Assessment of insect mortality

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NATURAL INFESTATION

If the insects infesting goods or premises are present in fair numbers, a reliable estimate of the success of a treatment may be made by collecting representatives of the species and stages present. If any of the insects are inside grains, seeds, nuts or other plant material, samples of these should also be taken. If possible, each sample taken from the different positions should weigh not less than 0.5 kg or 1 lb. It is important to collect from as many different positions as possible; small collections and samples from many positions give a better picture than large ones from only a few places. Where bags or packages are concerned, insects and samples should be removed both from the middle and the outside of as many containers as is practicable.

If sieving the infested material is necessary, it may be done at the actual fumigation site, but the insects collected in this way should be handled with care. Insects and samples of the infested material should be placed in small tins or pillboxes which are not absolutely air tight and taken to the office or laboratory as soon as possible. Here they should be transferred to clean jars or bottles containing some of the material they were feeding on. Fresh air must be available to the insects, and cloth, metal or plastic gauze should, therefore, be placed over the top of the new container in a way that will not allow the insects to escape. Petri dishes with the upper cover resting firmly on the lower dish are suitable because they usually do not permit escape and fresh air is able to reach the insects.

Appraisal of Mortality

It is unwise to attempt to appraise the results of the fumigation immediately in terms of insect mortality. With some fumigants, such as methyl bromide and ethylene dibromide, mortality may be delayed. Others, such as hydrogen cyanide (HCN), cause temporary paralysis and the insects may fully recover some time after they are restored to fresh air. It is advisable, therefore, to leave the insects in a warm place (20 to 30C) overnight before mortality counts are made and before definite conclusions are reached as to the success of the treatment. Some insects feign death. If gentle heat radiating from a 40- or 60-watt lamp is applied to the insects, the living individuals normally resume active movements within a short time.

Samples of the infested material, which may contain immature stages of the pests, (e.g. see Section 'Toxicity' under phosphine in Chapter 6) should be kept in 1 litre (2 pint) glass jars covered with 20 mesh to the inch (2.5 cm) screening. This material should be kept, if possible, in a warm room (20 to 30C) at not less than 70 percent relative humidity and examined or screened periodically to watch for emergence of adults.

TEST INSECTS

Some thought and care must be given to the way in which test insects are used, otherwise conclusions based on their mortality may be misleading. For instance, if the insects are brought from a warm room and exposed in a treatment conducted at a much lower temperature, they might, with certain fumigants, be more likely to succumb than if they have been conditioned to the temperature of the fumigation for several hours or days before it begins. This and related aspects of insect reaction to fumigants were discussed in Chapter 2.

As a general rule, test insects should be exposed as adults, unless special information is needed on immature stages. The insects should not be placed in bare cages or other containers used for holding them during exposure; they should be put in with some of the normal food used in the rearing culture. The containers should not be jammed full with either food or insects. A container three quarters full of food with about 30 to 50 insects is a good arrangement. A better interpretation of results is obtained by dispersing the available test insect population in many places in small numbers rather than by concentrating large numbers in only a few stations.

Test Capes

Cages or containers may be improvised from available materials or bought from dealers. If the containers are exposed in the free space of a fumigation system or placed between packages or bays, tins with capacity of 30 to 60 ml (1 to 2 oz) may be used. At both top and bottom of the can a hole 2 cm (0.75 in) in diameter should be punctured and the hole closed on the inside by fine mesh or plastic screening, which should be soldered or strongly glued to the metal. The mesh of the screening will depend on the insects being used; 20 mesh to the inch (2.5 cm) is suitable far most purposes and is recommended for the two species mentioned below. Experience will show the best way to use available materials. Some insects will bore through cloth or plastic screening.

Other types of cages may be made from cylinders of metal or plastic gauze, a good size being 12 x 6 cm (5 x 2.5 in). The seam may be soldered or some of the wire wound in a spiral around the gauze cylinder while it is rolled firmly on a metal rod (Brown, 1959). The ends of the cylinder may be plugged with corks or cotton batting (some species may become entangled in the cotton).

