FARMERS' FOES.

Clutha Leader, Volume XXXVIII, Issue 14, 22 August 1911, Page 7

FARMERS' FOES.

1 ♦ turnip and rape BLIGHT. a scientists' opinions. The farmer would probably esteem himself a fortunate man if the grass grub, bad as that pest is, was the only infliction he had to bear; but unluckily it is only one of many. In past years, it from any cause there was an unußttal shortness of feed in the grass paddocks when winter was approaching, the sheep-raiser could rely with a considerable degree of certainty upon his fine crops of turnips to carry him through the winter with the help of hay and; straw. But now it seems as if this re? source was in a fair way to fail the man upon the land, for the damage done by the blight and its accompanying disease of a bacterial nature, the "rot," throughout Canterbury and other portions of the Dominion last autumn i§ as bad, if not worse, than the ravages of the grass grub. The seriousness of the situation with regard to the turnip and rape crop, is greatly enhanced by the fact that so far practically no preventive o- curative measures have been round to be of any avail; to some extent the grass grub can be combated, not so the scourges that are described in this article. There is not much doubt that the dryness of the late summer was to a certain extent responsible for the severe occurrence of the blight in the turnip and rape crops; their growth was retarded, the plants were weakened and so were much less able to withstand any attacks of insects. Speaking to a representative of 'The Press,' Dr. Hilgendorf, the biologist of Canterbury Agricultural College, impatred some interesting information on the turnip and rape blight. "There is no doubt," he said, "that the extensive damage done to the crops of turnips and rape this season provides a very serious problem for the farmer to face. With very few exceptions all the turnip and rape crops have suffered more or less severely from the blight, and in only too many instances the crops have proved almost a total failure. This fact, combined with the shortness of grass feed in many districts, makes the situation decidedly unpleasant for the farmer who has breeding and other stock to keep through the winter months. Not only have the paddock turnips been attacked, but in the experimental plots at the collegej which contained over 30 varieties of turnips, only one bulb has been left. Though all the plants except this accidental survival are now dead, yet there were differences in the resistance to the attack of tfie aphis, the Imperial Green Globe holding out as long as, or longer than, any other variety, though it, too, succumbed at last. The Life History. The apperaance of the minute insect which causes all the damage is too well known to require any description, but firmer < may not be so well acquainted with its life history, which presents many very interesting characteristics. The aphides usually begin to make their appearance upon rape in December. These insects are about one-eight of an inch long, and ta this stage are all wingless females. These wingless females reproduce from six to ten young cii.ps alive, and a careful observer may see the parent insect in the leaf of the plant surrounded by its tiny young ones. A peculiar point is that these wingless females reproduce parthenogenetically, which means that no male insect is needed to fertilise them. This means a great saving of time to the aphides, who can in this manner multiply at a great rate; also every individual reproducers all being females. Within a week the young of the lirst generation—which are also wingless females—begin multiplying, and this goes on for about sixteen generations. A little calculation will show that after allowing a big percentage from losses the descendants of one wingless female will amount to billions in a season. When it is realised that there are probably hundreds of thousands to start with it is not surprising that all the plants in a big paddock may be literally smothered with aphides! The damage to the crop is caused by these countless aphides sucking the sap from the leaves, which naturally results in the leaves withering away. Appearance of Winged Aphides. In the autumn, about March or April winged females appear, and these insects drift about in the wind and sometimes in a bad season they may be seen literally in clouds. These winged females do not reproduce their young alive, but lay two different kinds of eggs —still without any male interference—and these eggs hatch out into winged males and winged females. The result of the mating of the latter description of the aphides is the laying by the female of eggs with thick, hard cases or shells. These eggs are designed to remain through the winter, and on the approach of warm weather they hatch out into the wingless females first mentioned, thus completing the life cycle of the pest. This history of aphis shows a marvellous adaptation for their particular purpose in life; the first method of viviparous reproduction makes for tl.o most rapid multiplication of the species possible, while the later laying of the hard-shelled eggs ensures that the species will not die out but will be enabled to endure until the next warm weather. "The whole of the damage done by the aphis is done in the wingless stage fhc, destruction of the leaves, especially of turnips, extends down to the crown of the bulb, causing injuries which allow of the ingress of bacterial disease such as the "rot." If the rot does not attack the roots, and if the roots are hard, like Swedes, it is possible that the plants may partially recover by the putting out of fresh leaves. This is done, of course, at the expense of sie root, and as a consequence it becomes woody and hard, and loses a great deal of its feeding value. However, even in this condition it is a great deal better than nothing, as it least supplies some feed." Control Methods Laoking. "I am sorry to say that methods of control are almost entirely laoking; not because that there are not many substance# tfctt will kill tiie »pbu, bat

