8.3 Calculating the dosage of insecticides in stored product pest control

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Great care should be paid in calculating the amount of insecticide to be used, because:

- if the dosage it too low, it means:

• lack of efficient control
• waste of money
• promotion of resistance

- if the dosage is too high, it means:

• danger to users and consumers
• uneconomic usage

Only the correct dosage guarantees optimum pest control whilst also keeping all risks to a minimum (see sections 8.1.5 and 8.1.6). The details as to the recommended application rates and to the active ingredient content are to be found on the label of the container in which the insecticide is sold.

Stored produce which is to be treated directly must be weighed.

Surface areas to be treated must be measured and calculated.

For organizations with a number of stores, it is advisable to introduce standardized stack sizes and issue technical instructions for treatments. These must include the following details on making up spray mixtures:

• Amount of water per standardized stack (in litres)
• Specification and amount of insecticide per standardized stack (in ml for EC formulations and g for WP formulations)

This simplifies carrying out treatments and avoids wrong dosages being made.

8.3.1 Calculating the Dosage for Surface Treatment Using Dust Formulations

Recommended application rates given in g/m²
(= g commercial product/m² surface area)

Details required for calculation:

• surface area to be treated (in m²)
• recommended application rate of insecticide (in g/m²)

The calculated surface area to be treated (in m²) is multiplied by the recommended application rate of the insecticide.

Example: A stack of bags has a surface area of 120 m². It is to be treated with a 5% dustable powder formulation. The recommended application rate is 10 g / m².

10 g/m² × l20m² = 1200g

of the 5% dust formulation is thus required to treat the stack.

8.3.2 Calculating the Dosage for Surface Treatment Using EC and WP Formulations

Two questions have to be answered in order to calculate the dosage for EC and WP formulations:

1. How much spray mixture is required to treat the surface area?
2. How much insecticide is required for the correct amount of spray mixture?

8.3.2.1 Amount of Spray Mixture for Surface Treatments

Details for calculation:

• surface area to be treated (in m²)
• recommended application rate of spray mixture (in l/100 m²).

The following basic principle applies: The smoother the surface, the less spray mixture will be required.

The following amounts are recommended for surface treatments of:

smooth walls: 3 - 51/100 m²
rough walls: 6 - 8//100 m²
jute bags: 8 - 101/100 m²
plastic bags: 3 - 5//100 m²

To calculate the required amount of spray mixture, the recommended application rate is multiplied by the actual surface area to be treated.

Example: A stack of jute bags has a surface area of 160 m². The recommended application rate is 8l/100 m².

8l/ 100 m² × 160 m ²= 12.8 l

of spray mixture is thus required to treat the stack.

8.3.2.2 Calculating the Amount of insecticide Required for the Spray Mixture

- Recommended application rate given in ml/l (EC) or g/l (WP)
(= ml or g of commercial product/l of spray mixture)

Details for calculation:

• amount of spray mixture (in l)
• recommended application rate of insecticide (in ml/l for EC or g/l for WP formulations)

The recommended application rate of the insecticide is multiplied by the amount of spray mixture calculated.

Example: 12.8 l of spray mixture is required to treat a stack of bags

a) it is to be treated with an EC formulation with 50% active ingredient. The recommended application rate is 20 ml/l.

20 ml/l x 12.8 l = 256 ml

of the EC formulation is thus required.

b) it is to be treated with a WP formulation with 40% active ingredient. The recommended application rate is 30 g/l.

30 g/l x 1 2.8 = 384 g

of the WP formulation is thus required.

