Insecticides
4.7 The Farmer/User Requirements
The decision to adopt insecticide applica- tion as a method of insect pest control will not only depend on the type and level of pest infestation but also on a number of other factors which relate to the potential user. Although individual circumstances will differ and the relative importance of each factor will vary, every user will have in common the need for insect control that is easy to use, is safe and economically viable (Fig. 4.13).
4.7.1 Ease of use
The extent to which insecticides have been used in the past tends to lend credence to
their effectiveness and to some extent their ease of use, since one would not expect a technique that was ineffective or too diffi- cult to use to gain its present level of acceptance. However, there are still obvi- ous practical problems with insecticide use, such as the hazard during mixing and application and hence the need for protec- tive clothing, difficulties in sprayer calibra- tion, the necessity for appropriate weather conditions, practical thresholds for timing applications and disposal of used contain- ers (Fig. 4.13).
Practical problems associated with insecticide application include the time and difficulty associated with transporting and moving large volumes of water. A large proportion of the time used applying an insecticide can be spent filling a tractor mounted sprayer (ADAS, 1976; Matthews, 1984), a fully loaded knapsack sprayer can weigh 21.5 kg (Pawar, 1986) and it can take up to three to four man days to spray a 1 ha field (Matthews, 1983). The effort required to carry the full sprayer, to refill it (this may be needed over 30 times to spray 1 ha), the continuous pumping required (if the sprayer is a hydraulic energy sprayer) is certainly sufficient to discourage many farmers from using insecticides, especially in a hot climate. Some of these problems have been overcome with the introduction of ULV sprayers, which as the name sug- gests, use less water than high volume sprayers and hence do not have to be refilled as often. The hand carried versions are light (4 kg; Pawar, 1986) and require less manual effort for application because they are battery powered.
Application equipment needs to be reli- able and simple in design so that it requires little maintenance. This simplicity and ease of maintenance has been one of the main advantages of hydraulic knapsack sprayers in developing countries. A further concern for a user is the calibration of the spraying equipment and calculation of application rates. If an insecticide is to be applied at the recommended dose then the user must know the rate at which the insecticide is delivered by the sprayer
which in turn requires measures of vol- ume, swath widths and walking speed to be made (Spencer and Dent, 1991). The user then has to calculate the amount of
insecticide needed for the total area to be sprayed at the required dose, the number of sprayer loads and the quantity of the insec- ticide required for each load. Calculations Fig. 4.13. Abiotic factors governing the use of insecticides as a control option.
such as this may seriously reduce the chances of an appropriate dose being applied (Dent and Spencer, 1993).
In addition, simple errors in application can also have a significant impact on whether or not the required dose of insecti- cide is actually applied. For instance, with knapsack sprayers an operator needs to practise walking at a constant speed whilst pumping the hydraulic sprayer, the speed at which he walks will influence the cover- age of the crop. For instance, with a swath width of 1 m and a flow rate of 0.5 l min21, then for a walking speed of 0.4 m s21 the application rate will be:
whereas if the walking speed is 1 m s21
Even when a farmer has correctly cali- brated the sprayer and determined the appropriate amount of insecticide to apply, unless the environmental conditions are suitable, an application should not be made (Table 4.4). Conditions under which insecticides can be applied are referred to as ‘weather windows’, times when: it is not raining; crop leaves are not wet with dew;
the wind speed is appropriate; the plant is not under moisture stress; temperatures are below a certain maximum and sunlight is not too intense (Smith, 1983). Hence, despite their value as crop protection agents insecticides, if applied correctly, are not easy to use.
4.7.2 Safety
Chemical insecticides are inherently toxic substances but they are only a hazard to man if used inappropriately. In developed countries, where users are aware of the need for care and caution when dealing with toxic substances, the hazard from pes- ticides arises mainly from carelessness and negligence. There is no reason to expect that procedures recommended for the safe handling of insecticides will be followed any more conscientiously than those from other sources of hazard (Barnes, 1976).
Provided users in a developed country are aware that they are dealing with a toxic chemical, have a knowledge of the correct procedures and proper protective clothing readily available, then they are totally responsible for their own safety. Accidents occur because users become complacent.
The situation is totally different in developing countries, however, where peo- ple may be unused to handling and using toxic chemicals. In some developing coun- tries the use of pesticides may be a commu- nity’s first contact with toxic chemicals (Barnes, 1976), and they will view the
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Table 4.4. Conditions under which pesticides should not be applied (from Smith, 1983).
