Natural enemies: One controlled study from the UK reported more natural enemies when insecticides were sprayed earlier rather than later in the growing season.
Pests: Two of four studies from Mozambique, the UK and the USA found fewer pests or less disease damage when insecticides were applied early rather than late. Effects on a disease-carrying pest varied with insecticide type. Two studies (one a randomised, replicated, controlled test) found no effect on pests or pest damage.
Yield: Four studies (including one randomised, replicated, controlled test) from Mozambique, the Philippines, the UK and the USA measured yields. Two studies found mixed effects and one study found no effect on yield when insecticides were applied early. One study found higher yields when insecticides were applied at times of suspected crop susceptibility.
Profit and costs: One controlled study from the Philippines found higher profits and similar costs when insecticides were only applied at times of suspected crop susceptibility.
This involves applying insecticides at different dates in the growing season or at different times during the cropping cycle. Sprays can be reduced or avoided during periods of natural enemy vulnerability to reduce impacts on the ecosystem service they provide, although many studies test different dates simply to time spraying with periods of likely pest abundance or crop damage. Some of the evidence relates to chemicals now widely removed from use, and readers should bear in mind that using more selective insecticides may also allow greater flexibility in the timing of applications (‘Use more selective pesticides’ will be included in future synopses). Using historical information on pest population characteristics and crop susceptibility to time insecticide applications is included here. Informing spraying decisions by monitoring pests or crop damage within the present cropping season is included in 'Use pesticides only when pests or crop damage reach threshold levels'.
Here we present evidence from five of 13 studies testing this intervention.
A controlled study in 1978 in pear Pyrus sp. orchards in Kent, UK (Solomon et al. 1989) found plots sprayed with permethrin in March had 2.9 flower bug (Anthocoridae) adults/beat, plots sprayed in July had 0.5 adults and plots sprayed in both months had 0.4 adults, when these natural predators were measured in August. Spraying in March reduced flower bug numbers from 0.05-0.10 adults/beat before spraying to 0.0 adults one month afterwards, while spraying in July reduced numbers from 0.8 to 0.5 adults. In late August, plots sprayed only in March had 9 pest pear psyllid Cacopsylla pyricola eggs/10 leaves, plots sprayed only in July had 55 eggs and plots sprayed in both months had 45 eggs. A 2 ha orchard was divided into four treatments receiving permethrin sprays (100 g a.i./ha) in March, July, March and July or no sprays. Predators were sampled by beating branches over a 0.3 m² funnel.
A randomised, replicated study in 1984-1985 in North Yorkshire, UK (McGrath & Bale 1990) found that barley yellow dwarf virus Luteovirus spp. created more patches of stunted barley Hordeum vulgare in plots sprayed on 13 November (averaging 11-16 patches/plot) than on 23 October (6-16 patches) or 31 October (7-14 patches). The area of stunted patches (ranging 1,124-4,087 cm²) only differed between spraying dates in barley sown on 6 September rather than 18 or 27 September, and showed an increase with delayed spraying date. Effects of spray timing on English grain aphid Sitobion avenae (a carrier of the virus) depended on insecticide type. On all spraying dates, deltamethrin reduced aphids with no reinfestation later in the season. Demeton-S-methyl reduced aphids but limited reinfestation occurred (affecting < 3% of plants) in plots sprayed earliest (23 October). Pirimicarb also allowed reinfestation (affecting up to 6% of plants), particularly when applied early (23 October) or to plots sown on 6 September. The authors suggest that spraying early was effective for persistent insecticides, but spraying later (after aphid migration) was more effective for other insecticides. Yield (ranging 6.6-7.1 t/ha) was not consistently different between spraying dates. Three spraying dates were tested in plots of 2 x 24 m, replicated twelve times across three blocks testing sowing date effects.
A replicated study in 1987-1989 in Nebraska, USA (Peters & Lowry 1991) found similar root damage from western corn rootworm Diabrotica virgifera larvae in plots receiving insecticide before planting (average damage rating of 4.2) and plots receiving insecticide after planting (rating of 3.8). Maize Zea mays yields were also similar between plots treated before (10.5 t/ha) and after (10.9 t/ha) planting. Treatment prior to planting comprised chlorpyrifos granules applied at 34 g/1000 ft (304.8 m) of row. Treatment after planting was timed to correspond with corn rootworm egg hatch and early larval development and comprised chlorpyrifos emulsion at 1.12 kg/ha. Treatments were tested in 48.8 m² plots replicated four times. Root damage was scored from 1-6 with 1 being minor feeding damage and 6 equalling three or more root nodes destroyed/plant. Yields were assessed by hand harvesting two 20 ft (6.1 m) lengths of row in each plot.
A controlled study in November to March 1996-1997 and 1997-1998 in Nueva Ecija, Philippines (Aganon et al. 1997) reported a lower impact of insecticides on ladybird and other insect natural enemies (Coccinellidae and Hymenoptera) following the strategic use of insecticides (during times of critical crop susceptibility to pests) compared to conventional practice. Net profit and yield of eggplant Solanum melongena crops were US$481 and 3.3 t/ha (respectively) following strategic applications, compared with US$54 and 2.7 t/ha for conventional practices. Similar patterns were found for stringbean Phaseolus vulgaris profit and yield under strategically timed (US$718 and 6.9 t/ha) and conventional (US$576 and 6.7 t/ha) insecticide applications. The number of sprays was reduced from 13 for conventional practice to 10 for strategic application in aubergine, and from 13 to seven in stringbeans. The costs of production were US$875-1,072 for the strategic treatment and US$982-1,179 for the conventional practice. Insecticide applications in the strategic treatment were timed according to peaks in pest invertebrate population profiles monitored before the study in 1993-1996. No other details of experimental set-up or insecticide type were provided.
A randomised, replicated, controlled study in 1993-1994 in northern Mozambique (Davies 1998) found similar numbers of stem borers (Noctuidae) in plots of maize Zea mays treated with insecticide at 0-40, 40-80 and 80-120 days after planting, and between 120 days and harvest (1.1, 0.1, 0.5 and 1.0 borers/plant, respectively). Plots treated at 0-40 days after planting had more stem borers than controls treated throughout the growing season (0.03 borers/plant), but plots treated at other times had similar pest numbers to continuously treated controls. There was no difference in the percentage of stems infested (15-39%) or plants lost (42-48%) to stem borers for any of the treatments and controls. Yield was greater in plots treated after 40-80 days (4.8 t/ha) than in plots treated at other specific times (2.5-3.9 t/ha), but was similar to continuously treated controls (4.5 t/ha). Plots treated after 0-40 days (2.5 t/ha) and between 120 days and harvest (2.6 t/ha) had lower yields than continuously treated controls. Cyhalothrin insecticide was applied weekly in each time period. Each treatment was replicated four times in plots of four maize rows, 5 m long. Stem borer larvae and pupae were counted on 10 plants/plot at 120 days after planting.