THE CHAFF-MITE.

New Zealand Journal of Agriculture, Volume XII, Issue 5, 20 May 1916, Page 372

THE CHAFF-MITE.

METHODS OF CONTROL

A. H. COCKAYNE,

Biologist, and, R. WATERS, Biological Assistant

It is only at certain times that chaff-mites belonging to the genus Tyroglyphus cause sufficient apparent damage to arouse any particular interest in their suppression. These mites, however, are of widespread occurrence in many agricultural products such as bran, flour, chaff, hay, cereals, grass-seeds and brassica seeds. When present in large numbers they cause great damage by actually destroying the material in which they are found. In many cases, however, the damage is not properly recognized, and no effort is made to keep the pest under control. On the other hand, especially with regard to chaff, the presence of mites in large numbers renders the fodder unacceptable to stock, and it becomes quite useless for feeding purposes. In seed-stores mites are often extremely troublesome, especially amongst turnip and rape seed.

During August of. last year a very severe attack of chaff-mite occurred in the fodder belonging to the Defence Department, stored in Wellington and awaiting shipment. The mites were present in such enormous numbers, especially in the bran and chaff, that it. was feared that unless some special efforts were made to control the pest the whole of the fodder would be rendered useless for shipping. The material had been in store for some months, and it appeared as though the mites had been introduced in certain lines of chaff, and had spread from them until the whole of the shed was badly affected. A careful examination of the : material was made, and it was found that the main damage was being done to the chaff and bran. The hay and oats, although affected, did not appear to be depreciating at the same rate as the bran and chaff. This was due to the fact that the hay, and more especially the oats, did not offer to the mites the same facilities for securing food.

As very little was known regarding methods of mite-control that could be applied on a large scale, a series of experiments were at once carried out to determine what, in the circumstances, was the best procedure to adopt. Many of the experiments were of a negative character so far as the immediate reconditioning of the many thousands of sacks of fodder were concerned, but as they

throw a considerable amount of light on the life-relations of mites they are here detailed. EXPERIMENTS WITH VARIOUS METHODS OF CONTROL. Heat. The investigations into the penetrative power of heat through infested material, and into the powers of heat-resistance manifested by the mite, brought to light certain principles that could be applied in connection with the production of chaff and

its subsequent management. A sack of chaff was subjected in a heating-room to temperatures ranging from i6o° to 178° F. Separate thermometers were placed respectively in the heating-room and at 11 in., 3 in., in., and 6 in. depth into the sack. The test was commenced at noon and terminated at 8 p.m. 'After the sack had been subjected for eight hours to a temperature of 170° F., the temperature i|in. within the sack was 114 0 and 6 in. within the sack 92 0 . While the differences between the temperatures of the thermometers at i| in. and at 6 in. in the sack had gradually

increased up to 5- p.m., such - difference had begun to diminish by 8 p.m., and would doubtless have eventually become nil : some time after the thermometer at i| in. deep had attained its maximum temperature. The difficulty of raising sufficiently the temperature of a sack of bagged chaff by firstly covering that , stack with tarpaulins and thereafter introducing -into it a continuous and large supply of heated and dehydrated air was thus. manifested —- chaff being such a poor conductor of heat. ■ ' ... The mites, being very watery and soft-bodied, showed very little power of heat-resistance. A quantity of the pests subjected directly to the comparatively low atmospheric temperature of .140 0 F. all died in five minutes. Again, a quantity of mites exposed for one hour to the direct rays of the sun (temperature 80° F.) all died. In this case the destructive effects of light may also have been at work. In this experiment, as in those with light, the mites showed a tendency to shelter in shaded positions. As the mites succumb at such comparatively low temperatures, the application of heat in combination with sieving and blowing or with chaffing is a matter that may prove worthy of consideration if the pest should appear in such numbers as to demand special treatment. The destructive effects of sunlight, on the other hand, would doubtless suggest to producers troubled with this pest some simple methods in their practice which would serve to reduce or exterminate mites in straw or chaff. If the necessity for treatment were sufficiently pressing, an appliance could doubtless be devised by which even bagged chaff could very rapidly be unbagged and passed through a heated chamber, and delivered back into sacks free from living mites or mite-eggs. Cold. A sack of chaff, mite-infested, was placed in a chamber at 27 0 F. at noon on the Ist September. At noon on the 2nd September the temperature at the centre of the chaff in the sack was 40° F., with the mites living. . The sack was then taken into a chamber at zero. At 10 a.m. on the 3rd September the temperature at the centre of the chaff was 17 0 F., and no living mites could be found. Light.— A quantity of mites were taken into a dark room and were there exposed to the rays of a 250 candle-power arc lamp. The atmospheric temperature about the mites was 64° F. All were quite dead in two hours. - Pressure.— Only negative results were obtained from experiments under this head. Half a dozen sacks of chaff were subjected in a wool-press to ■ pressure rising gradually to 35 tons, when the sacks burst— still living in large numbers. Hydrocyanic - acid Gas (Prussic - acid Gas, HCn). —-As is well known, this is one of the most destructive gases to animal life. It

