THE POTATO MOTH.

Oamaru Mail, Volume XXXIX, Issue 10762, 9 May 1911, Page 6

THE POTATO MOTH.

(By A. H. Cockayne.)

The Journal of Agriculture contains the following interesting article on the potato moth by Mr A. H. Cockayne, the Government expert: A very severe outbreak of the potatomoth (Gelochia operculella) lias been experienced in the South Island, and the present crop lias in certain localities been very materially affected. This insect is especially prevalent in North Otago and Canterbury, two of the most important potato-growing districts of the Dominion. Recently I examined on tlie Wellington wharf several lines of potatoes, totalling some seven hundred sacks, which had been shipped irom Oamnru. .All these lines were badly infected with the moth, in various stages from newly hatched to adult and pupating larva'. Many other samples of affected potatoes have been received from localities as widely separated as Christcliurch and Oamaru, showing that the pest is well distributed and not restricted to any special district. Some seventeen years ago the South Island crop was seriously affected, hut between that date and .this season the potatomoth lias not been reported as causing anv appreciable loss in the South Island, although it has long been recognised a.s a .serious and annually recurring potato trouble in the North. —The Distribution of the Moth —- . The potato-moth is especially serious in the Mediterranean regions, South Africa, California, and Australasia. In Tasmania and many parts of the Australian mainland it annually occasions great loss and seriously depreciates the crop, especially after it has lieen stored. In Xew Zealand it is now widespread throughout the North Island, and in the South' as far as Dunedin. The important potato-growing areas in Southland are apparently, as yet free, but it is only a matter.of time before, the moth will be prevalent as far south as the potato is grown. —Damage Caused The tubers are the parts chiefly affected, becoming tunnelled with the galleries of the larvte of the moth and rendered unfit for., consumption". Some may be affected before the tubers are lifted, but the majority of the damage is caused after the crop has -been harvested. Affected tu.bers- frequently rot in storage, especially if they are not kept quite dry, and the rot may be spread rapidly under humid warm conditions and involve tubers that would otherwise remain sound. If no methods for control are adopted it is not unusual for the entire crop to become riddled and rendered completely useless, 'lhe potato-moth can live on other plants besides the potato, such as many solanaceous plants like the black nightshade, native poroporo (Solatium aviculare), and tobacco. In parts of Australia and the southern United States where it Is a common tobacco-pest it is designated as the tobacco-leaf miner. :—Life-History.— . . . . In haulm - infection the larva; bore dowii the stem, and, it is said, in this way enter the tubers. So far as tlie present South Island;, outbreak is. concerned, the, majority of tji.at has occurred dur-. ing the growth of the plants has taken place by eggs" deposited directly on the tubers. This is ' evidenced by the fact of the eve end " being . invariably attacked.

"When once .the larvfpluvve entered the tubers tliey tunnel, galleries in all. directions, aria wlieTi'£ potato is cut across the whole interior may be found to be riddled,, larvae are about Jin, _ iii' -length, of a dirtywhite color bfSenVtjflgea with a pinkish or greenish. itho' head is dark and iu . full-grown,"specimens quite black. They may rennun in the tubers for about eight weeks'when they work their way out and seek a suitable place to pupate. They often pupate on depressions on the tubers, or on the sides of the sacks, in crevices in the store-house, or attach themselves to the straw coverings of the pit.-. - . ~The \ wings are light brown, mottled- with dark brown or black ; the hindwings are narrow, blu'e-grev in. color, - with a deep . wide fawn-colored fringe. - -. *. There appear, ito . be several broods throughout the year*' but in all probability the last", brood develop pup® that, reinaiii dormant. for. ; rseveral months, y the winter is at< all. severe. The broods overlap one another,; and this ,renders control methods more difficult itli'an if tliey we're well defined, —rControl Methods.^— Many methods have been suggested, for the control. of, the but little concerted aetioii is_ey(jrtaken by farmers in,anjr.country for,the suppression .of th^^'pest. l; .' The Jipatliy of the farmer towards, controlling the .cropenemies v so large a toll of. hisjjrodiicts.is almost,incredible.: The efficjency of Bordeaux mixture in controUing.irisl^,potato-blight, and the almost complete disregard of .this treatment by tlie average farmer, is a case in point. . . . . . It is, perh'ajpV; too much to hope that the suggestions that ifre here given will be earned out, but the thorough adopttion of 'them' would' to reduce the effects of the potato-moth to a minimum.' /Tliemethods to a.dppt naturally. f ifie, jtwo'^ii^ngs—the protection of,tlie grqwipgcroj),. and the protection of tbe tubers' after digging.