As mentioned in Chapter 6, phospine reacts strongly with some metals, more especially copper. Copper, or copper containing alloys, should not be used in the construction of test cages to be used with this fumigant. Stainless steel is suitable if it is low in copper content. Generally, it is better to use plastic cages and screening for test purposes when working with phosphine.

Metal Probes

For the purpose of testing the reactions of insects inside a closely packed commodity, metal probes, available commercially, may be used. These have narrow, cylindrical pointed heads, which may be thrust deep into dense materials. These heads are small chambers 5 or 8 cm (2 or 3 in) long with narrow slits on three or four sides to allow the fumigant to diffuse in. The heads are threaded to the main stem of the probe so that the insects with their culture material may be put in and taken out. Probes may also be specially made in a laboratory or industrial machine shop.

Interpretation of Results

The same considerations already discussed for the handling of insects infesting the material, and for the interpretation of mortalities observed, apply also to test insects. As suggested, judgement must be exercised in assessing the results obtained. Until an observer has had considerable experience in using test insects to indicate the success or failure of commercial fumigations, he must be especially wary of undue optimism based on a few favourable results.

Species Used

For general fumigation work, the granary weevil, Sitophilus qranarius (or other species of Sitophilus) and the confused flour beetle, Tribolium confusum, are easily reared and are suitable to use for test purposes. Also, as Table 16 shows, they vary in their response to different fumigants. For instance, S. qranarius is more resistant to HCN and chloropicrin and T. confusum is more resistant to CS2, ethylene oxide and methyl bromide. If both insect species are available, it is advisable to test the one that is more resistant to the particular fumigant or mixture of fumigants being used.

TABLE 16. CONCENTRATION x TIME PRODUCTS* OF CERTAIN FUMIGANTS REQUIRED FOR THE CONTROL OF VARIOUS SPECIES OF INSECTS

Rearing Test Insects

For general use, the two species mentioned are best reared at 25C and 70 percent relative humidity in an incubator box. If an incubator is not available, rearing may be done in an ordinary office room in which the temperature does not fall below 20C. If necessary, moist conditions can be provided by placing the culture bottles in a box or small cupboard with a small fan blowing across a flat pan of water. Other methods of maintaining a reasonable humidity may be devised. The humidity is important because Tribolium, especially, does not rear well under dry conditions.

The insects are reared in screw-top glass jars of 1 litre (2 pint) capacity. The glass lids are replaced by discs of plastic or metal screening of not more than 20 mesh to the in (2.5 cm), which are fitted tightly by means of the screw tops after the insects are introduced.

Granary weevil. Place 350 9 (0.75 lb) of soft wheat in the glass jars. Introduce from 100 to 250 active granary weevils (100 adult weevils occupy approximately 1 ml in a measuring cylinder) and leave them on the wheat for two to three days. If only small numbers are available, they should be left for a longer time; and it may take several generations before populations adequate for test purposes are built up. After five or six weeks, weevils will start to emerge. At this point it is a good idea to sieve and transfer the insects to fresh jars of wheat once a week. The date of transfer is marked on the fresh bottle. Regular weekly transfers provide for the starting of new cultures. The weevils are best used for test between two to four weeks after emergence from the kernels.

The granary weevil is able to survive for over a month at temperatures between 0 and 5C and is tolerant to temperatures as low as -10C for about two weeks. It is, therefore, suited for use as a test insect in fumigations at low temperatures.

Confused flour beetle. This insect is reared on whole wheat flour. The culture jar or bottle should be only about half full of flour. On the flour, place between 100 and 200 adults, which should be taken off by sieving through a 20-mesh screen after a week and placed in another jar. The new generation of 1 000 to 2 000 adults will begin to appear after about one month and these insects may be used for test purposes two to four weeks after emergence.

Adults of T. confusum should not be allowed to become too crowded in the culture bottles, because the insects emit a vapour which may be selftoxic. Overcrowding is indicated when the flour turns pink from reaction with this yes. In view of the fact that autointoxication is possible with the confused flour beetle, great care must be taken to make sure that test cages are well ventilated when in transit to and from the fumigation site.

The confused flour beetle is sensitive to low temperatures and is best used at temperatures above 10C.

