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.
4.7.3 Economic viability
Insecticides are without doubt an effective means of killing insects, quickly and on demand. No other control method provides users with an immediate and visibly effec- tive means of response to signs of impend- ing pest outbreaks. This ability has meant that risk-averse farmers spray at the first signs of insect pests, whether the numbers observed are indicative of a serious out- break or not, and some may even spray insecticides regardless of the possible need as an insurance against a perceived risk.
Obviously, there is a cost associated with application of an insecticide. The insecti- cide itself tends to be inexpensive relative to the value of the crop; for instance in the UK, in apples pesticide costs £747 ha21,
8.3% of total income (Webster and Bowles, 1996); in field beans purchase of Pirimicarb to control black bean aphid costs £12.50 ha21providing a yield benefit of 3.5% on a crop yielding 5 t ha21 (Parker and Biddle, 1998). Pesticide application costs are esti- mated to be around £10.00 ha21 (Anon, 1993; Oakley et al., 1998) although the operating costs of the application equip- ment used will differ (Table 4.7).
In general, the use of pesticides is prof- itable (e.g. Oakley et al., 1998) or their use is perceived to be profitable through reducing the risk of damage to a crop. Farmers may use pesticides to reduce the risk of pest attack without establishing whether or not control is justified economically. Such pro- phylactic approaches to control usually Table 4.7.Operational costs with knapsack and hand-carried CDA sprayers I and II (actual costs of
equipment and labour will depend on local conditions; the cost of the chemical, which is also affected by the choice of formulation, is not included) (from Matthews, 1992).
Manually Motorized
operated knapsack Hand-carried sprayer
knapsack mistblower I II
Initial capital cost (£) 60 350 45 45
Area sprayed annually (ha) 20 20 20 20
Tank capacity (litres) 15 10 1 1
Swath width (m) 1 3 1 3
Life in years 3 5 3 3
Hectares h21spraying* 0.36 1.08 0.36 1.08
Overall ha h21(% efficiency)* 0.18 (50) 0.65 (60) 0.31 (85) 0.97 (90)
Use (h annum21) 111 30.8 64.5 20.6
Annual cost of ownership (£) 20 70 15 15
Repairs and maintenance† (£) 6 35 4.5 4.5
15% interest on half capital (£) 4.5 26.3 3.4 3.4
Total cost of ownership (£) 30.5 131.3 22.9 22.9
Ownership cost per hour (£) 0.27 4.26 0.36 1.11
Ownership cost per hectare (£) 0.76 3.95 0.99 1.03
Labour costs per hectare††(£) 1.38 0.39 0.80 0.25
Operating cost including batteries**(£ ha21) – 0.68 2.2 0.74
Labour costs to collect water***(£ ha21) 1.38 0.92 0.13 0.04
Total operating costs per hectare (£) 3.52 5.94 4.12 2.06
* Assuming walking speed is m s21, actual efficiency will depend on how far water supply is from treated area, application rate and other factors.
† 10% of capital cost.
†† Assumes labour in tropical country at £2 per 8 hour day.
** Assumes batteries cost 50p each and a set of 8 will operate for 5 h with a fast disc speed. Fuel for mistblower at 44p l h21.
***Water required for washing, even when special formulations are applied at ULV.
Battery consumption is less on some sprayers with a single disc and smaller motors. The ‘Electrodyn’ sprayer uses only 4 batteries over 50+ hours, so the costs of batteries on a double row swath is 0.6 instead of 2.2.
involve spraying insecticides at set times or intervals during a cropping season, i.e. cal- endar spraying. In contrast to this insecti- cides can be applied responsively, according to need in terms of the level of pest attack.
This normally depends on the availability of an appropriate monitoring and forecasting system. It is generally considered preferable for farmers to switch from prophylactic cal- endar based applications to responsive need based applications, simply to improve tar- geting and to reduce the environmental impact of unnecessary insecticide applica- tions (Dent, 1995). However, the extent to which farmers are willing to make this change is dependent on a number of factors.
The difficulties associated with a choice between the two strategies, prophylactic (calendar) spraying and responsive, moni- toring and spraying programmes are illus- trated by the hypothetical net revenue lines
depicted in Fig. 4.14 (Norton, 1985). In years when the level of pest attack is very low the farmer may benefit from not moni- toring or controlling the pest, but with mon- itoring still providing a more profitable option than prophylactic applications. As the level of pest attack increases the benefits of monitoring and spraying outweigh those of both no control and prophylactic control.
However, in situations where pest attack is more frequent the need for the monitoring programme is reduced, and the costs exceed the benefits, making prophylactic control the most financially attractive option (Fig.
4.14; Dent, 1995).
4.8 Insecticide Resistance
Insecticide resistance is the result of the selection of insect strains tolerant to doses of
Fig. 4.14. Hypothetical net revenue curves for a prophylactic and a responsive-spray strategy at different levels of pest attack (after Norton, 1985; calendar spraying).
insecticide that would kill the majority of the normal insect population (Cremlyn, 1978). These strains tend to be rare in the normal population, but widespread use of an insecticide can reduce the normal suscepti- ble population thereby providing the resis- tant individuals with a competitive advantage. The resistant individuals multi- ply in the absence of intraspecific competi- tion, and over a number of generations quickly become the dominant proportion of the population. Hence, the insecticide is no longer effective and the insects are said to be resistant.