- Recommended application rate of insecticide given in %
(= % of active ingredient in spray mixture)

Details for calculation:

• Amount of spray mixture required (in 1)
• Active ingredient content of the insecticide (in %)
• Recommended application rate of the insecticide (in %)

In this case, the amount of insecticide required can be seen from the following per cent table by means of the concentration of active ingredient in the commercial product and the recommended rate of the active ingredient in the spray mixture. The amounts of insecticide listed are in ml/l for EC and g/l of spray mixture for WP formulations. The dosage can be calculated in four steps with the aid of the % table:

1. The top row shows various application rates in %. Find the correct column for the amount stated!

2. The left column shows various concentrations of active ingredients in insecticides in %.

Find the correct row for the concentration of the active ingredient stated on the label of the insecticide used!

3. Find the intersection of the chosen row and column! The figure listed there is the amount of EC or WP formulation in ml or g required for 1 litre of spray mixture.

4. Calculate the amount of insecticide to prepare the actual amount of spray mixture required!

Example: 12.8 litres of spray mixture are required. A SO EC insecticide is to be used (concentration of active ingredient = 50%). The recommended application rate is 0.25% (concentration of active ingredient in the spraying mixture).

The point where the 50% row crosses the 0.25% column shows the amount required for 1 litre of spray mixture: 5 ml of the EC50 formulation.

5 ml/l × 12.8 l = 64 ml

of the insecticide are thus required for 12.8 litres of spray mixture.

The increase in volume of the spray mixture resulting from adding the insecticide to the water can be disregarded. Calculations of WP formulations are made as for EC formulations.

Table for the calculation of the amount of insecticide needed for 1 litre of spray mixture

Example
a) To make a stack treatment you need to apply 12 1 of spray mixture.
b) The available insecticide is an EC-formulation containing 50% a.i
c) The recommended application rate is 0.5%.
d) How much of the EC-formulation is required for the treatment?
e) Find the intersection of the 50%-line with the 0.5%-column!
f) Calculate the amount of EC needed for 12 1: 10 ml x 12 = 120 ml
g) 120 ml of the EC-formulation is required to be mixed with 12 l of water.

8.3.3 Calculating the Dosage for Fogging

The dosage of a togging concentrate depends on the volume of the free space in the store. It is thus necessary to first determine the total volume of the store and deduct the volume of the stacks from this figure.

Recommended application rates of ready-to-use fog formulations are generally stated in ml/100 m³ volume.

Example:

A store which is 40 m long, 15 m wide and 8 m high contains 10 stacks of bags of the same size, all measuring 5 m x 5 m x 4 m. Infestation with moths is to be dealt with using Dichlorvos, a ready-to-use commercial fog formulation. The recommended application rate is 100 ml/100 m³

Calculation of the volume of empty space in the store:

Store: 40 m x 15 m x 8 m = 4 800 m³
Stacks: 5 m x 5 m x 4 m = 100 m³ x 10 = 1 000 m³
Free space: 4 800 m³ - 1000 m³ = 3 800 m³
100 ml/100 m³ x 3 800 m³ = 3 800 ml = 3.8 l

of the fog formulation are thus required.

8.4 Precautionary measures

Insecticides constitute greater or lesser dangers to humans and to all other living organisms. In order to minimize the risk of damage being caused, precautionary measures must be strictly adhered to when dealing with insecticides. Even apparently inconsequential violations of safety regulations may have serious results, many of which will not be immediately recognizable.

8.4.1 Storing Insecticides

When storing insecticides, it is imperative:

• that any danger to humans, animals or the environment be excluded
• that the insecticides remain effective as long as possible

Figure 115

The following points must therefore be observed:

• Keep insecticides locked away so that no unauthorized persons have access to them! For small amounts, a poison cabinet in a well-ventilated room will suffice; a pesticide store will be necessary for larger amounts.
• Store insecticides away from other commodities and never in offices or other rooms where people often spend time!
• Store insecticides only in their original packaging in order to prevent any possibility of confusion! Never fill insecticides into empty bottles or tins!
• Store insecticides in cool, dry and shadowed places!
• Purchase only amounts which can be expected to be used up in a single storage period. Especially dustable powders degrade fast under tropical climate conditions and loose their effectivity. Additionally this practice allows to change active ingredients regularly in order to prevent the development of resistance.
• The 'first in - first out! 'rule must always apply to insecticides.