Other test species. Other species of insects, more particularly species of beetles which are fairly easily reared, are suitable as test insects. The Khapra beetle is recommended by Brown (1959) because it is resistant to methyl bromide. This insect should be used only in countries where the species is endemic. Spider beetles (Ptinus spp.) are useful for tests at temperatures below 10C, but they are not as easily reared as the species mentioned above.

Information on the rearing and handling of many insect species suitable for use as test insects is given by Campbell and Moulton (1943).

Fumigation failures

The effectiveness of a fumigation depends on many factors - the properties of the fumigant, the gas tightness of the chamber, the nature of the commodity, the condition of the environment and the type and condition of the target organism all have direct effects on the treatment. Occasionally, well established treatments do not give the expected control or other unexpected effects may occur; very often the reasons for such results are not immediately apparent.

Throughout this manual many potential problems with the fumigants themselves or with the fumigation procedure have been referred to. Most of these problems can be avoided or overcome by having a good working knowledge of the fumigants and by continually exercising care in their use. Some of the usual reasons for failures include:

- inappropriate choice of fumigant;

- improper application;

- uneven gas distribution;

- poor penetration of goods;

- loss of fumigant through leakage;

- temperature variations, especially low temperatures;

- excessive sorption by materials;

- insect resistance.

A fumigation treatment may be considered as a failure if the pest organism is not controlled or the commodity is damaged.

LACK OF CONTROL

Any insects that survive a fumigation do so because they have not absorbed enough of the toxicant for it to be effective. Dosage schedules are designed to cope with most variabilities, such as species, stage or condition of the insects, and they are made for a range of temperature conditions. Difficulties usually arise from unobserved or unusual conditions that prevent the fumigant from reaching the insects. Loss of gas through leakage or absorption can be a major reason for lack of kill and poor penetration into a commodity can be another.

Most problems concerning loss of gas can be avoided by proper preparation for the treatment and analysis of gas concentrations at appropriate locations during the exposure period. Information on gas concentrations gives the fumigator a high degree of control over the operation; it can give a good indication of gas dispersal, maximum levels attained, penetration into materials or loss from the space. Problems can usually be overcome by extending the period of fan circulation, by adding more fumigant or by extending exposure time.

Temperatures within a commodity or at different locations in a structure can vary greatly so that insects in areas of lower temperature may survive. Ranges of 10 - 20C or more in commodity and free space con occur, particularly in structures exposed to strong sunlight. It should be noted that at temperatures around freezing the effectiveness of a fumigant is quite variable and may not give control (see Chapter 2). Since fumigant effectiveness is closely related to temperature, the dosage must be adjusted to the lowest temperature for complete control.

Insect resistance (see Chapter 2) is a matter of great concern to those using fumigants because of the small number of fumigants available and because of the lack of effective alternatives. Any indication of resistance to fumigants should be thoroughly investigated. If control procedures become ineffective when conducted according to recommendations, a careful check should be made for resistant insects. The FAO(1975) method for detection and measurement of resistance can be used. If resistant insects are found, measures should be taken to eradicate the population completely and thus prevent dispersal to other areas.

DAMAGE TO THE COMMODITY

Although recommended treatments are designed to kill pest organisms without any effect on the commodity, unexpected injury to plants or damage to other materials does occasionally occur. Many of the different effects that fumigants can have on materials are given in Chapter 6. Information on the tolerance of various plants and plant materials are also given in the dosage schedules.

Some of the types of damage that should be guarded against include corrosion, fire or explosion and changes in odour or taste.

Corrosive effects. Damage may result if methyl bromide comes in contact with flame or hot wires to form the corrosive hydrobromic acid. Phosphine can damage electrical equipment with copper wires, particularly in conditions of high humidity.

Odour and taste effects. Methyl bromide occasionally reacts with flour to produce offensive odours during baking or in hot loaves of bread. A list of other materials that are adversely affected by methyl bromide is given in Chapter 6.

Fire and explosion. Although treatments are always designed to avoid any condition that will produce fire or explosion, accidents do occur at times. Containers of phosphine fumigant may occasionally flash when first opened due to release of accumulated phosphine gas. For this reason, they should always be opened out of doors away from any flammable material. Fires have occurred on grain ships when phosphine producing formulations that had been laid on the surface of the grain cargo shifted during the voyage to accumulate in one corner of the hold. Such formulations should always be used so as to avoid any possibility of high concentrations of phosphine accumulating in confined spaces.