Resistance was recognized as a phenom- enon as early as 1911 when citrus scales treated with hydrogen cyanide acquired a certain level of tolerance; however, resis- tance only really became a concern in the late 1940s with the use of the organochlorine insecticides. The effects were noticed first among pests of medical importance (Busvine, 1976) where the insecticides were being heavily used to control insect vectors of human diseases. The incidence of resis- tant species of medical importance increased quickly from the late 1950s, and this was fol- lowed by a concomitant increase in the numbers of resistant insects of agricultural importance. The number of confirmed resis- tant insect and mite species continued to rise (Fig. 4.15), to a level of 447 recorded by 1984 (Roush and McKenzie, 1987).
The increasing level of insect resistance to chemical insecticides has been one of the driving forces for change in insect pest management. The development of insect resistance prompted the search for alterna- tive means of control both as a substitute to chemical control and as a means of delay- ing the establishment of resistance. Faced with the unequivocal fact that no insecti- cide is immune to resistance, far greater emphasis is being given to insecticide resistance management and to evaluating resistance risk prior to approval of new toxicants (Denholm et al., 1998). The eco- nomic costs of resistance to the agrochemi- cal industry can be very high. The cost of a single non-performance complaint can negate the value of 10–1000 individual
sales (Thompson, 1997). Costs can include sales and technical service staff time, replacement products, legal costs and crop yield replacements as well as loss of future product sales and damage to the reputation of the company. Thus, agrochemical pro- ducers have been increasing their commit- ment to confronting resistance problems through, for instance, evaluating the risk of cross resistance to new insecticides as part of the product development programme.
Such cross resistances may occur when a new product shares a target site or common detoxification pathway with compounds already in widespread use against the same pest species. Resistance generally origi- nates through structural alteration of genes encoding target-site proteins or detoxifying enzymes, or through processes (e.g. gene amplification) affecting gene expression (Soderland and Bloomquist, 1990).
Fig. 4.15. The increase in the number of insect species known to be resistant to at least one insecticide (after Georghiou and Mellon, 1983).
Insecticide resistance can be conferred on individuals by one or a number of genes, but major genes are responsible for the 10–100-fold increases in resistance that are often observed. The major gene inheri- tance follows the classical Mendelian laws providing individuals with definite geno- typic differentiation. Polygenic inheri- tance, involving cumulative resistance due to many small changes in minor genes, can occur in the field but this type of resistance can be superfluous once a large increase in resistance is conferred on individuals by major genes. Polygenic resistance often occurs in laboratory selection experiments and has been responsible for some confu- sion over mechanisms of inheritance of resistance (Whitten and McKenzie, 1982).
Such laboratory based studies of the inheri- tance of insecticide resistance therefore have little value, since they are not repre- sentative of field induced mechanisms of resistance.
Traditionally resistance studies have identified only two phenotypes, resistant (R) or susceptible (S), and simplifying assumptions have been made about inheri- tance of resistance (Daly and Trowell, 1996). The process has been seen as one involving simple major gene inheritance where in the absence of selection resis- tance genes would exist at very low fre- quencies; the homozygous resistant genes RR would be very rare compared with the heterozygous RS (resistant/susceptible) and the homozygous susceptible SS very common. The presence of the RS gene is maintained purely by a balance between mutation and selection. Once the insecti- cide is applied the resistant individuals have a selective advantage and their genes rapidly spread through the population (Fig. 4.16; Roush and McKenzie, 1987). In reality, however, the evolution of resis- tance can involve more than one major gene, the genetic basis of resistance can Fig. 4.16. Four ranges of insecticide concentrations: (A) no mortality of any genotype and hence no selection; (B) mortality of SS only (with R resistance dominant); (C) mortality of RS and SS (resistance recessive); and (D) all genotypes killed and hence no selection (after Roush and McKenzie, 1987).
change over time and the expression of resistance can vary throughout the insect life cycle.
Techniques for detecting insecticide resistance in insects have also advanced greatly in the last ten years. Traditionally resistance was monitored in field popula- tions using bioassays of whole insects but there are many problems associated with the use of such techniques (Brown and Brogdon, 1987). More recently a range of biochemical and molecular methods have become available such as enzyme elec- trophoresis, enzyme assays and immuno- assays (Symondson and Hemingway, 1997) which have revolutionized the means by which resistance can be detected and man- aged. It seems likely that DNA based detec- tion systems will become available for field detection in the near future. These moni-
toring tools will complement the range of tactics available for the management of resistance.
There are relatively few resistance man- agement tactics that have been proposed that are sufficiently risk-free and intuitive to stand a reasonable chance of success in the majority of circumstances. Foremost among these are: (i) restricting the number of applications over time and/or space; (ii) creating or exploiting refugia; (iii) avoiding unnecessary persistence; (iv) alternating between chemicals; and (v) ensure targeting is against the most vulnerable stage(s) in the pest life cycle (Denholm et al., 1998).
However, experience would suggest that the most difficult challenge in managing resis- tance is not the identification of appropriate tactics but ensuring their adoption by grow- ers and pest control operatives.