When it is raining When leaf is wet with dew
When plant is dry; under moisture stress Under windy conditions
When temperature is over 18°C When temperature is below 10°C In intense sunlight
When mixed with non-compatible materials On open blossom
To resistant pathogens or pests
To plants to be consumed within 14 days To untested varieties
To uncleared minor crops When water supply is hard
Without adding cationic or non-ionic wetter Before reading instructions on can
After material has been exposed to frost To rapid growing lush foliage
If unable to convert pints per acre to litres per hectare If unable to calibrate sprayer
If unable to identify the target
When ground is too wet to carry machine When unsure of effectiveness of material If you have no field experience
When customers are looking If unable to afford cost
problems of hazard and toxicity from a totally different perspective. The safe use of insecticides under these circumstances can only be achieved through education and training. The difficulty is exacerbated because non-users such as family members will inevitably come into contact with insecticide deposits as they wash clothes, prepare food or re-use discarded insecti- cide containers. Even illness caused by insecticide poisoning may not be diag- nosed as such or associated with their use by an unsuspecting community doctor.
The level of hazard associated with dif- ferent insecticides varies markedly. The World Health Organisation classifies chem- icals in terms of hazard according to acute oral and dermal toxicity data for solid and liquid formulations. Thus, good quality granular materials are less hazardous than sprays of the same chemical (Matthews, 1992). The classification in four categories:
Ia, Extremely Hazardous, e.g. phorate; Ib, Highly Hazardous, e.g. endosulfan; II, Moderately Hazardous, e.g. deltamethrin;
and III, Slightly Hazardous, e.g. malathion, provides a guide to the care that should be taken in dealing with the chemical and the need for a type of protective clothing that should be worn.
For most insecticides it is advisable to wear rubber gloves, overalls, face mask and boots. However, in tropical conditions heavy protective clothing becomes unbear- able with high temperatures and humidi- ties. Often no protective clothing is worn (Srivastava, 1974) either because it is not available or it is too expensive for farmers to purchase. At least a light overall or nor- mal clothes should be worn. Many farmers normally walk barefoot, which could seri- ously increase their exposure to the insecti- cide; shoes or boots must be worn. A handkerchief tied over the face can be used as a face mask. All clothes used for insecti- cide application should be washed with soap and detergent after completion of use of the insecticide (Matthews and Clayphon, 1973). Operators should under no circum- stances apply insecticides without clothes covering the legs, torso and arms. However,
in reality, this can often occur, thus expos- ing the spray operator to contamination.
The greatest risk to the user comes from dispensing and mixing the chemical con- centrates with the dilutent in the spray tank. A number of solutions have been developed to reduce the operator exposure during mixing. Low level mixing units eliminate the need to climb on a tractor mounted sprayer tank to pour insecticide into the tank; closed chemical transfer sys- tems (CCTS) couples the pesticide con- tainer directly to the sprayer or mixing unit, and injection systems mix water with the pesticide concentrate as it is applied (Matthews, 1997a). The agrochemical com- panies have also agreed to standardize con- tainer apertures, which is promoting the development of CCTS.
Other problems with safety aspects of insecticide use concern packaging, labelling and disposal of insecticides and containers. In developing countries adul- teration of insecticides can and does take place at repackaging and distribution cen- tres. In Manila’s rice bowl 70% of pesti- cide bottles at local retailers were found to contain chemicals adulterated to more than twice the acceptable standard of devi- ation (Goodell, 1984). Regulations govern- ing the packaging of insecticides need to be closely controlled, but while monitor- ing tests for adulteration remain expensive and difficult to carry out this is unlikely to happen, and the dangers from use of such insecticides remain. Labelling of packages needs to be in the local vernacular, using symbols and diagrams meaningful to the user. The label should include the brand name, details of the active ingredient and materials used in the formulation, intended use of the product, full direc- tions for correct and safe use and how to dispose of the container (Matthews and Clayphon, 1973; Matthews, 1992).
The usual recommendation for the dis- posal of insecticides is to puncture the con- tainer and to bury it, preferably in clay soils at a depth of at least 1.5 m (Matthews and Clayphon, 1973). In the tropics dis- posal of containers in this way is unlikely
to occur since the farmer is being asked to dig a pit 1.5 m deep to rid himself of a con- tainer that has so many potential and valu- able uses to him. The disposal of
insecticide containers is something that needs serious re-evaluation; perhaps the container needs to be changed to a type that has no intrinsic value after use.