Case Study: The effect of trap spacing on trap catch of Erioishia brassicae (Finch and Skinner, 1974)
2.4 Monitoring Strategies and Objectives
Monitoring strategies can be roughly classi- fied as:
1. Surveys.
2. Field based monitoring strategies.
3. Fixed position monitoring strategies.
The particular sampling technique used in each of these monitoring strategies
depends largely on the precise objectives of the proposed programme (Fig. 2.10).
2.4.1 Surveys, field based and fixed position monitoring
A survey may be carried out to study the distribution of a pest, or it may involve a study of both the distribution and the abun- dance of a pest species. The aim of a distri- bution survey is to locate and map the geographical distribution of a pest species.
This may be used to assess the pest status of a particular insect, the spread of an intro- duced species or the spread of a mobile endemic pest that is extending its range.
Surveys measuring both the distribution and abundance of a pest can be used to assess the relative level of pest infestation and pest migration. A survey can identify areas of relatively high infestation and may show up seasonal patterns of occurrence in
Fig. 2.10. The different monitoring strategies and factors affecting their use.
different locations. Such seasonal patterns may be related to differences in environ- mental conditions and may provide some understanding of factors influencing pest population dynamics. Levels of infestation and/or environmental factors in particular regions may be shown to be indicative of impending pest outbreaks and hence used in regional forecasts.
Survey techniques can also be used to establish the presence of the seasonal or sporadic movements/migration of some pest species. A knowledge of likely source populations and pest movement from these areas may have potential uses in pest fore- casting. Pest migration may also be related to weather systems.
The aim of a field based monitoring strategy is to provide the farmer with a decision-making tool. The field based sampling technique, which could be an insect trap or a method of counting eggs or larval stages, is used to collect data on local pest population changes in a partic- ular crop or field. This information is then used by the farmer to make decisions about the implementation of control mea- sures. Preceding this, in research terms, field based monitoring may form part of a study of pest population ecology.
Information gained in this way may then be used to produce a practical monitoring strategy on which management practices can be based.
Sequential monitoring is carried out with the objective of measuring the way in which pest numbers change over time. It can be done in different crops, different crop growth stages and over different sea- sons, and may identify the development times, mortality and generation times of pest populations under different condi- tions. The data may be used to determine rates of population development which in turn may be used to produce pest forecasts.
The damage caused may be related to lev- els of infestation and used to determine damage thresholds.
Field based monitoring can also be car- ried out to follow the progress of popula- tion development up to a pre-defined
number of insects or action threshold. This may be any number of insects from one, and may indicate that insecticide applica- tion is necessary, or it may be used in developmental models which predict the most appropriate time to apply insecticide.
Fixed position monitoring is usually restricted to research stations where a trap, e.g. a suction or light trap, is maintained in a fixed position and is used to sample insects over a number of seasons and years. The method can be used to identify the pest insects present at a given location, and to compare relative changes in pest numbers between seasons. It can provide a general overview of pest abundance and may be useful, if related to other pest esti- mates, for forecasting outbreaks on a regional basis.
2.4.2 Biology and nature of attack The biology of a pest and the nature of its attack and damage it causes will influence the type of monitoring/forecasting strategy that is eventually adopted. The level of damage that can be tolerated, the number of insects that cause this and the rate of population increase are all important con- cerns. However, the most important biolog- ical criteria for determining which strategy to adopt will be the frequency and distrib- ution of pest outbreaks.
Infrequent but widespread outbreaks would be most effectively dealt with using a centralized regional monitoring/
forecasting strategy since this would not involve farmers and agricultural extension workers in a large amount of unnecessary sampling during non-out- break years. Regional centres (normally research institutes) can continuously monitor pest population levels and issue warnings to farmers when an outbreak is predicted. Infrequent outbreaks may be caused by influxes of highly mobile migrant pests on weather fronts or spo- radic disturbances, bringing insects from source areas that may be located in other regions, or countries. Forecasts of such outbreaks would depend on coordination and cooperation between monitoring
centres and this would be best achieved through research institutes.
For pests that reach outbreak levels reg- ularly each season, and for outbreak years of ‘infrequent pests’ then a field level mon- itoring/forecasting strategy, or at least a farm level strategy, will be most appropri- ate. Farm level monitoring will be satisfac- tory where there is little variability in infestation between fields or perhaps crops.
Where infestation is highly variable in space then field by field monitoring would be most appropriate for optimizing insecti- cide use between fields, provided that the costs of extensive monitoring did not out- weigh their usefulness.
In situations where pest outbreaks are infrequent and patchily distributed then the chances of developing any form of monitoring/forecasting strategy is remote.
In such cases the level of damage and hence the economics of crop production would be critical factors in assessing the potential value of a monitoring pro-
gramme. Where outbreaks were frequent but patchily distributed across a region then a farm level monitoring/forecasting strategy or a combination of a farm and regional centre strategy would be most appropriate.
