Using thresholds and risk assessment for pest management

Transcription

1 Using thresholds and risk assessment for pest management J. N. OAKLEY 1, D. B. GREEN 2 1 ADAS Rosemaund, Preston Wynne, Hereford, HR1 3PG 2 ADAS, Woodthorne, Wergs Road, Wolverhampton, WV6 8TQ Summary The use of economic action thresholds to decide on the need for treatment against pests is described and illustrated through examples relating to those pests of wheat and oilseed rape for which discretionary treatments are most often applied. Recent or ongoing research has led to improvements in these decision-making processes that should result in both reduced losses and decreased treatment costs. Avoidance of unnecessary insecticide application allows the specific natural enemies to survive and assist in preventing future outbreaks. Introduction Integrated Pest Management systems for crops are based on the use of Economic Action Thresholds for pests, which identify the minimum number of pests that need to be present for a worthwhile response to be obtained from controlling the pests. The pest assessment methods that can be used as the basis of thresholds are constrained by the need for their cost to be less than that of treatment and to be applicable before the treatment needs to be applied. Due to these limitations, risk assessment by the use of thresholds is an approximate process. It is best applied to those situations where the overall economics of pest control are improved by targeting control measures. When combined with a likely forecast of pest incidence, three different scenarios can be identified: Low probability of pest damage. Cost of assessment is greater than potential damage. The best option is to ignore damage unless noted at an early stage during routine crop walking, when levels can be assessed against threshold and fire engine control measures applied, if justified. Moderate probability of pest damage. Cost of assessment (less savings on unnecessary control measures) is less than cost of treating all fields at risk. Use rotational and other factors to determine which fields to assess at the correct time and apply planned treatments where justified by pest numbers above threshold. High probability of pest damage. Cost of assessment is not justified by savings on treatment. Apply planned treatments where indicated by high risk, e.g. due to annual incidence or time of sowing. Paper 11.1

2 The environmental benefits of using thresholds are thought to be considerable, but have not been quantified. Herbivorous insects form the greatest proportion of the invertebrate biomass in crops and the main food source for insectivorous birds as well as insects that prey on them. Leaving sub-economic numbers intact should improve the survival of predators and result in fewer cases of pest populations getting out of natural control. The use of a threshold for assessing pest numbers allows the observer to assess whether natural controls have been effective, without needing to find and assess the many types of natural enemy that may be involved. The appropriate threshold to use within risk assessment systems varies according to the type of pest and the opportunities available to assess their numbers prior to needing to apply the treatments. A selection of pests is discussed to illustrate the basic principles of setting and interpreting thresholds. Summer aphid problems on wheat The control of aphids in summer poses problems in so far as numbers may increase quite rapidly in warm, settled weather, but may just as quickly collapse due to the effects of heavy rainfall or natural enemies. The thresholds used are based on the numbers of aphids that can be tolerated with minimal yield loss. Losses increase if numbers increase above these levels and are then proportional to the accumulated total of aphids on the crop, known as aphid days (the sum of the numbers of aphids/day throughout the infestation). As there is a close relationship between the numbers of aphids present and the percentage of tillers infested by aphids, the latter measure is used as it is more easily assessed. A threshold value of 50% of tillers infested (2 aphids/plant) is used before flowering when infestation may reduce grain number as well as size. A threshold of 66% tillers infested (5 aphids/plant) is used after flowering when the sole effect is on grain size. These threshold values only apply if the infestation is thought to be increasing at the time of treatment; if in doubt it is suggested that the crop is reassessed a few days later to see if numbers have started to decline aphids / tiller 10 Threshold A B C D May 31-May 7-Jun 14-Jun 21-Jun 28-Jun 5-Jul 12-Jul 19-Jul Figure 1. Aphid numbers recorded on four winter wheat crops. Paper 11.2