Figure 116

8.4.2 Handling insecticides

When treating produce with insecticides, it is particularly important that the user is protected. Due to the great responsibility connected to this activity, treatment must only be performed by people who are sufficiently familiar with techniques and possible dangers. When less-qualified personnel apply insecticides, they must be instructed and supervised by a qualified technician.

Figure 117

The following rules must be observed:

• Always read the label and follow the manufacturer's instructions!
• Pay attention to the warning signs on the packaging and take them seriously! Highly toxic products bear the "skull and crossbones" symbol. Less toxic ones bear a cross:
• Simple graphic designs without words (pictograms) have been devised to communicate key safety informations to people of varied levels of literacy. Their meaning is shown on the next page.
• Preplan for emergencies: inform a nearby doctor about chemicals used, be aware of first aid measures and always have plenty of water and soap and medical charcoal at hand!
• Check that the equipment used for applying the insecticides is in good condition (dusters, sprayers, fogging machines)!
• Mix spraying liquids in the open air, not in the store!
• Avoid any contact with the insecticides!
• Do not inhale any insecticide vapours!
• Never use your hands to mix insecticides! Always use a clean stick!
• Never use your mouth to blow into blocked nozzles!
• Always wear protective clothing when mixing and applying insecticides!

Figure 118

Figure 119

Protective clothing consists of:

• Overall (made of light cotton material in the tropics) or trousers and long-sleeved shirt
• Hat (preferably with a rim)
• Respirator or a face shield with a fine dust filter
• Rubber gloves
• Wellington boots or firm leather footwear (no sandals)

Figure 120

• Do not drink, eat or smoke when working with insecticides
• Do not drink any alcohol either directly before or directly after insecticides, as this accelerates the body's intake of toxic substances.
• Use buckets solely for mixing insecticides and never for any other purpose, not even if they have been thoroughly cleaned!
• Never spill any left-over spray mixture!
• Only mix up as much spraying liquid as required in accordance with the calculated dosage. Small left-over amounts of the mixture can always be used for places in the store which are particularly endangered or for treating empty pallets.
• Dispose of all empty insecticide packages! They still contain traces of the insecticide even after being thoroughly cleaned.

Figure 121

The safest way of disposing is by destroying them (crushing cans, cutting up plastic containers, breaking bottles). Bury them in waste land, far away from wells, settlements or cultivated areas.

It is not advisable to burn them, as dangerous toxic gases may be produced on doing so.

Figure 122

Figure 123

• Thoroughly clean all material and machines used! Rinse spraying equipment, buckets, measuring cups, etc. using plenty of water.
• Wash protective clothing with sufficient soap (or washing powder) and water! Wash separately from other clothes!
• Take care that wells or other water sources do not become contaminated when you wash your clothes!
• Take a shower or wash thoroughly after using insecticides for treatment!
• Always put on clean clothes after washing!
• If necessary, fix warning signs (e.g. after fogging) and lock buildings which have been treated to prevent any danger to other persons!

8.4.3 Poisoning and First Aid

If insecticides are used correctly, poisoning is not likely to occur. Most accidents are due to carelessness and disregarding rules and regulations.

Figure 124

Contamination with insecticide will take place:

• by swallowing (oral contamination)
• by absorption through the skin (dermal contamination)
• by inhaling the fumes of insecticides (respiratory contamination).

In addition, insecticides may directly enter the bloodstream through open wounds.

A distinction is made between two types of poisoning:

• Acute poisoning, when symptoms can be seen after absorbing the insecticide a single time.
• Chronic poisoning, when symptoms do only become apparent after absorbing the insecticide a number of times. Chronic poisoning may not be detected in some cases until years later.

