The application of a demeton-S-methyl spray and a granular application of phor- ate were tested for their effectiveness against the black bean aphid, A. fabae, on pre-flowering and post-flowering field beans. The granules were compared with the spray formulation in order to determine whether the granules, which are less harmful to bees, provide adequate control of the aphid. The pre- and post-flower- ing periods were also chosen since there is less bee activity in the crop at these stages. The spray was applied at 0.25 kg a.i. in 370–562 l of water per hectare
and 10% phorate granules at 11.2 kg ha21to randomized plots replicated five or
six times and repeated over six consecutive years. The value of the treatments was considered in terms of the size of the aphid infestation, the yield and the economic viability.
In general, the early treatments significantly decreased the number of aphids compared with the untreated plots, while early spray and granule applications appeared equally effective. The early spray and granule applications were better than the late treatments for controlling the aphids and plots sprayed both early and late had the greatest reduction in aphid numbers (Table 4.10).
Table 4.10. The effects of insecticide treatment on aphid infestations (Aphis fabae) on field beans. The values in the table are the means of all weekly counts between the end of the primary migration in June and the decline in numbers in late July or early August. The counts are expressed as infestation categories on a logarithmic scale: 0 = no aphids, 1 = 1–10, 2 = 11–100 etc. (from Bardner et al., 1978)
Aphid infestation
Treatment 1968 1969 1970 1971 1972 1973
Sprayed early 0.08 0.41 0.16 0.05 0.41 0.23
Sprayed late 0.39 0.74 0.49 0.08 0.53 0.41
Sprayed early and late 0.06 0.23 0.06 0.04 0.27 0.18
Early granules – 0.45 0.12 0.05 0.37 0.23
Late granules – – 0.14 0.07 0.73 0.48
Untreated 0.76 1.09 1.60 0.04 0.78 0.60
SE of differences ±0.188 ±0.109 ±0.072 ±0.188 ±0.225 ±0.237 There were significant increases in yield in treated plots in all years except 1971 and 1972, although there were large fluctuations within treatments. This was probably caused by variability in aphid abundance and the inherent variabil- ity in bean yields. The largest yield increases were the result of early granule application while the combined early and late spray treatment and the early spray treatments were often as good. Lower yield increases relative to untreated plots occurred with both late spray and granule application (Table 4.11).
An economic analysis of these experimental results shows that two sprays were more effective than a single early spray but the extra spray decreased the overall profit (Table 4.12). The application of early granules was the most prof-
itable treatment (£32 t21) even though the application of a spray costs less than
granules.
Table 4.11. The effects of insecticide treatments on the yield of field beans (t ha21).
Yield of field beans
Treatment 1968 1969 1970 1971 1972 1973
Sprayed early 2.13 2.79 1.48 2.16 3.11 4.21
Sprayed late 1.99 2.79 1.38 2.30 2.97 3.90
Sprayed early and late 2.23 3.00 1.39 2.26 3.15 4.27
Early granules – 2.76 1.60 2.42 3.19 4.38
Late granules – – 1.48 1.94 2.90 3.65
Untreated 1.67 2.56 1.07 2.15 3.16 3.70
SE of differences ±0.188 ±0.109 ±0.072 ±0.188 ±0.225 ±0.237
Table 4.12. The value of mean increase in yield of field beans with insecticide treatments. Mean Value of increase Value of increase, less cost of yield increase at £130 t21, less treatment, if treatments applied Treatment (t ha21) cost of treatment (£) only in 1968, 1970 and 1973 (£)
Sprayed early +0.26 23.32 24.55
Sprayed late +0.17 11.41 12.63
Sprayed early and late +0.33 21.73 20.72
Early granules +0.34 32.01 26.48
Late granules 20.03 15.90 5.60
A great deal of research effort is now directed towards the screening of insecti- cides for their selectivity so that chemicals which are less toxic to natural enemies can be recommended for use. The physiologi- cal selectivity of an insecticide depends on either a decreased sensitivity in the natural enemy at the target site or an enhanced rate of detoxification compared with the pest insect. If the natural enemies have dif- ferent detoxification pathways from the pest, then this could be exploited by using insecticides that can be detoxified more
readily via the pathway used by natural enemies. Insect herbivores and omnivores rely heavily on oxidative detoxification pathways in developing resistance to insecticides (Georghiou and Saito, 1983) whereas entomophagous arthropods appear to use esterase and transferase activity to detoxify insecticides. Thus, the design of insecticides that are primarily detoxified non-oxidatively or activated oxidatively should produce chemicals that are favourably selective for natural ene- mies (Mullin and Croft, 1985).
These results show that pre-flowering treatments were the most effective and pre-flowering granular applications provided the greatest profit while minimiz- ing the risk to foraging bees.
There is a great need for research in this subject area to ascertain the biological, physiological and biochemical differences between pests and natural enemies, so that screening for physiological selective insec- ticides can proceed at a greater pace.
4.10.2 Dosage and persistence The insecticide dose applied should be suf- ficient, but no greater than the level required, to provide satisfactory control. The insecticide manufacturer will set the dosage level to an amount that ensures an acceptable level of control, produces acceptable levels of residue and maximizes the return per unit of formulated insecti- cide. Insecticide doses reduced from recom- mended rates have been shown to provide adequate levels of control (Hull and Beers, 1985). There are a number of advantages in reducing the dosage level of an insecticide that, both directly and indirectly, benefit the user. The direct benefit would be the lower costs due to the reduced level of chemicals required, while the indirect ben- efits would be the extra control exerted by natural enemies that may obviate the need for further applications, and a slowing in the rate of development of insecticide resis- tance if pest populations are only reduced rather than decimated.
The benefits from natural enemies might be diminished, even with lower
doses, if a persistent insecticide is used. Mobile natural enemies may avoid an insecticide application or move into a pre- viously treated area, or if present during an application they may survive as a resis- tant stage or within sheltered refuges. However, if an insecticide is persistent the chances of contamination of natural ene- mies by insecticide residues are greatly enhanced. Since persistent insecticides are more likely to contribute to insecticide resistance and have a greater effect on nat- ural enemy populations, the advantages of persistence in terms of improved pest con- trol are short lived.
4.10.3 Selective placement
Few insecticide application procedures distribute the insecticide efficiently. With less than 1% reaching the intended insect target a large amount of the applied insecti- cide is wasted. It is vitally important that the target for the insecticide be precisely defined, and with an understanding of insect biology and behaviour the properties of the insecticide deposit at different appli- cation and dosage rates can be optimized (Section 4.5). Selective placement for a par- ticular target can reduce the amount of insecticide applied so that only those sur- faces most likely to mediate transfer of a deposit to the insect are treated.