Pest population studies are helpful in pinpointing the factors that bring about numerical changes in the natural population and also in understanding the functioning of the life system of the pest species
Pest population studies are of two types:
(i) Extensive Studies:
These studies are spread over a large area and are needed to understand the distribution pattern of a population, to predict the damage it is likely to cause, to initiate control measures and to relate changes in the population to certain climatic or edaphic factors. A particular area is observed once or at the most a few times during the season and counts are made of a particular developmental stage of the pest.
(ii) Intensive Studies:
These studies involve repeated observations in a given area when it is desired to determine the contribution of various age intervals to the overall rate of change in the population or the dispersal of species. In this case the numbers of successive developmental stages are counted, and life-tables and budgets are prepared for determining the key factor(s).
For a correct and scientific understanding of a pest population, it is of fundamental importance to develop sound methods of population estimation. This involves two considerations. First, the life stage (egg, larva, pupa or adult) at which counting can be made most advantageously; and secondly, the actual process of counting.
The ideal approach to population estimation would be to count all the individuals. However, it is not possible to count most of the pest species over an area large enough to be of use in a practical study and hence some method of sampling becomes necessary.
The amount of time and effort required to obtain absolute counts even on a limited area is so large that it is often uneconomical and unproductive. Thus, although we wish to have information on the true population, we are forced to take smaller collections (samples) and use these to make inferences about the total population. Based on the goal of sampling population, sampling plans in IPM can be grouped into three categories.
First, detection sampling is used when pest is not present in the field. These are planned to avoid the chance that the organism is erroneously missed. Second, estimation sampling when the actual population of pest is estimated with desired levels of precision. It is mainly used in research and also to evaluate the effectiveness of any IPM module or pest control strategy developed to manage the insect pests in the farmers’ field.
Third, decision sampling based on which the decisions are made when to intervene with management tactics. In this sampling, objective is not to quantify the actual abundance of pest, but to decide the correct timings when control measures should be adopted or not. Various sampling techniques used for this purpose have been developed.
Measurements taken to estimate pest population density fall into three categories, viz. absolute estimates, relative estimates and population indices.
1. Absolute Estimates:
The total number of insects per unit area (1 ha, 1 m row length, 1 m2 quadrat, etc.) is the absolute estimation. The numbers per unit of the habitat (per plant, shoot or leaf) indicate the density of population. The estimates of absolute population and population density are used for preparing life tables, study of population dynamics of field populations and to calculate oviposition and mortality rates.
The following methods are commonly employed for estimating absolute population:
(i) Quadrat method:
Small areas or quadrats are chosen at random from a large area which contains the population. A large number of quadrats are required for over-dispersed population than that in the randomly distributed population. Stratified sampling is followed in the case of aggregated distribution, i.e., the population is divided into different strata and varying number of samples is taken from each stratum.
From a quadrat, the insects may be counted or collected directly as in the case of fairly immobile but relatively large insects such as cutworms, caterpillars and grasshoppers. In case of tissue borers such as sugarcane borers, maize borer, etc., the estimation is done by first removing the infested plants from the quadrats and then counting them after splitting open the plant parts.
(ii) Capture, marking, release and recapture technique:
This technique is generally used for estimating the population of flying insects. The losses or gains in a population over a period can be determined with the help of this method.
The estimate of density fluctuations and the death rate can also be made for making comparisons between different forms of pests under varying environmental conditions. The different types of markers used are paints and dyes, labels, mutilation, radioisotopes, etc. This method has been used for estimating the absolute population of butterflies, grasshoppers and beetles.
The total population is estimated by using the following formula:
P = N × M/R
Where P = Population of insects
N = Total number of insects caught
M = Number of marked individuals released
R = Number of marked individuals recaught
For effective application of capture-recapture technique in population estimation, the following assumptions have to be met:
i. The marking of individuals does not lead to changes in their behaviour or longevity and the marks do not get lost easily.
ii. The marked individuals, after being released, become completely mixed up with the unmarked individuals of the population.
iii. The population is sampled randomly with respect to its mark status.
iv. The method of marking should be such that it should distinguish between different dates of capture.
v. The population under study must be reasonably stable and not subject to rapid fluctuations in numbers.