Depending on the type of contamination, the toxicity, the amount absorbed, the insecticide formulation and the constitution of the person affected, the following symptoms may appear in varying degrees:

Should any of these symptoms occur, even slightly, first-aid treatment should be given immediately and the person concerned should be taken to the nearest doctor as soon as possible. Note that severe acute poisoning can lead to death!

The following first-aid measures should be carried out without any delay in the case of poisoning:

Skin contact:

• Remove any contaminated clothing!
• Wash the affected part of the body with plenty of water and soap!
• Should there be no water immediately available, wipe the insecticide off using a cloth and look for water!
• if the insecticide has come into contact with the person's eyes, rinse under

Figure 125

Figure 126

Inhalation or swallowing:

• Put the person affected in a shady place, open any tight clothing and lay him in a comfortable position until there is a possibility of taking hint to a doctor!
• If the person affected is unconscious, lay him on his side and take care that he is able to breathe freely (clear the respiration tract if necessary)!

Figure 127

Figure 128

• Make any affected person who is not unconscious vomit (by putting his fingers down his throat or giving him salt-water to drink (I teaspoon of salt in a glass of water) in order to remove any toxic substances still in his stomach!
• Apart from water and, if available, activated carbon, do not give the affected person anything to eat or to drink! Water will dilute any toxic substances ingested (important for caustic substances) and activated carbon will absorb most toxic substances.

Never offer affected persons eggs, milk, alcohol, etc! All of these substances accelerate the body's intake of toxic substances.

Figure 129

Figure 130

• In every case of poisoning or suspicion of poisoning, visit a doctor, even if the symptoms are not present! Symptoms often do not appear until several hours later, and important time may be lost for treatment.
• Take the packaging or the label of the insecticide with you in order to provide the doctor with all necessary details!

8.5 Equipment

Equipment for the use of insecticides includes:

• Application equipment (dusters, sprayers, fogging machines)
• Buckets
• Measuring cups (1 - 2 litre, 100 ml)
• insecticide scale
• Stirring stick
• Tape measure
• Approved insecticide
• Clean water
• Protective clothing, consisting of:

• light overall
• headwear
• respirator or face shield with dust mask
• goggles
• rubber gloves
• boots

8.6 Further literature

ANONYMOUS
Problems of Pesticide Residues in Stored Grain, GASGA Executive Seminar Series No. 3, 24 pp

APPERT, J. (1985)
Le stockage des produits vivriers et semenciers Maisonneuve et Larose et A.C.C.T., Two Volumes, tog. 225 pp.

BAUR, F.l., ed. (1984)
Insect Management for Food Storage and Processing, St. Paul, 384 pp.

CHAMP, B.R. & C E. DYTE. (1976)
Report of the FAO Global Survey of Pesticide Susceptibility of Stored Grain Pests, FAO, Rome, 297 pp.

CHAMP, B.R. & E. HIGHLEY (1985)
Pesticides and Humid Tropical Grain Storage Systems, ACIAR Proc. No. 14, Manila, 364 pp.

DAVIES, J.E., V.H. FREED & F.W. WHITTEMORE (1978)
An Agromedical Approach to Pesticide Management, USAID, Washington., 320 pp.

GOLOB, P. (1977)
Mixing insecticide Powders with Grain for Storage, TPI, London, 14 pp.

JOURDAIN, D. & E. HERMOUET (1989)
Regional Agro-pesticide index Asia, Pacific, Africa, CIRAD, Bangkok, 3 volumes, 566, 548 and 498 pp.

OUDEJANS, I.H. (1982)
Agro-Pesticides: Their Management and Application, Bangkok, 205 pp.

ROBBE, P. & G. MAURIN (1988)
Index Phytosanitaire Afrique, Paris, 255 pp.

NELSON, I.T. (1987)
Grain Protectants, ACIAR, Canberra, 448 pp.

THEISSEN, J.-G. & R. PIERROT (1994)
Food Crop Protection in West and Central Africa, MCP, Paris, 525 pp.

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