2.4.3 Availability and suitability of monitoring techniques
Monitoring techniques fall into two groups:
those that are only suitable for use at research centres (where there are the neces- sary resources available) and those poten- tially suitable for use by farmers.
Monitoring techniques appropriate only to research centres are techniques such as light and suction traps that require an elec- trical supply, or other traps that also require trained personnel to sort and iden- tify the insects caught. Insect-specific traps and crop counts of insect stages that are easily identified represent potential moni- toring techniques for use by farmers, pro- vided that the technique (which will Fig. 2.11. A pegboard for counting insect numbers and producing a recommendation on insecticide application. The pegboard is divided into three parallel strips painted different colours, each of which is drilled with 24 holes. At one end is a black band with holes for pegs prior to use. One of the painted strips is used for counting the number of plants sampled on a diagonal transect across the field. When the line across the strip is reached the farmer starts sampling along the second diagonal. The other two strips are used for counting the number of eggs or larvae sampled for two insect species. The line across the strip indicates the action threshold for insecticide application (after Beeden, 1972).
primarily have been used by scientists developing the monitoring and forecasting scheme) can be modified appropriately.
Monitoring techniques to be used by farm- ers must be simple to operate or carry out and the data easily interpreted, traps must cost little to run and be readily main- tained. Monitoring and forecasting meth- ods that involve the graphing and charting of the progress of infestation are to be avoided unless there are opportunities for extensive farmer training.
Techniques that require training and supervision stretch the resources of exten- sion workers. It cannot be stressed enough that the monitoring must be appropriate for the situation to which it is to be applied. Farmers in developing countries may be illiterate and/or enumerate so that even simple counts of insect numbers may provide a problem. Beeden (1972) devised a simple pegboard for counting insect numbers and for making an insecticide rec- ommendation, a method which involves no reading or writing (Fig. 2.11). The peg- board provides a simple practical applica- tion of a quite sophisticated monitoring and decision making technique that is appropriate for farmers (Beeden, 1972); it represents a simple application that has potential for development and extensive use in insect pest management.
The distribution of an actual or poten- tial insect pest species may be determined through the use of an appropriate survey programme. The sampling technique used in a survey programme will depend on the biology and life cycle of the insect in ques- tion. Since the distribution of an insect species is usually dependent on the mobile winged stage of the life cycle, various aer- ial trapping techniques are often used in distribution surveys. Traps or other tech- niques that have a high degree of pest specificity and can be readily checked and maintained are helpful in reducing the costs of sample surveys. Pheromone traps are particularly useful in this respect and have been used successfully in monitoring the spread of a number of pests in Europe (e.g. Bathan and Glas, 1982).
The survey should adequately cover the area of interest and individual sample loca- tions should be chosen on the basis of what is known about insect host preference and general biology. The data collected from a trap network or a field count distribution survey can often be used to determine the relative levels of abundance in different locations. Such surveys may provide some understanding of the driving variables behind regional population changes.
The amount and type of information that can be gained from surveys carried out at different locations often depends on the type of supplementary data that is collected or available. On their own, data of relative levels of infestation can provide informa- tion about seasonal changes in abundance in different locations and may identify areas and time of high infestation. However, this information has limited value unless it can be related to other variables.
If the sampling technique used is a trap that catches mobile adult insects, a rela- tionship between this catch and the levels of damage, or abundance of the insect, in adjacent crops or the surrounding area may be required. The method of collection of additional information, such as crop counts, damage estimates, crop types and growth stages must be standardized between locations. The sampling may need to be continued for a number of years before it can be used in any regional fore- casting programme. The importance of correctly evaluating the type of informa- tion that is going to be required in a fore- casting programme at the start of the sampling survey cannot be over-empha- sized. Time and effort can easily be wasted in collecting inappropriate data. It is not unheard of, after a few years of a survey, for it to be realized that relevant data have not been collected.
The identification of an appropriate sampling and monitoring technique in order that the data generated can be used for forecasting requires clear objectives and planning. Various types of trapping device are used extensively for monitoring, but rarely are the ultimate uses of the data to
Fig. 2.12. Research pathway for developing traps as monitoring devices.
be generated considered at the outset. The use of traps for the monitoring of insects is fraught with difficulties mainly caused by the variability of trap catch data. This vari- ability makes interpretation of the data dif- ficult and this in turn reduces the value of such traps as monitoring devices. It is important if traps are to be used for moni- toring that they are developed for the spe- cific purpose and conditions for which they are needed. In an ideal situation a trap would be developed and evaluated along the lines of the scheme in Fig. 2.12.
The most critical phase of the development occurs after the initial field test. If success- ful then full scale trials over a number of sites and seasons could take place. This could not be justified if the initial trial were not successful, but if results were partially successful the temptation always exists to expand the trial with the hope that more data will clarify the situation. A successful monitoring procedure will pro- vide consistent and reliable forecasts; if the data are variable then the value of the pro- cedure will be diminished.