3 In the example (Figure 1) the counts of aphids in three crops of wheat are shown. When graphed on a log scale the lines for increasing aphid populations tend to be parallel, with a three-fold multiplication per week. All of the fields were sprayed at GS 61 in early June regardless of the aphid numbers at the time (Oakley et al. 2005). In field A, numbers increased above threshold levels by early June and continued to increase unchecked through the grain filling period; yield response to treatment in early June was around 4 tonnes/ha. Field B suffered a late immigration of aphids that were not controlled by the earlier treatment which gave no yield response. In field C, the low initial levels increased to pass threshold levels during grain filling; the earlier treatment gave a yield response of 0.3 t/ha. Field D attracted significant numbers of hoverflies that controlled the aphid infestation, keeping it below threshold levels; no yield response was obtained. The main conclusion from the series of experiments from which these examples were taken was that prophylactic sprays applied before threshold levels were exceeded, were often ineffective, even if numbers later increased. In field D, a pyrethroid insecticide when applied at GS 61 killed the hoverfly larvae, but not all the aphids, resulting in a yield loss. Should treatment be deemed necessary at low population levels, then a low rate of a selective aphicide, such as primicarb, may be most appropriate to protect the natural enemies (Oakley, 2000). Wheat blossom midge The orange wheat blossom midge (owbm) poses particular problems for assessment as, under the right evening conditions, clouds of midges may fly between fields and invade previously uninfested crops. The thresholds used are based on the number of egg-laying female midges that, on average, would lay 5 eggs/ear on seed or milling crops or 10 eggs/ear on feed crops. Each midge larva reduces the size of the grain of a susceptible variety by 38% (Oakley et al., 2005); Hagberg falling number on Hereward is reduced by 5.7 for each grain damaged in a hundred; and damaged grain are unlikely to germinate successfully (Oakley, 1994). The resulting thresholds for adult midges are 1 midge/6 ears for seed or milling and 1 midge/3 ears for feed crops. These numbers need to be assessed on any suitable evenings through the vulnerable ear emergence period. The impracticality of assessing all vulnerable wheat crops has led to the development of trapping systems to indicate when midges are active and give some indication of numbers. Yellow sticky traps provide an indication of the numbers of both male and female midges flying within a crop but catch many other species of insects, so it may be hard to spot the approximate threshold of five wheat blossom midges per trap. Pheromone traps have now been developed within a LINK project sponsored by the Sustainable Arable LINK programme of Defra and the HGCA. The pheromone traps catch only the male owbm, but can catch large numbers of these. The traps are currently being calibrated to determine how to interpret the numbers caught. Paper 11.3

4 larvae / ear trap catch Figure 2. Relationships between pheromone trap catch through the ear emergence period and larval numbers on heavily infested farms in 2004 and The results obtained in 2004 contrast clearly with 2005 data (Figure 2). In 2004, on the heavily infested farms used there were large migrations between fields, with the immigrant midges contributing 25 larvae/ear, on average, to final infestation levels. Differences in the numbers of immigrants arriving resulted in a large variation in the infestations predicted by traps placed in the fields themselves. In 2005, the weather was marginal for flight and little migration between fields occurred. The within-field traps gave a more accurate estimate of the infestation, with the regression line passing through the origin. For accurate identification of risk it will be necessary to trap in a range of fields on the farm. An action threshold based on the catch in the target field would be sufficient if flight conditions were marginal. If conditions for migration were good the same threshold would apply to the sum of the catches in the target field plus those in surrounding fields. Conditions for flight and egg-laying within the field may be suitable when the evening air temperature and wind speed within the crop canopy meet the above 15 C and below 10 km/hour threshold levels. These same conditions would need to be met above the crop canopy for migration between fields. Wheat bulb fly The wheat bulb fly is a difficult pest to assess as both seed and egg hatch spray treatments need to be applied before the damage can be seen in the field in the early spring. Soil sampling methods can be used to detect egg numbers in the soil before sowing, but are too expensive to be used on a field-by-field basis. The numbers of eggs laid differ considerably from season-to-season, mainly due the previous season s Paper 11.4

5 numbers and to variations in the availability of the saprophytic fungal spores in wheat crops on which the adults feed. Delays to harvest due to wet conditions tend to increase and prolong the availability of spores. Young & Ellis (1996) reviewed the applicability of the thresholds using the results of insecticide efficacy experiments conducted between 1970 and They found some correlation between category and the probability of a worthwhile response to treatment, but the relationship was inconsistent. The risk of economic loss is increased by late sowing and harder winters. Category Probability of economic response to treatment Low < 1 m eggs/ha 25% Moderate m eggs/ha 62% High m eggs/ha 75% Very high >5 m eggs/ha 78% The HGCA are now supporting a survey of autumn incidence levels that may assist in deciding whether seed treatments or egg hatch sprays are justified. The key indicator is the proportion of fields with egg numbers over the threshold (250 eggs/m 2 ) for normal sowing dates. This varies considerably from year to year (Figure 3) % fields above threshold Figure 3. Percentage of fields sampled in England each year with owbf egg numbers greater than 250/m 2. When the risk of damage is less than 25% of fields, the best advice is not to use a prophylactic treatment and wait to check deadheart levels in the spring. If the risk is 25% or more of fields, then a seed treatment becomes cost effective. If the risk is more than 40% of fields, then a seed treatment plus an egg hatch spray is the best Paper 11.5