2. Relative Estimates:
In relative population estimates, the samples usually represent an unknown constant proportion of the population. A given amount of labour and equipment is utilised to yield much more data than is possible for absolute estimates. Such estimates are useful in making comparisons in space or time. These are useful for studying the activity patterns of a species or for determining the constitution of a polymorphic population.
The relative estimates are influenced by following factors:
(a) Variation in behaviour of an insect with change in age
(b) Variation in level of activity of the pest as influenced by its diurnal cycle
(c) Variation in responsiveness of sexes to trap stimuli
(d) Variation in efficiency of the trap or the searching method, besides the pest population.
The methods employed for relative estimates include the catch per unit time or effort and the use of various types of traps.
(i) Catch per unit time or effort:
Various types of collection nets are available for use in different habitats and the sweep net is the most widely used for sampling insects from vegetation. Only those individuals on the top of the vegetation and those that do not fall off or fly away on the approach of the collector can be caught with the sweep net.
The efficiency of catch with a sweep net will be influenced by changes in the habitat, species and vertical distribution of the pest species, variation in the weather conditions and effect of diet cycle on vertical movements. The number of sweeps necessary to obtain a mean that is within 25 per cent of the true value will depend upon the pest species, its spatial distribution and the diurnal periodicity.
(ii) Line-transect method:
If one walks in a straight line at a constant speed through a habitat, the number of individuals can be counted. This technique is used for quantitative comparisons both between different species, and between different occupiers of habitats.
This technique was originally employed by botanists for estimating the population of plants, but could not be easily used in animals owing to their mobility from place to place. However, the data based on number of encounters have been used for estimating the absolute population of locusts and grasshoppers.
The number of organisms per unit area or their density can be calculated by the formula:
D = Z/2R (V̅ + W̅) 1/2
Where,
D = Density
Z = Number of encounters between the observer and the organism in a unit time
R = Radial distance within which the organism must come in contact with the observer to affect an encounter
V̅ = Average speed of the observer
W̅ = Average speed of the organism
The following pre-requisites are necessary for application of this formula in the study of natural populations:
i. The number of organisms to be counted should remain reasonably constant during the period of observation.
ii. The average speed of the observer (V̅) needs to be determined before the actual observations. The time spent in locating the organisms and in making records has to be taken into account while working out the average speed.
iii. The average speed of the organism (W̅) should be determined by taking a large number of small samples.
iv. The effective radius I of the organism is determined on the basis of recognition distance which can be visual or auditory.
Further, the following conditions must be satisfied for getting valid estimates and comparisons:
i. The observations should be made in the same phase of the life-cycle.
ii. Weather conditions, time of the day and season of the year, should be reasonably comparable.
iii. The features of the environment influencing recognition must remain uniform.
(iii) Shaking and beating:
Some insects can be collected on ground by shaking or beating the plants. A piece of cloth or polythene may be laid out under the plants and the plants are vigorously shaken. The insects fall on the cloth or polythene and need to be counted immediately before they disperse. The gram pod borer, Helicoverpa armigera (Hubner) larvae may be sampled by vigorously shaking the chickpea plants.
(iv) Knockdown sampling:
The insecticides such as pyrethrum or other pesticides with safe chemistry may be sprayed on plants enclosed in a polythene envelope. The insects will be knocked down due to the insecticides. The plants are shaken and the insects which fall on the ground or cover sheet may be counted to estimate the population.
Other approach is heating the sample which forces the insects to come out of their dwellings. The plant sample may be placed in a special device, the Berlesse funnel that heats the sample. The insects come out, fall through the funnel and can be counted in a receptacle.
(v) Use of traps:
In this case, it is the insects rather than the observer that would make the action leading to their enumeration. Traps are of two types, viz., interception traps (that catch the insects randomly) and attraction traps (which attract the insects in some way). The interception traps provide indices of absolute population more easily than those of attraction traps, as there is no variation due to attraction.
Various types of interception traps are flight traps, aquatic traps, pitfall, light and other visual traps. The flight traps that combine interception and attraction include sticky traps and water traps. The traps that attract the insects by some natural stimulus or substitute include the shelter traps, trap crops, bait traps, chemical attractants and pheromones.
