Transactions of RHASS Volume 1940 - Page 062
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| Year | 1940 |
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| OCR Text |
INSECT AND OTHER l’lCS’l‘S or .1939. yet achieved ware size. On badly infested farms the yield of potatoes was at least halved. Whilst it is not always easy to discover the underlying cause or causes of an insect outbreak, it is perhaps no exaggera— tion to say that there are no other insects which are so sensitive to changes of weather as are aphids. \Varm dry weather in early summer supplies a stimulus to reproduction, which greatly increases its rate and induces an outbreak in late summer. Cold wet weather has just the opposite effect and is probably even more important in stemming an increase of aphid numbers than are predatory or parasitic insects. As Fig. 9.—Bi'evicoryiie brassicze. .llraly ('rtblmgrAp/ii's. ll'iuyul l'irgiu Female. x 12. From nature. between the aphid and its insect enemies there is usually a lag, so that the latter cannot cope with the much greater rate of increase of the former once an outbreak has commenced. Life-history and Habits—Both the Mealy Cabbage Aphis (Fig. 9) and the Potato Aphis (Fig. 10) spend the winter 011 the stems and lower leaves of Brussels sprouts, winter cabbage and cauliflower, the former as an egg, the latter a Wingless female. The eggs of the Mealy Cabbage Apliis, which are black, hatch about March or April and produce young that develop into wingless females. The progeny of the Wingless females of both kinds of aphid represents the nuclei of colonies which proceed to increase on the winter Brassicas. As the season advances winged migrants appear iNsEeT AND OTHER PES’i‘s OF 1939. 113 which fly to the plants of the new crops and give rise to small colonies in the heart or crown. The aplnds spread over the young leaves, first the upper surfaces and later extending to the lower surfaces as well. In June Winged forms of the Pota o Aphis migrate to the potato crop, where they produceban abundance of Wingless forms. In Aiigiist and Septem er, When the potatoes are mature, Winged forms are again pro- duced which return to the winter hosts, there to_g1ve use once more to Wingless forms that enter hibernation along with the eggs of the. Mealy Cabbage Aphis, which are laid during October and November. Damage—Both aphids are responsible for widespread damage to Brassicas by draining the sap of leaves and stems. Fig. 10.—Myzus persicze. Potato Aphis. li'iny/ed Virgin Female. X 12. From nature. ) nts make little headway in their growth and may 3231212345113 stunted. The leaves become malformed, swollen, curled and discoloured, and hearting may be inhibited. Ever: should the plants proceed to develop they become overspregf with sticky honey-dew excreted by the insects, mth the result that they are rendered unmarketable. In swedes t e destruction of the leaves materially cuts down the yield. . Depredations of the Potato Aphis do not end With Brassicas as do those of the Mealy Cabbage Aphis. It extends its aetiVi- ties to the growing potato crop, where by sheer force of numbers it may completely destroy the fohage before the crop Iripens, not to mention the risk of its infecting the plants With the ' Aaf roll and mosaic. _ - Vlrgrslislfigfiet gardener whose crops were particularly heavin |
| Title | Transactions of RHASS Volume 1940 - Page 062 |
