Population Estimation Methods of Insects by M.Salman
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This document discusses various population estimation methods for insects. It describes both absolute methods that directly count every insect, like using quadrats or emergence traps, and relative methods that estimate populations from samples, like sweep nets or light traps. It also discusses factors to consider for different sampling techniques based on insect behavior and distribution. The purpose of population estimation is outlined as determining pest levels to control problems and monitor changes over time and space.
Population Estimation Methods of Insects by M.Salman
- 1. Department of Entomology BZU Multan Pakistan
- 2. Muhammad Salman Roll # 34-E 5th Semester Entomology
- 3. Population Estimation Methods of Insects Total counts of insect populations are in most cases labour intensive and time consuming. Parameter-estimating sampling or census or total counts gives an accurate estimate of a population. Decision-making sampling or monitoring or sample count allows precise estimates.
- 4. Sampling Insect Populations on Single Plants Insects are small and often numerous. They can hide under leaves, at the base of stems and other places that make them hard to see and count. Estimating their numbers on a small plant or in a large field can be quite a challenge. Researchers solve this problem by counting the individuals in a "sample" part of the plant or habitat and then use this sample to estimate the total population.
- 5. Sampling Insect Populations on Single Plants Consider the behavior of the insect being counted before choosing a sampling method. Note the overall distribution of insects to be counted (base of plant, under leaves, at new growth, etc... In addition, the time of day and/or the season can affect results.
- 6. Purpose of Population Estimation To determine a pest species, its population distribution, change in population in space & time. To determine local or newly introduced population. To monitor pest level to control and recommend when, where & how to deal with specific problem.
- 7. Using Randomly Selected Leaves For evenly distributed species, count the insects on three randomly selected leaves on each plant. The "randomly selected leaf" will be your sampling unit. Average the results and multiply by the number of leaves on the plant to get an estimate of the total population.
- 8. Using a unit area or Quadrat When individuals are clustered on one part of the plant, sampling a unit area within that part of the plant may be necessary. For example, cabbage aphids form tight clusters on the underside of large cabbage leaves. By counting the aphids within a square centimeter placed within the cluster of aphids and then measuring the diameter of the cluster, you can estimate the number of aphids within a cluster of that size.
- 9. Using a unit area or Quadrat Perform a similar calculation for the other clusters on the plant, add them all together in order to obtain an estimate of the total population. In this case the unit area is the sampling unit.
- 10. Using a leaf Position Another method for sampling insects that tend to favor a particular part of the plant, is to use a leaf position as the sampling unit. Select a leaf position that appears to have the highest density of insects on most of the plants in your study. Whatever leaf position you choose, be consistent and count the same leaf position on each plant. Note that this method does not give you an estimate of the total population. It is best used to determine the relative level of insect population growth on a plant.
- 11. Absolute Methods Absolute methods yield estimates in density per unit such as locusts per land area, eggs per leaf or pupae per tree. Every insect per unit has to be counted. Most reliable data can be gathered from insect counts in square plots or standardised rings of 0.1 m² area that are placed along a transect line. The results of absolute methods can be directly compared with the results of previous assessments or assessments at different locations.
- 12. Absolute Methods However the price of this advantage is literally very high not only in terms of the required time and labour. Suitable absolute methods are leaf-counts, manual collection, the use of suction devices, rotary nets, emergence traps and Berlese extractors.
- 13. Capture-Recapture Method An interesting absolute method for the assessment of the population density is the capturerecapture-method (Peterson-Lincolnindex or proportionality method). Insects are captured randomly and marked, for instance with coloured nail polish. After marking the insects are released. Care must be taken to ensure, that the insects intermingle with the rest of the population before recapturing is carried out.
- 14. Capture-Recapture Method An estimate of the population density can be calculated from the ratio between the number of marked and released insects, the recaptured insects and the marked recaptured insects. A microscopic dye was offered to the termites and the number of termites feeding upon this. These animals eventually became coloured.
- 15. Relative Methods Relative methods are less labour and time intensive, therefore commonly used by entomologists. Most catching and trapping methods are relative methods such as visual searches, fixed time collection, sweep-net catch, shaking and beating, vacuum traps, Malaise traps, window pane traps, sticky traps, pitfall traps and traps using attractants like pheromones.
