Efficacy testing

The techniques used to evaluate insecti- cide efficacy have been reviewed by Matthews (1984, 1997a) and Busvine (1971). The experimental process starts with laboratory evaluation and then pro- gresses with small plot trials and then large scale testing on research station fields (Reed et al., 1985). Following this trials will be carried out as multi-location experi- ments on the fields of cooperating farmers.

Laboratory bioassays to establish insec- ticide efficacy include standardization of insect species, stage, sex, age and physio- logical and behavioural condition (Dent, 1995), since all of these factors will influ- ence the susceptibility of a pest. Extrinsic factors such as temperature, humidity, feeding and time of treatment, density of treated insects and illumination also have an impact on susceptibility and need to be standardized. The insecticide may be applied topically to the outer surface of the insect using micro-pipettes or special syringes, as a residual film applied to a suitable surface (e.g. glass slides, filter papers, leaves) or systemically through the xylem of treated laboratory plants to evalu- ate efficacy against sucking pests such as aphids and whitefly.

The results of a bioassay (the mortality recorded over a range of insecticide con- centrations) are plotted as dosage mortality curves. These are analysed by Probit analy- sis (Finney, 1971; Gunning, 1991) (Fig. 4.3). Toxicity, quoted in milligrams of active ingredient for each kilogram of body weight (i.e. parts per million of the test organism), is measured as the dose at which 50% of the test insects are killed, in a specified time (often 24 hours) and is

referred to as the LD₅₀ (LD = lethal dose)

(Busvine, 1971; Finney, 1971).

Field trials can be costly, hence there may be situations where smaller scale investigations are needed to more precisely determine the treatments to be used in larger, more formal field trials (Matthews,

1997a). Field trials methodology is dealt with in Section 3.8 (see also Perry, 1997) but there are a number of aspects of field trials methodology that are specific to the testing of insecticides, particularly the size and shape of plots, methods of preventing spray drift and the use of control untreated plots. The size of plot used in insecticide trials will be dependent on the type of insecticide application equipment used to apply the treatments. Quite small plots can be used for granular insecticides, plots treated with knapsack sprayers need to be

at least 10 3 10 m in size, while plots of 30

3 30 m have been used with hand held spinning disc sprayers (Matthews, 1984; 1992). Smaller plots than these need to be shielded in some way to prevent drift cont- amination between plots. Rectangular plots can go some way to preventing drift conta- mination if the long axis of the plot coin- cides with the direction of the wind at the time of application, but if the wind direc- tion is at all variable this approach is unsatisfactory. Square plots have a rela-

tively low perimeter length and reduce the area sacrificed to guard rows (Reed et al., 1985). Small plots can be physically shielded from drift particles with erection of portable screens but these require extra staff to move them between plots as each plot is treated. The most reliable method of preventing drift contamination is the use of sufficiently large plots or spraying systems which minimize drift (Matthews, 1981).

The use of untreated control plots can promote undesirable inter-plot effects. In insecticide trials untreated control plots can sometimes be replaced with a standard check insecticide treatment. The check treatment could be the current recommen- dation for the insecticide and its rate of application. The experiment would then determine the value of differing dosages, timings of application and different insecti- cides from those currently used. Such an approach would reduce, although not remove, the inter-plot effects. Reed et al. (1985) suggested the need for maintaining standard unsprayed crop areas well away from insecticide sprayed crops to provide information on seasonal fluctuations in pest numbers, unaffected by insecticide treatments, however, there are probably few research stations that could afford this luxury, despite its obvious value.

The insecticide treatment may be applied using conventional spray equip- ment or, because field trials are often car- ried out over relatively small areas, a number of specialized sprayers have been developed as plot sprayers (e.g. the Oxford Precision Sprayer) (Matthews, 1997a).

The evaluation of treatments is ultimately concerned with the measurement of crop yield but measures of insect damage and numbers may also be considered relevant. The ease with which these variables can be measured depends on many things but the simplest trials will involve univoltine pests for which only a single application of insec- ticide is required to reduce yield loss. Where multivoltine pests reinfest a crop after insec- ticide application, repeated applications may be needed which may require estima- tion of the extent of pest re-invasion.

Fig. 4.3. A hypothetical example of a regression line (solid line) used to calculate an LD₅₀through probit analysis. The dotted line is a provisional line from which the expected probits are determined. Response % is the percentage of insects killed; probits are a transformed estimate of the % response (after Matthews, 1984).

The size of an insect infestation can be measured before and after insecticide application to quantify the effectiveness of the treatment and to measure the subse- quent rate of reinfestation. Problems may arise with this method with slow acting microbial insecticides because of the time needed to kill insects (e.g. Prior et al., 1996). Counts immediately after applica- tion would probably not differ from counts in controls or be greater than those from a check standard chemical treatment. The time taken for insect mortality to occur in the field would need to be known before counts could be usefully employed as a measure of effectiveness. Puntener (1981) provides a number of formulae using insect counts before and after treatment to calcu- late insecticide efficacy. Two formulae based on counts of surviving insects are included here. The Henderson–Tilton for- mula should be used when infestation between plots is non-uniform before treat- ments are applied:

where:

Tb and Ta are the sizes of infestation in the

treated plots before and after application.

Cb and Ca are the corresponding infesta-

tions in the control or check plot.

If the infestations in plots before treatments are uniform then the Henderson–Tilton for- mula reduces to Abbott’s formula:

since Tb = Cb = 1.

Measures of yield are considered in Chapter 3.

4.5.2 Spray characteristics and droplet