is the gas commonly used in the fumigation of imported fruits, vinehouses, insect-infested dwellings, &c. Its power of penetrating sacks of chaff, however, was unsatisfactory, though its effects when brought directly, in contact with the . pests would no doubt be destructive. A number of experiments were made upon three sacks —one being opened at the mouth. Finally the large amount of 6 oz. of cyanide of potassium, with the requisite amount of sulphuric acid and water, was used in 100 cubic feet of space for two hours, when the mites were found to be still living. Sulphur-fumes (SO 2 ).The same three sacks as had been used for the hydrocyanic-acid-gas experiments were . subsequently submitted to the fumes of two sulphur candles for. twelve, hours in the same airtight chamber— still living.. The relatively low penetrative power of this gas was also doubtless the cause of failure, as the mites when exposed directly to these fumes succumbed in about one hour. . < , ' Carbon Disulphide (CS 2 ). —Three sacks were taken, and 6 oz., 12 oz., and 18 oz. respectively of the material was poured into the open mouths of the sacks, which were then sewn up and wrapped in a tarpaulin. After twenty-four , hours some of the • carbon disulphide was still unvolatilized, and many living mites were found in each sack. Seventeen hours later the following results were observed: The carbon disulphide was volatilized, there being a strong smell of the gas. Twelve tests with, a trier were then made at various parts of each sack. ■ Two living mites were withdrawn from the top of the sack treated with 6 oz., two from the top of the 12 oz. sack, and- one from the bottom of -the 'iß oz. sack. The sacks were standing upright, and the penetrative power of the heavy gas in these circumstances was much more satisfactory. The opening of all infested sacks was a matter which time did not permit of prior to shipment. ... » Carbon Dioxide (CO 2 ). —Laboratory experiments' showed that' this gas was destructive when applied . directly' to the mites. Mechanical Measures. —-It was noted that the mites were often particularly abundant in the material of the sacks themselves. This no doubt accounts for the fact that an improvement in the condition of infested chaff may be seen when it has ' been transferred from one place to another, the mites being shaken off' the outside of the sacks in large numbers, and egg-laying' also thus being reduced.

Trials with hand-sieves showed that if infested chaff received a similar shaking to that applied to seeds in seed-dressing, the mites, as well as any dust present, separated readily. No mites were found in infested chaff after it had been well shaken in a sieve composed

of round holes of i millimeter in diameter. 8 oz., or 227 grams, of infested chaff yielded 1-52 grams of dust and mites. METHOD EMPLOYED FOR BULK TREATMENT OF CHAFF. After the comparatively satisfactory results resulting from carbondisulphide fumigation, it was decided to employ this method on about sixteen thousand sacks that represented the worst-affected of the lines which could not be safely shipped owing to their extremely mitey condition. The application of heat or cold would, of course, have necessitated special accommodation and apparatus, and for so large a quantity of material would have been extremely expensive. Any method, too, that required the rebagging of the chaff was clearly out of the question for this occasion. About ten days before the proposed shipment of the chaff the shed was made as airtight as possible, and some 40 gallons of carbon disulphide was poured into the various stacks from the top. The liquid was well distributed and volatilized rapidly, necessitating considerable speed in the application of the material. The sides of the stacks were also sprayed with carbon disulphide through a Bordeaux nozzle, but this part of the work had to be curtailed owing to the overpowering effects of the fumes. After the carbon disulphide had been applied the only exit from the shed was battened up. Owing to the extremely inflammable nature of carbon disulphide every possible precaution was taken, and the shed was carefully guarded throughout the fumigation. The fumes escaping from the shed were quite strong five hours after the application of the material, and when the place was opened next day the smell was distinctly noticeable in the vicinity of the treated stacks. In fact, nearly a week afterwards, when the stacks were being broken down, the disagreeable odour of carbon disulphide was present. The effect on the mites appeared to be that many were driven to the outsides of the sacks, where they perished. While it could not be claimed that the fumigation exterminated all the mites, yet it certainly was instrumental in limiting any increase of the pests and rendered the chaff perfectly safe to ship. The whole of the chaff, with the exception of some 150 sacks picked out as likely to cause recontamination, was shipped, and arrived at its destination in excellent condition. The cost of the fumigation was extremely low, being less than is. 6d. per £lOO worth of material treated. TREATMENT OF INFESTED BRAN. The mites were found particularly abundant in a large stack of bran, in which they appeared to thrive, owing to the large amount of food easily available to them in a powdery form. The stack

