckoned that he had lost one—quarter of his cruciferous crops, whilst another described how he had sown a field seven times with M‘Ewan’s Early Cabbage, and the aphids had destroyed the plants soon after they came through the ground. In the second week of August the potato shaws began to flag and were soon destroyed by the attacks of M yzus persicae, althOugh the tubers had not |
| Title | Transactions of RHASS Volume 1940 - Page 061 |