Fumigation and dust explosions

Concern over a possible connection between the presence of fumigant in grain and grain dust explosions has been expressed; however, it must be pointed out that the matter is uncertain at the present time. Some tests have given evidence to suggest that there may be significant interactions between fumigant vapours and explosive dusts; tests with a mixture of 80 percent carbon tetrachloride and 20 percent carbon disulphide, which by itself is nonflammable, were found to lower the explosive concentration of commercial flour dust appreciably (Atallah, 1979). However, another investigation with three fumigant mixtures, containing carbon tetrachloride and ethylene dichloride or carbon disulphide, showed that there was no increase in the severity of grain dust explosions and in some cases the vapours actually suppressed the explosion (Tait et al, 1980).

The uncertainty about any connection between fumigation and dust explosions will remain until further information is available. However, due precautions against creating a hazardous combination may be warranted, particularly with fumigants that have relatively low flammability limits.


15. Training in fumigation

The effective and safe use of fumigants is dependent on a good knowledge of both the chemicals that are used and the procedures employed in fumigating. Present standards and restrictions on the use of pesticide chemicals also require the fumigator to know many things that were disregarded in the past. Personnel using fumigants should have practical knowledge of the pests they are dealing with, they should know the hazards involved in using fumigants and they should be familiar with methods of detection and analysis. Also, they should know safety and first aid procedures and have some knowledge of official regulations governing the use of these materials.

Many national or local governments require that fumigators be trained and licensed before they are permitted to apply fumigants. A working knowledge of the technical and business aspects of pest control can be best achieved by a period of study and training. On-the-job experience is important, but this approach alone can lead to mistakes and offers fewer opportunities for career advancement (Osmun, 1976). Training courses should include instruction on the basic principles of fumigation and pest control, practical demonstrations on procedures and a period of apprenticeship with an experienced fumigator. Competence for certification is determined on the basis of written examinations and performance testing. Refresher courses are given in some countries to ensure that high standards of competence are maintained and that applicators become familiar with new developments as they take place.

From the viewpoint of the truly professional fumigator, every applicator should want to become certified, as this is the only way he is likely to be exposed to sufficient training to become professional. For the employer, the value of good training for his employees will be evident in their competence and ability to carry out the operations in an effective, safe and professional manner.

The information given in this book can serve as the basis for a course of instruction in fumigation. Chapters dealing with principles of fumigation, properties of fumigants, methods of analysis, safety precautions and protective devices are particularly important. A comprehensive course on fumigation would include the following subjects:

1. General introduction to the principles of pest control, including a brief account of methods of food storage and preservation, infestation and destruction of food and other commodities by pest organisms, methods of prevention and control of infestations, integrated pest management programmes.

2. Principles of fumigation

a. Definition of a fumigant; choice of fumigant; vaporization of fumigants; latent heat of vaporization; law of diffusion; specific gravity and distribution; mechanical aids to diffusion, sorption and desorption; chemical reactions; residues.

b. Dosage and concentration; calculations for conversion of concentration values; concentration x time (c x t) products.

c. Toxicity to insects; bioassay techniques; comparative toxicity of various fumigants; effect of temperature, humidity and other gases on toxicity; fluctuations in insect susceptibility and effects of nutrition, species, stage and climatic changes on these; resistance testing.

3. Methods of detection and analysis - chemical methods, instrumental analysis, types of instruments available for field and laboratory use, instruments used for health safety.

4. Safety precautions and protective devices.

a. Threshold limit values; hazards; acute and chronic toxicity; detection of fumigants in the atmosphere; symptoms of poisoning; first aid measures and medical treatment.

b. General precautions in handling fumigants - in preliminary preparations; during application; during treatment; post treatment; protective clothing; respirators - self contained, air-line and canister type.

5. Properties of individual fumigants

a. General characteristics, including corrosive nature and inflammability; toxicity to insects; effect on seeds, bulbs, growing plants, dormant plants (phytotoxicity) and plant products including fresh fruit and vegetables, cereal products; effect on animal products; residues.

b. Detection of vapours; field analysis; methods of application; methods of sealing structures; methods of aeration.

c. Safety precautions - specific health hazards, methods of detection for health safety, symptoms of poisoning, respiratory and general body protection, first aid, medical treatment.