(vi) Remote sensing:
Remote sensing technology has long been used for monitoring insect infestation in field crops. It is based on the principle that the absorbance and reflectance of plants in response to pest attack changes and these changes are recorded by a device from far away. Remote sensing platforms can be aircraft, satellites or ground based. The remote sensing techniques include full-colour photography, infrared (IR) wavelength and multiband spectrometers.
In full colour photography, the coloured photographs are examined for chlorosis or other symptoms. In IR wavelength, the changes in leaf temperature in relation to differential moisture content induced by pest activity are measured using remote thermometer. In multiband spectrometers, the reflectance at specific wavelength is measured to record different types of vegetation.
The insect infestations on beet and cabbage plants were monitored by aircraft based multispectral imaging. The negative correlation between beet armyworm, Spodoptera exigua (Hubner), hits and transformed normalized differences in vegetation index values was reported.
Based on these, future sampling plans and site specific management techniques can be developed. The hyper spectral remotely sensed data with an appropriate pixel size have the potential to portray greenbug, Schizaphis graminum (Rondani), density on winter wheat and discriminate its damage to wheat with repeated accuracy and precision.
The early identification of damage due to two-spotted spider mite, Tetranychus urticae Koch, on greenhouse pepper (Capsicum annuum) can be obtained by multispectral means as it can be spectrally detected in the reflectance of the visible and near- infrared regions. The hyperspectral reflectance data of greenhouse pepper leaves were transformed into vegetation indices allowing early two-spotted spider mite damage detection by separation between leaf damage levels.
With advancement in spatial information technologies such as Global Positioning Systems (GPS) and Geographical Information Systems (GIS), remote sensing is finding more practical application for monitoring and management of insect pests. The three-dimensional real time observations on insect population can be achieved using remote sensing in conjunction with ‘silicone oil-carbon black powder suspension squeeze’ (3S) technique.
3. Population Indices:
Population indices do not count insects at all, but rather they are measures of insect products or effects. Under field conditions, it is not possible to estimate the absolute population in most of the cases. It, therefore, becomes necessary to establish a relationship between absolute estimates and population indices or the relative estimates so that the latter two types of estimates could be converted to absolute terms by using certain correction factors.
(i) Insect products:
In some cases, a species that is difficult to sample creates products directly that are easily sampled by absolute methods. The insect product most often sampled is frass or excrement of lepidopterous defoliators. The rate at which frass is produced can be estimated from the amount falling into a box or funnel placed under the trees. The size and shape of the frass pellets is rather constant for a given species and instar; this allows one to identify the species and age composition of defoliators.
(ii) Plant damage:
The amount of damage caused by insects to crop plants is a function of the pest density, the characteristic feeding or opposition behaviour of the species and the biological characteristics of the plants. Different methods have to be adopted for measuring damage by direct and indirect pests.
(a) Direct pests:
These pests attack the produce directly, destroying a significant part of its value. Bollworms on cotton and fruit borers in fruit and vegetable crops are examples of such pests. The damage by direct pests is sampled on the basis of absolute or relative numbers of damaged unit, e.g. number of damaged bolls per plant, apples per tree, pods per meter row length, etc.
(b) Indirect pests:
Damage by indirect pests may be measured by estimating the extent of defoliation in case of defoliating pests like lepidopterous caterpillars, leaf beetles, grasshoppers, etc.
Many devices like planimeters, photoplanimeters, leaf area meters, etc. are now available for quantifying the extent of defoliation. In case of sap-sucking insects like aphids, jassids, white flies, etc. damage may be estimated by change in coloration of the leaves, plant vigour and size. For root feeders, damage to root system may be evaluated by percentage of nodes below the soil surface having injury, force needed to pull a plant from the soil, etc.
For a correct and scientific understanding of a pest population, it is of fundamental importance to develop sound methods of population estimation. This involves two considerations. First, the life stage (egg, larva, pupa or adult) at which counting can be made most advantageously, and secondly, the actual process of counting.
The ideal approach to population estimation would be to count all the individuals. However, with most of the pest species it is not possible to count them over an area large enough to be of use in a practical study and hence some method of sampling becomes necessary. For devising a satisfactory and successful sampling programme, consideration has to be given to the nature of the sample, mode of sampling, the size and number of samples.
The precision of a pest population estimate based on a given sampling technique would depend upon both the properties of the population in terms of its density and degree of aggregation and upon the characteristics of the sampling plan, i.e. the number and size of samples.