—The Protection of the Growing

. Crop.—-: Selection of Land.—Fields that have carried a diseased crop should not be planted with potatoes for some seasons. Mangels, turnips, grass, or corn crops can, however, be grown on affected land without , any. danger. A proper, croprotation should always be adopted where insects or fungi are at all troublesome. Liming the Soil. —It is 6aid that liming the soil before planting with airslaked lime applied at the rate of about one ton per acre helps to ward off an attack, but the. reason for this is not very obvious. In Europe liming of the soil is regularly practised in regions where the closely allied insect, the sugar-beet, moth* (Gelechia ocellatella) is troublesome.

Planting.-r-Greafc care should be exercised in the,,use of .clean seed., Diseased sets are not only liable to carry infection, . but also in many cases by having the eyes eaten out will not develop... Tubers, with tunnels in them also allow the free entry of putrefying bacteria and fungi, and often rot rapidly when, planted, especially if wet weather is experienced. The sets should be planted rather deeper than usual, provided the land is well drained find not too heavy. Early crops appear to escape attack better than the late ones, so that it would seem a good practice to get in the main crop as early as possible and not unnecessarily delay planting. Cultivation. —One of the main field treatments is to keep the plants well moulded up. This should be done very carefully, so as to avoid any tubers becoming exposed during their growth. From this it follows that the ground should be in good tilth, and that the presence of large clods, between which the moths could crawl, is a decided objection. For ease and efficiency i" high moulding the rows should be kept well apart, 30in..being little enough to allow. Tlie fields should also be kept as free from weeds as possible. Spraying the Tops.—French and Kirk have suggested the spraying of the tops with tar-water as a deterrent towards egg-laying, but 1 am unaware whether this has ever been carried out on a large scale. The spraying of the tops several times during the season with Bordeaux" mixture for. the eontrol of Irish blight, a proceeding that should never be neglected by any potatogrower. might also to a certain extent ward off ovipositing. A combination . spray of Bordeaux arsenate, as regularly used by fruitgrowers, might prove much superior to ordinary Bordeaux.

—Protection of the Tubers after Digging.—

Protection before Storing.«-A great deal of infection is liable to occur' between the period of digging and storing. Any tubers that are dug and left on the* ground overnight are liable to have eggs deposited on them by .the, moth, which is ehieflv, a night flier. Bags standing in the fields with j the tops covered with haulms may also have the potatoes badlv infested .before removal to the store or pits. Potato-bags used in the field are also often full of holes, and easily allow the entry of the moth. For this reason the. use, of clean new bags is advocated, and they should be carefully sown up as soon -as, they are filled. If heaps of potatoes have to he left exposed they should bo thoroughly covered by a thick layer of earth well patted down so that- there : w ( ill be no crevices through could reach the tubers. Cases' have arisen where potatoes placed under the shelter

of a tent-fly liavc become badly effected after being dug. Sorting Over. —Before being stored or sent from the farm all tubers which contain any grubs, the presence of which can generally be recognised by the little masses of frass at or near the eyes, should be.treated. The best method is either to soak in water for some days or boil. They can then bo fed to pig s, but as all the affected tubers have to be dealt with at once oidy a certain amount can thus be made use of.