6. Biology, life history and identification of insects and other pest organisms. This is particularly important in insect control since control measures may vary with different species.

PRACTICAL TRAINING

Demonstrations plus active individual participation in performing the various procedures involved in fumigation are considered to be essential parts of any course of instruction. The practical training should demonstrate with step-by-step directions how to carry out various procedures. Similarly, it should emphasize the importance of systematic planning and conduct of every stage of a fumigation operation from initial planning to final clearance of fumigant. Planning should include the following steps:

1. Preliminary inspection of facilities to be fumigated.

2. Arrangements with officials and personnel, plus notification of appropriate authorities - fire, police departments, etc.

3. Materials required for the fumigation.

4. Duties to be performed by each individual of the fumigation crew.

5. Preparation of the facility to be treated.

6. Pre-application procedures.

7. Fumigant application and operations to be performed during the treatment (surveillance for gas leaks, analysis of concentrations).

8. Aeration and post-fumigation procedures.

A check list that gives a record of all equipment required and all duties to be performed should be made.

Demonstrations

Practical demonstrations of the following types may be considered.

1. Methods of storage; handling fumigants - solids, liquids, gases; proper disposal of used containers and end products remaining after a treatment.

2. Demonstration of equipment used for detection and analysis, e.g. halide leak detector, thermal conductivity analyses, glass detector tubes, interference refractometer plus infra-red analyses and portable gas chromatographs, if available.

3. Proper care and use of protective devices; respirators - self contained, air-line and canister types; safety blouses; first aid procedures.

4. Bioassay - treatment of test insects with fumigant; assessment of mortality; determination of LD(50) and LD(99).

5. Stack fumigation to include step-by-step planning of the various procedures involved in a fumigation - choice of fumigant; use of test insects; covering and sealing the stack with a gas-proof sheet; determination of volume of space to be fumigated and quantity of fumigant required; application of fumigant; analysis of fumigant; calculation of c x t product; safety measures; aeration. Factors such as sorption and desorption, effects of temperature and other atmospheric conditions should also be noted.

6. Other demonstrations and projects requiring active student participation can be designed to fulfill individual needs, depending on the nature of fumigation operations that are anticipated. For large-scale fumigations, such as warehouses, mills or ships, the importance of systematic planning and conduct of all aspects of the treatment should be emphasized.

APPRENTICESHIP

A period of on-the-job training with an experienced fumigator will enable the student to observe first hand many of the principles and practices described above. A period of at least six months, hut preferably one year, should provide enough experience to allow the new fumigator to use fumigants effectively and safely and to adapt his treatments to the varying situations that he may encounter.


Fumigation schedules

The treatments listed in these schedules have been successfully applied in practice. Nevertheless, they are given here for the purpose of reference only. Modifications may have to be made, after preliminary trials, to suit local conditions. Although the schedules represent a wide range of treatments, they do not include all possible applications. Rather, an attempt has been made to select representative schedules from different parts of the world.

Instructions for application of fumigants are usually given on the label affixed to the container in which the fumigant is supplied. In some countries the information given on these labels, including dosage recommendations, is carefully controlled by legislation and, therefore, the use of the fumigant should conform to these national requirements.