was sorted over, and all seriously affected sacks were made up into five small stacks each about five or six sacks high. These were stacked somewhat loosely, to facilitate the percolation of the gas. Each stack was well covered all round with tarpaulins, which on top were folded back preparatory to the application of carbon disulphide. As soon as the liquid had been applied these top covers were drawn over, and the. stacks were left overnight to fumigate. As with the chaff, it was noted that after treatment the mites came out through the sacking in considerable numbers for some days afterwards and fell to the ground, all such being swept

up daily and destroyed. Owing to the denser nature of bran it was considered that the gas would not penetrate as readily as through chaff, this being the reason for making up smaller stacks. The fumigation of the bran certainly resulted in a very appreciable reduction in the numbers of the pest, sufficient at least to enable the military authorities to thereafter dispose of the fodder according to their requirements. GENERAL RECOMMENDATIONS. It is well recognized that the subject which is being , dealt with is important enough to warrant very much closer attention than

there has been opportunity so far to devote to it, and the intention is to further pursue the subject of treatment as the opportunity offers. In the meantime the remedial measures that have been suggested by the previously mentioned experiments are now outlined for general information. The mere removal of chaff from one place to another is in itself a ready method of improving its condition so far as mites are concerned. Thus large stocks that are intended to remain a long time in one position are much more liable -to a bad outbreak than smaller ones that are being constantly removed and replaced by fresh supplies. It should here be noted that the mites are very generally present in great numbers in the fabric of a badly infested sack of fodder material, and that with practice the condition of that sack may be readily, determined by the animal, smell on the hand after it has been rubbed , over the sack. By this action, moreover, the crushing of numerous mites lends a distinctly wet and sticky feeling to the sack. ■ . . The fine hairy parts of a sack . are • frequently found to be covered with mite-eggs ; hence the use of sacks which have previously held fodder or meal of any kindparticularly flour, and the like —should be abandoned as far as possible. ■ The exposure of infested sacks to the sun would doubtless greatly improve them, but so far little is known as to the capacity of the mite-eggs for resisting unfavourable conditions. The use of carbon disulphide as a fumigant for empty and infested sacks is to be recommended. , One of the simplest methods of dealing with a badly infested line of chaff is. to sieve it. With sufficient shaking the mites and many of the eggs may be separated from the larger particles to which they are clinging or attached, or fall through with the finer dust and hairy matter. In the experiments carried out a . sieve made of circular holes I millimeter in diameter was used, but doubtless a large mesh could be employed with advantage and without undue loss. A purchaser of a bad line might well use this method; if necessary, leaving. the chaff so treated a short time to sweeten before offering it to live-stock. The conditions under which fodder of all kinds, including hay, are stored is a matter that deserves close attention. The admission of an abundance of circulating air and of light is highly desirable, but most important is the need for good dry conditions in the store. One of the chaff - stacks that was dealt with as above recorded was built round a hydraulic jigger, at the base of which was an iron drain to carry away any water that might fall from the joints higher up. This drain had become . blocked by the chaff that had fallen through several rat-holes in the surrounding sacks.

There was consequently a constant uprising of water-vapour from the overflowing drain and wet floor below. Nearly all the sacks exposed to these damp conditions had to be held back from shipment, owing to the extent to which they were infested by mites, particularly on their exposed surfaces. Carbon disulphide is commonly used in grain-mills for fumigation purposes at the rate of about i lb. to every 1,000 cubic feet of space, but for the treatment of stacks of bagged fodder a much heavier application is preferable to ensure quicker penetration. In the method finally adopted material up to 5 lb. per 1,000 cubic feet was used. The gas into which this clear liquid volatilizes is heavy as compared with the atmosphere; applications should therefore always be made to the tops of stacks. The more' limited and confined the area to be fumigated can be made the better small stacks may be covered with some suitable material. The buildings in which extensive fumigation is being done' should be made as airtight as possible. The longer the gas. is allowed to remain undisturbed the further it may be expected to penetrate into the sacks. The main objection to the use of this fumigant is its very highly explosive nature, and every precaution must be taken that no spark of any kind of fire or heat shall come in contact with the gas. The flash at the switch of an electric-light button has been known to create a . big explosion. - A diversion from the ordinary method .of stacking could well be made in order to facilitate the fumigation of chaff-stacks. A scheme was devised by which smaller stacks were practicable, and stacks provided with air-channels throughout might be formed to this end. The use of insulated meat-vans as fumigation-chambers was proposed, but, as described in the case under notice, large tarpaulins were used for confining the fumes about the smaller and worse-affected stacks of fodder such as bran. NATURAL ENEMIES. ' Little is at present known of the various kinds of mites to be found in . infested chaff-samples. Besides the smallest and commonest hyaline mites, other much larger and variously. marked mites were observed. It is not improbable that some of these larger kinds are parasitic on the smaller. As with all the other work performed in connection with this particular outbreak, however, the extent to which the observations and experiments were carried was limited by the fact that the best possible conclusion had to be arrived at and acted upon in the space of about fourteen days, when the military authorities were obliged to ship.