because of the -difficulty of practical .application, For instance, a kerosene emulsion will infallibly slaughter every aphis it falls Upon, but the difficulty is to get the insects. ' One year at the College we had a crop of jkohi-habi badly infested t*3thi apiis, tried spraying with kerosene emulsion; the attacks of the insect cause the leaves of the plants to. curl up, and we found that we should almost have to uncurl every leaf to insure the spray fluid getting on to the aphides. If a fairly clean sweep is not made of the insects, it may easily be realised by those who have read the above'life history that in a week or so they will be as bad as ever, owing to their rapidity of reproduction. Also, as' rape, is meant to be fed off, there is a probability that if the leaves were flavored with kerosene the stock might refuse to eat it, and if poisonous insecticides were used on turnips or rape there would be a possibility of killing some of the stock. Swedes Susceptible. Swedes are particularly susceptible to the attacks of aphides; I have sometimes noticed that where a swede seed or two has got mixed with turnips and the swede has grown, it has formed a base for the plight which attacks it first .and from thence spreads to the rest of the crop. When the blight comes in early, say in December or January, the swedes generally have a bad time and as often as not get killed out entirely. Last year it came in early and at the college we ploughed in the rape as useless in January. This year the blight came late, not making its appearance until February, and a s a consequence, while the leaves of both turnips and sweeds were entirely killed, the roots of the swedes were so big that they were able to make a partial recovery. They are certainly only halfgrown, but the bulbs are sound and should provide a good quantity of feed. The turnips on the other hand l , were entirely destroyed, as they suffered froc l the rot after the aphis had gone. Still, I think that in an average season, that is when the blight appears at the asual time, turnips have a bet- ■ ter chance of surviving the aphides than swedes have. Other crops that might be mentioned are kohl-rabi and cabbage; unfortunately both are very susceptible to the attacks of aphis, but a better crop still would be kale. This is very resistant and we have never failed to get a good crop of kale at the College. The trouble with these crops it that they require considerably more labor to grow. I doubt if they would succeed well on light land which would grow fairly good crops of turnips or rape. An immense amount of feed can be got from a crop of kale and it stands weeding well and so far as its feeding value is concerned is compares not unfavorably with rape. Silver Beet Recommended, There is another plant that I should particularly like to draw attention to, and that is the silver beet, as suggested by Mr A. L. Joseph. lam of the opinion that there is a strong possibility that this plant may replace, to some degree, both rape and turnips. It will stand repeated feeding down, and seems but to grow better for that process, and it is aphis resistant. Sheep are very fond of it, and when I turned some into, the experimental plots at the college, the sheep went for the silver beet first, though there were over fifty varieties of grasses, clovers, and different fodders for them to choose from. The beet was put in in October, and I found that after it had 1 been fed down the first time in February, it grew more in a fortnight after than it had done in the two months previous. It was fed off again in early April, and again grew to about 6in high. At the present time I notice that the sparrows are going for it wholesale, which is a good argument for it palatability. As for its feeding value, it certainly has a slightly higher water context than rape or turnips, but I do not attach much importance to that point; the main thing is it supplies plenty of feed of a fairly nutritious character. It is easy to grow, and any good mangel manure will suit it admirably." "From what I have already said you will gather that I think preventive measures against aphis attack impossible and curative measures very difficult. A great deal depends on the season ; if the turnips are sown at a lucky time they may survive, but usually the aphides will prove too much for them. I don't think for a moment that turnips and rape are doomed; the blight has certainly been bad for some seasons, but these visitations appear to be somewhat periodic in their nature, and we may have future seasons almost free from them. Breeding Resistant Varieties,

There is a fair hope that seed-breed-ing from turnips or rape plants that have been found to have resisted attack in an infested paddock may produce a resistant variety, but the uncertain point is this, the fact that they have resisited attack probably is because they have an acrid and unpalatable flavor, and plants bred from them might prove as unpalatable to stock as to the aphides." Natural Enemies. "The aphis has a large number of natural enemies. It is fed on by the larvae of Houer flies, Ladybirds, and Lacewing flies, and by adult Ladybirds. It is, moreover, preyed upon by Ichneumons, and on the underside of a rape or turnip leaf are always to be seen many J dead aphides, larger than the living specimens, and a shiny, pearly grey in color. These are specimens inside which the parent Ichneumon has laid ' its egg, and the grub is within the aphis feeding on its substance. In many of these swollen specimens a round hole, as if made by a pin, may be observed on the quarters of the dead aphis—that is the hole by which the Ichneumon that hatched out has escaped. There is also a fungoid parasite attacking the aphis, and this accounts for several yellowish, powdery dead insects among the others. All these six natural enemies, however, cannot reduce the number of the aphides during the height of the breeding season, and it is not "until early autumn that their united efforts, combined with the cessation of parthenogenitic reproduction diminishes the numbers of the pest,, although it is probaWy dim to iWr effort* th*t thw«

occurs the sudden total disappearance of the aphis towards the middle of March. There is no hope of any parasite coping with the aphis. That pest dies out during the winter, so that the parasite could die out, too, and then when summer comes again the aphis breeds so quickly that no parasite could keep up with it. It was different with the gum-tree scale; that was present all the year round and l its parasites could keep themselves in food both summer and winter, and so increase even more radidly than the scale. Nor do I know of any birds that will willingly oat the aphis, and though several species may be found in aphis infected paddocks, they are chiefly after diamond-back moth and caterpillars."