- 16. Direct & Indirect Assesment Direct assessment aims at the assessment of a pest population causing particular damage. This is, however, in many cases not possible because the insects might be hidden in the plant, like termites, or the pest may be living in the soil and emerges only during night. Therefore, the assessment is often indirect and actually does not count the number of insects, but their representatives such as frass, the number of cocoons, exuviae, egg shells, etc.
- 17. Damage Assessment An agriculturalist’s or forester’s major concern is the actual loss of a crop rather than the number of insects causing the damage. The damage to a crop is another indicator of the number of pest insects present on the particular crop. Therefore, the damage or yield loss assessment is an indirect method for the estimation of the insect pest abundance.
- 18. Damage Assessment The degree of defoliation of a host tree is a useful measure of the damage and is commonly indicated insect Population. The use of satellite images and aerial photos allows an effective damage assessment of large scale plantations. The damage caused by termites can be assessed by tapping the base of an infested tree. A hollow sound is audible where the trunk has internal termite galleries. Radiography is a sophisticated, therefore less appropriate, but interesting method of visualising the damage caused by wood and bark boring insects inside the trunk of a tree.
- 19. Collection and Trapping Methods There is a large variety of methods available for catching and trapping insects, each being suitable for a particular assessment method and group of insects. Traps usually immobilise insects and are either active or passive. The time and the location are important for successfully collecting and trapping insects. Some common trapping devices and collection methods are:
- 20. Manual Collection Manual collection is definitely the method of choice for many purposes. Beetles, most moths, all sorts of larvae can be easily collected by the use of hands, without any further devices.
- 21. Visual Searches During visual searches all individuals of the species to be assessed are counted or collected from the leaves of the host plant, from the stem, from the litter below the plant, from under rocks and fallen logs, etc.
- 22. Fixed time collection Fixed time collection is a relative method that uses visual searches carried out for a certain time. For example, the pests on a particular host plant are counted for ten minutes.
- 23. Nets Nets are the ultimate devices for catching flying insects like butterflies, that are difficult to catch by any other means. Once the insect is trapped the net is flipped over its rim so that the insect cannot escape
- 25. Light traps Light traps are used to catch nocturnal insects like moths, beetles and many more. Those insects are actually attracted to light because they become completely confused by an artificial source of light.
- 27. Malaise traps Malaise traps and quick traps are tent-like or soccer goal-like devices that are commonly used for the relative assessment of agricultural pests. The traps are open on one side allowing access for the insect and are funnelled into a collecting vessel attached to highest point.
- 29. Vacuum or Suction Traps Vacuum or suction traps and aspirators are suitable for the relative assessment of ground dwellers and of insects on low vegetation. Suction devices suck insects into a net from which the trapped animals can no longer escape. The current of air is either produced by a hand-held vacuum cleaner-like device or simply by the lungs of the collector
- 31. Barber traps or Pitfall traps Barber traps or pitfall traps are used as a relative method for the assessment of ground dwelling insects, mostly beetles or other arthropods, that walk on the ground and accidentally fall into the pit. Pitfall traps can also be lured with attractants or baits.
- 33. Windowpane traps Windowpane traps consist of a transparent plexiglas screen mounte vertically above a trough containing a suitable preservative. When an insect hits the invisible screen it drops down into the pan containing preservative and drowns. This method is also suitable for determining the flight direction of insects
- 35. Combined light and suction traps Combined light and suction traps attract nocturnal insects to the source of light. When an insect is close enough, it is sucked into the trap by a current of air. The trap is equipped with a funnel with smooth and steep walls from which a trapped insect slips into a killing jar. A disadvantage of this kind of trap is that larger specimens like moths are easily damaged in the trap
- 37. Emergence traps Emergence traps are used for the absolute assessment of insect larvae and pupae hidden in soil or litter. The cage-like device is placed over the respective site and left there to trap the adults emerging from the soil.
- 39. Shaking and Beating The method is suitable for catching insects associated with lower vegetation like smaller trees and shrubs. A beating tray, canvas or piece of cloth is held or placed below the plant to be assessed. Then the stem of the plant is beaten or vigorously shaken so that insects fall on to the beating tray from where they can be collected. Fast moving insects however might easily escape from the tray or canvas, therefore an aspirator can be very helpful.
- 41. