FARMERS' FOES.

Dominion, Volume 4, Issue 1203, 11 August 1911, Page 8

FARMERS' FOES.

• THE DIAMOND-BACK MOTH. . Tho damage done, by the diamond-back mot.h every year must run into thousands of pounds, but, according to experts, this.pest, bad- 'aa it is, is not as serious as tho' aphis, which, with the. turnip-rot, is the. great destroyer of valuable fodder crops. The life, history of the motlris described, as under.by Dr. Hilgendorf, of Lincoln Col lose: — "The nnths themselves are of a Rreyisti colour, about three-quarters of an inch long, and dowji 'tho interior edges ol us wings arc 'two- irregular .white markings whigh, when the wings aro closed, form diamond-shaped spots, from which tho insect gets its name. They may bo easily distinguished on> the turni]> crops and on cabbages, etc., and have, a characteristic flight. They never, by. any chance, fiy high, but always low down, a few inches above tho top "of the leaves, and it must be by .their vigorous and rapid .flight, Tather than being blown by the- wind, that they manage to get so quickly and widely: spread about tho country. The insects Jive in the moth stage for about fifteen days, andduriu;: tills time the fema'cs lay about eighteen cg?s. Those egss usually take eight days to hatch, and tho little caterpillars immediately start to feed, and continue, to dp to for about twenty-two days. By this time they may •be about'half an inch, nr. more, lone and all -tlia time they have been feeding Tavenously on the under surface of tlic leave?, just eatiiipr the soft green part and leaving the upper skin of the leaf intact, which act.; as a kind' ot' transparent shel■fe.r. When they. aro. full.fed ■' they ench spin themselves a'.'cocoon, of. a. silky material, in which -they pupate, or go through the chrysalis stage. This stage takes about seventeen days, and then tho perfect moth, emerges, from.. the cocoon, thus completing the lifc-cyclo of the insect. Jt. will be noticed that their life lasts tbout fifty-three days, or, say, eight weeks, from the egg to the egj, and this'fact has aii"important bearing upon measures that can be taken to limit their number." "The diamond-back caterpillar does not only exist upon turnips, but will afco livo upon rape, cabbage,, kale, Ewudes, wild turnip, and, besides these, .npon garden, stocks and wall-flowers." .

'Smokingthe Plants. After dealing with various experiments, . Dr. Hilgendxu'f says- he has never found smoking the pianls fail; to make- all the caterpillars loo?o their hold, ami from this fact ho thinks gardeners could evolve a good plan to secure a clean crop of winter cabbages, which at , the present time are not at all .easy. to grow freo ■ from iusscts. It would bo very little • trouble for a market gardener to gq along tho rev;s of his cabbage patch wi Hi an ordinary smoker, os used by beekeepers, pmoke' the underside. of tho, plants and then follow it up with a good brushing and booing. This would break all the caterpillars' threads, and bury them, and tho odds arc it would effectually kill i them. "The only method of. dealing widi tho math'and its caterpillars in a wholesale manner , is' based upon a knowledge of its life history. The pupae flint survive the winter hatch out about September 1; half of •.these (the females) lay 20 eggs each (approximately), k> ' that for one moth at September. 1 wo have 10 at .November 1. ,'llalf. qt" these again Jay.'.'iO eggs each, r6 that for each one at November 1 wo lhavfi 100 at-January-i.' This is the time when they begin to attack the turnip crop, btei- they are not really noticeable until the riex! bre-od hatches about March March 1, when tiiore will be 1000 moths for%ver,v one that hatched out at September 1. Now it is clear that if there was no food for the lot that hatched out. on September 1, that the bulk of them would die of starvation. But they manage to live on garden cabbages, etc., and ns they become more numerous, they get distributed" to the turnip fields. If thtra wero no other food for the insect but the main turnip crop, then the caterpillars worth! certainly die for want of sustenance in the early summer, and even if the hatching out of the pupae that survived the winter was postponed, until January, we should only have one-huii-drcdtli of the number ofoatexpillars that we have now. Consequently if widespread action were taken tq cease growing plants -tint would afford the pest food in tho .early summer its ravages would be very jßTcatly reduced.' This theory necessitates the supposition that the moths are. capable of travelling considerable distances, because, tnrni|> crops that are not neo.r ;gardens aro imiueutly badly attacked; end a study of the. agile movements of the moth indicate that they do so travel, and supports the theory. Such, a plan, would be useless in a district whore there was β-quantity of wild twnius growing, but ]and which usually.carries wild turnips is generally of a class that is' not vwy suitablo for growing turnips, as it ig heavy land. ProJectivo Plant. "Tlio moth werns very susceptible to .changes of climate, aud it is niiite. probable tliat there will come years in -which it will not bo nearly so bad. We liave had some wild winters lately, and it will bo interesting to note if tho turnip and rape crops are-freer from pests next year after all this cold and wet. I thought at one time that I had found out a plan to protect winter cabbages from the attacks of both moths and aphis, and that was by planting the. cabbages between two vows of parsnips. One season when most crops of winter cabbages were ruined the plants set out at tho College between rh-o parsnips were sound and'free from blight, and from the caterpillar as well. This season thny wp-ro quite free i'rom Wight, but diamond-back caterpillars upon them were as bad o.s ever. It may bo the peculiar scent of tho parsnip plant that protects tho cabbages from aphis, or it may be that'it is merely tho shelter, afforded by the parsnips that docs so. At any rate, it is a method that is well woriny of trial by gardeners, and if the parsnips keep oft' the blight, and tho smoking and holing destroys the caterpillar*,- it should no longer be impossible to. grow cabbages, etc., for winter us? Acting on similar lines I planted sheep's parsley (allied to the parsnip, and. frequently used o.s' a sheep fodder); with rape, hoping to protect the latter from tho aphis, but the rape grew.so fast that it smothered the parsley, audthe experiment failed. ■ ... "The conclusion of the whole mutter (says Dr. Hilgendorf) is: M.il props of turnips cannot by any moans at present known, be saved from the diainond-Wx moth, because'no matter how success! ill any of these methods might be, tne trouble and expense of applying them" to field crops is absolutely prohibitive. "Wo have had.a.long run of seasons bad with this caterpillar and the time may bo near nt hand for one of its occasional cessations. This is our present hope* but I lielievo that our efforts should be vigorously directed towards finding suitablo fubsljl.ules for rape and turnips, whose successful cultivation has become so rare."