Economic thresholds in practice

Stern et al.'s (1959) economic threshold is more commonly referred to as the spray or action threshold and the economic injury level as the economic threshold (Blackshaw, 1995). In practical terms there are different types of economic threshold depending upon their flexibility and how they have been determined (Table 3.3; Poston et al., 1983; Morse and Buhler,

Table 3.3. Types of thresholds employed in crop protection (from Morse and Buhler, 1997; Poston et al., 1 983).

Type of threshold

Description

Nominal

(e.g. subjective ET) Simple

(e.g. subjective ET)

Comprehensive (e.g. objective ET)

Based on field experience and logic Values remain static

Calculated from crude quantification of the 'average' pest-host relationship in terms of pest damage potential, crop market value, control costs, and potential crop yield Generally inflexible to change over time

Based on interdisciplinary research incorporating the total production system on a given farm and including factors such as multiple pest and crop stress effects Very flexible to change over time Fixed ET: set at a fixed percentage of the EIL

Descriptive ET: includes projections of pest population growth based on simulation models Dichotomous ET: based on samples taken over time and classifying the population as economic or non-economic as a result of analysing the sample data

1997). At one end of the scale are the subjective ETs (nominal and simple thresholds), which are more or less fixed figures representing an average of the pest density at which the cost of control is warranted, and at the other end of the scale are objective ETs, which are based on comprehensive research (Morse and Buhler, 1997). The objective ETs are based on estimated ETs and are flexible over time whereas the subjective ones are typically derived by experience and are often no more than 'rules of thumb' or 'guestimates'. In practice, it is the subjective ETs that predominate (Pedigo, 1996) and amongst these the 'action threshold' is very common.

Action thresholds have been calculated for a number of insect species, e.g. Amblyomma americanum on cattle (Barnard et al., 1986), Aeneolamia varia in sugar cane (Norton and Evans, 1974), Keiferia lycopersi-cella in tomato, Acyrthosiphon pisum on green peas (Yencho et al., 1986), Tipula species in barley (Norton, 1976; Blackshaw, 1994), Epiphayas postvittana in top fruit (Valentine et al., 1996) and aphids in cereals (Elliott et al., 1990). Composite thresholds have been calculated for Helicoverpa armígera, Earías vittelli and Pectinophora gossypiella in cotton (Keerthisinghe, 1982) and for Trichoplusia ni, Plutella xylostella and Pieris rapae in cabbage (Kirby and Slosser, 1984).

Considerable effort has gone into economic thresholds (e.g. Table 3.4 for pests of soybean) but this may have reached its limit for some crop pest complexes. Such a large number of variables makes it extremely difficult to obtain economic thresholds that are generally applicable. Hence, although the economic threshold concept serves as a basis for decision making in insect pest management, the determination of such thresholds has proved to be one of the weakest components in management programmes, with the result that very few research based thresholds have been developed (Poston et al., 1983). Ultimately some situations are just too complex or, irrespective of the suitability of the data or how good the understanding of damage functions, some pest-crop complexes are just inherently uncertain (Table 3.5). Thresholds represent only one way of assisting decision making and should not be seen as a universal solution (Mumford and Knight, 1997).

Table 3.4. Economic injury levels (EILs) for selected foliage- and pod-feeding arthropods on soybean at growth stages R3 to R5. Values based on average recommendations for the major soybean growing regions of the USA, which may differ for different growing regions of the world (Sinclair et al., 1997).

Table 3.4. Economic injury levels (EILs) for selected foliage- and pod-feeding arthropods on soybean at growth stages R3 to R5. Values based on average recommendations for the major soybean growing regions of the USA, which may differ for different growing regions of the world (Sinclair et al., 1997).

Pest complex

Representative species

Economic injury level

Foliage-feeding arthropods at growth stages R3 to R5

Lepidopterous Anticarsia gemmatalis

20-25 larvae (>1.2 cm)/row m + 15% defoliation

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