Dipping the Tubers.—lt has been suggested that all apparently sound potatoes soon after digging as possible be thoroughly dipped in some material that will kill any young larvpe that may develop from the eggs that may have been deposited on tlieni. The dipping in corrosive, sublimate at i the rate of loz. to 8 gallons of water has generally been recommended. One ounce of the corrosive sublimate is dissolved in, 1 gallon of water, and then 7 more gallons is added (metallic vessels should not be used). This should be allowed to stand for some hours, being occasionally agitated before the tubers are dipped. An immersion of about fifteen to twenty minutes will- be found satisfactory. I believe that dipping in arsenate of lead at the rate of lib toi 50 gallons will be found even better, but this has not been previously suggested. It has the advantage that iron vessels can be used in which to mix. The idea is to cover the potatoes with a thin film of poison, as is done for the contruf of the codlin-moth. Both these mixtures are poisonous, especially the sublimate, and if the potatoes are used for food they, should 1 be well washed in cold water before being peeled or fed to stock. Owing to the poisonous nature of these washes great care would have to be exercised in their use, but as corrosive sublimate has been recommended by certain writers it is as wiell to mention the procedure to adopt. The use of formalin has also been suggested, one pint to 3o gallons of water. This would tend to destroy any eggs that were on the tubers at the time of application, but would not form a protective coat, so that the process would have to. be repeated from time to time. Treatment of Field after Digging.— All the haulms in an infected field should lie raked up and burnt after the tubers have been, removed. The fires may, with advantage, be lighted at night, as they will then help to attract the moths that are flying about,. and large numbers may in this way be destroyed. The ,■ -ttiriiiag-iin of pigs or sheep into the fields should also be useful in getting rid of all the odd potatoes that may have been overlooked, and which otherwise would' help to carry infection. After this the fields should be deeply ploughed so 5s to get rid of anv weeds that might act as hosts. Protection after Storing.—ln storing the potatoes the stores should have all the crevices stopped so as to make them as moth-proof as possible. It is a good plan to tack buttercloth in front of any windows or ventilators that may be present, and tightly fitting doors are essential. In stores where there is bad- ventilation the doors may be left open during a portion of the day, but should be always closed nt night. Fumigation. —It is, of course, more than likely that,. Rio matter how, carefully the, potatoes have ..been sorted: over, tubers' with the grubs already," in the tissues may find their way into ,the stores. The potatoes should from time to time be .and any. affected! or rotting ones be removed and buiait. 'lf any-moths or pupa-eases-are noticed the whole store should be fumigated with carbon-bisulphide, using 20 fluid ounces for every 1000 cubic feet involved. This material,. being heavier than air, should be placed in shallow earthenware'dishes on top ,of, the potatoes ; saucers placedhere .and; there will be found convenient. Carbon-bisulphide is extremely inflammable,. ,md' .lip. naked lights should be used' during fumigation. This.operation..may have to be repeated -several times -at intervals of about a Ifcittniglit, on account .of ;fche overlapping jof.:..,the, broods;;and more or less.conr tiriuous emergence of the insects. .After the potatoes have been removed it is a good plan to give a final fumigation and to burn all refuse that may be lying about. . .... r Pitting not advised.—The pitting Of tubers likely to be affected is not recommended, as when once the pits, are closed it is difficult .to the amount of infection that' lias' occurred. When the pits are opened all the affect- - e.d potatoes- should be Jburnt, and'.all.rer ' from' the pit-, Especially the straw ■ cover irigy Which - will harbor-large nu™" bersof the dormant insects, should be dealt with. This.is important, and should help to lessen, infection year. . —Conclusion.— The' methods of dealing with the tubers are rather troublesome to adopt, but -some Such 'system...will",have to becarried out if the moth is liable to .continue to be as destructive as it lias proved-; this year in the South Island. The keynotes in control lie in the >us'e of clean seed, high moulding, destruction of all affected, material, storing_m moth-proof stores, and carbon-bisulplnde fumigation where necessary..,