A. Bulk fumigation of grain in upright storage.

B. Bulk fumigator of grain in flat storage.

C. List of plants that have sustained injury from fumigation with methyl bromide.

D. Methyl bromide fumigation of actively growing plants.

E. Methyl bromide fumigation of foliated, dormant plants.

F. Methyl bromide fumigation of non-foliated, dormant plant material.

G. Methyl bromide fumigation of orchids.

H. Methyl bromide fumigation of fresh fruit at atmospheric pressure.

I. Methyl bromide fumigation of fresh vegetables.

J. Ethylene dibromide fumigation of fresh fruit.

K. Ethylene dibromide fumigation of fresh vegetables.

L. HCN fumigation of fresh fruit.

M. HNC fumigation of dormant nursery stock.

N. Fumigation of flower bulbs and corms.

O. Methyl bromide fumigation of cut flowers and greenery.

P. Atmospheric and vacuum fumigation for the control of pests infesting packaged plant products.

Q. Fumigation of mills, empty structures, and tobacco warehouses.

R. Local (spot) fumigants for mills.

S. Fumigation of seeds.

T. Fumigation for controlling rodents and other mammalian pests, snakes, birds, snails, ants' nests, wasps and termites.

A valuable source of information for plant quarantine treatments is the Plant Protection and Quarantine Treatment Manual (USDA, 1976 with updated inserts) issued by the Plant Quarantine Division, United States Agricultural Research Service, Washington 25, D.C.,which is kept up to date by periodic revisions.

Guide to schedules A and B

INSECT SPECIES

Schedules are for all stages of both internal and external grain feeders.

Exception. With fumigants containing bromides or chlorides (halogenated hydrocarbons and chloropicrin) and with ethylene oxide, double the dosage for any species of Trogoderma (Dermestid beetle genus, which includes T.qranarium, the Khapra beetle) and for the cadelle, Tenebroides mauritanicus, should be applied.

STRUCTURES

Schedules given are for concrete and metal structures. For wooden bins, only direct mixing and surface application are applicable, and for these the dosages should be doubled.

CARRIERS

Treatments of bulk grain may be made in carriers such as closed railway freight cars and road trucks. With due allowance for the tightness of these vehicles, dosages and exposures may be based on the recommendations in Schedule B. Liquid or solid-type fumigants may also be applied to grain in open vehicles if the grain is covered with gas-proof sheets after application of the fumigant (see Chapter 10).

ABBREVIATIONS

Fumigant mixtures by percentage composition (see Chapter 7)

Example. EDC75:CT25 means a mixture of 3 parts ethylene dichloride, 1 part carbon tetrachloride. All proportions are by volume unless other wise stated.

DOSAGES

Dosages in Column 2 are for major cereals, including shelled maize, unless otherwise stated under "Remarks." For Grain sorghum multiply rate given by 1.5 to 2 times. Dosages are based on full bins or storages. Special allowance must be made for partially filled structures.

MIXTURES

The fumigant mixtures given are representative of many combinations marketed. Other ingredients include chloroform, methyl allyl chloride and trichloroethylene. These are usually mixed with the more toxic ingredients: carbon disulphide, chloropicrin, ethylene dihromide or ethylene dichloride. Select the dosage from a similar mixture in this schedule containing the same more toxic ingredient.

TEMPERATURES

Schedules are given for grain temperatures of 21 to 25C. For other temperature ranges apply factors as follows:

10 to 15C Multiply dosage by 1.5
16 to 20C Multiply dosage by 1.25
25C and over Use 0.75 of dosage given

It is inadvisable to attempt fumigation when the grain temperature is below 10C. Hydrogen cyanide (HCN) or calcium cyanide should not be applied at grain temperatures below 15C.

Fahrenheit equivalents can be obtained by reference to the Table in Appendix 2.

DOCKAGE

If there is dockage present, or the grain is "tough," add 25 percent to dosage already calculated.

SEED GRAIN

Where grain is to be used for seed, do not fumigate if moisture content is above 12 percent. Also, exposure periods for seed should not exceed 24 hours with fumigants containing methyl bromide or chloropicrin or 72 hours with any other fumigant. Do not use ethylene or propylene oxides (see Chapter 6 for effects of individual fumigants on seeds).

FORCED DISTRIBUTION

As stated in Chapter 10, forced distribution may be used without modifying existing aeration systems. Usually dosages given in the schedules for recirculation should be multiplied by 1.5.

HOT SPOTS

Treatments of localized areas of infestation (hot spots) in a L, rain mass are discussed in Chapter 10. A good material for this type of treatment is the mixture EDB 70: methyl bromide 30, by weight, applied at I kg/ 36 m (36 oz/1,000 bu) of grain. The fumigant is applied through tubes or metal pipes into the region of the hot spot.

FURTHER REFERENCES

For more detailed information on schedules for grain fumigation, consult USDA, (1982). Comprehensive information on fumigant residues is given by Lindgren et al (1968).


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