6 strategy. The risk may vary according to the level prevailing in a particular area and the previous cropping; local knowledge can be used to modify the overall advice. A lower threshold (100 eggs/m 2 ) is used for late-sown crops which are likely to be pretillering at the time of attack. In 2005 there was a geographic split in the risk with no crops in northern England at risk, but half of those in East Anglia above threshold. The highest counts were found in row crops of sugar beet, potatoes leeks or onions rather than in bare soil situations after fallow or vining peas. This situation is typical of a dry August when bare field situations become too hot and dry for egg survival and the flies prefer to lay on shaded soil within row crops. These findings allow for an improved identification of seasonal risk factor, targeting late sown crops after row crops in East Anglia this winter. The threshold for deadheart sprays in the spring is adjusted according to growth stage and assumes plant populations of at least 200 plants/m 2. The threshold starts at 10% of plants attacked at GS 20, increases to 15% at GS 21 and to 20% at GS 22 onwards. Cabbage stem flea beetle The cabbage stem flea beetle continues to spread across the country from its original home in the traditional brassica seed and mustard growing areas of East Anglia. Damage tends to be most severe in newly infested areas prior to more routine control measures being applied. Insecticides provide very good control removing the problem temporarily, but once control measures cease, numbers return to damaging levels very quickly. For assessment of larval damage, Purvis (1986) developed an action threshold of 5 larvae per plant; treatments were shown to give a mean yield response of 0.34 t/ha to the use of carbofuran granules at this level of infestation. Walters et al. (2001) demonstrated that an assessment of leaf scarring may be used instead of dissecting plants to count larvae and that a threshold value of 50% of petioles showing scarring is equivalent to the 5 larvae/plant threshold. Sampling to assess numbers of larvae has never been adopted by the industry because of the expense of sampling and the consequent delay in treatment, which may mean missing the opportunity to tank mix a cheap pyrethroid insecticide with a herbicide or fungicide. Larvae continue to hatch through into the spring so that initial estimates of larval numbers may underestimate the final total. The threshold has not been reassessed to reflect the economics of cheap tank-mixed pyrethroids, which give control equal to or better than that given by the much more expensive carbofuran granules used in the 1980s work. At 2 larvae/plant the regression would suggest a response of 0.16 t/ha, worth 21/ha. A project was conducted for the HGCA looking at the possibility of using water or sticky traps to catch adult flea beetles and give an indication of risk in time for tank mixes to be used. Water traps proved to be more useful than sticky ones. The water trap catches gave a good correlation with larval numbers (Figure 4). Paper 11.6

7 70 60 total trap catch larvae / plant Figure 4. The relationship between water trap catches of CSFB adults and numbers of larvae. What should the action threshold be? The economics for a tank mixed spray suggests that two larvae/plant would be more appropriate giving an action threshold of 40 CSFB adults in a water trap. Once the system is validated and the thresholds reconsidered it can be absorbed into the pest DSS that forms part of the PASSWORD Package (Morgan et al., 1998). Acknowledgements We thank the HGCA for sponsoring all of these studies and the Sustainable Arable LINK programme of Defra for that on owbm. We also thank all of the farmers who assisted in providing sites and our staff who conducted the studies. References Morgan D., Walters K. F. A., Oakley J. N., Lane A. (1998). An Internet-based decision support system for the rational management of oilseed rape invertebrate pests. The 1998 Brighton Crop Protection Conference - Pests and Diseases, 1, Oakley J. N. (1994) Orange wheat blossom midge: a literature review and survey of the 1993 outbreak. Research Review No. 28, Home-Grown Cereals Authority, London. Oakley J. N., Cumbleton P. C., Corbett, S. J., Saunders P., Green D. I., Young J. E. B., Rogers R. (1998). Prediction of wheat blossom midge activity and risk of damage. Crop Protection, 17, Oakley J. N. (2000). Appropriate aphicide doses for summer aphid control on wheat. Project Report No Home-Grown Cereals Authority, London. Oakley J. N., Young J. E. B. (2000). Economics of pest control in cereals in the UK. The BCPC Conference Pests and Diseases 2000, Oakley J. N. (2003). Pest management in cereals and oilseed rape a guide. Home- Grown Cereals Authority, London. Paper 11.7

8 Oakley J. N., Talbot G., Dyer C., Self M. M., Freer J. B. S., Angus W. J., Barrett J. M., Feuerhelm D., Snape J., Sayers L., Bruce T. J. A., Smart L. E., Wadhams L. J. (2005) Integrated control of wheat blossom midge: variety choice, use of pheromone traps and treatment thresholds. Home-Grown Cereals Authority, London. Oakley J. N., Ellis S. A., Watling M. A., Walters K. F. A., Young J. E. B. (2005). Responses of cereal aphid populations to reduced rate aphicide applications in large field plots of winter wheat. Agricultural and Forest Entomology. 7, 1-8. Purvis, G. (1986). The influence of cabbage stem flea beetle (Psylliodes chrysocephala (L.)) on yields of oilseed rape. Proceedings of 1986 Brighton Crop Protection Conference - Pests & Diseases, Walters, K. F. A., Lane, A., Cooper, D. A., Morgan, D. (2001). A commercially acceptable assessment technique for improved control of cabbage stem flea beetle feeding on winter oilseed rape. Crop Protection, 20, Young J. E. B., Ellis, S. E. (1996) Impact of changes in arable agriculture on the biology and control of wheat bulb fly. HGCA Research Review No. 33, Home-Grown Cereals Authority, London. Paper 11.8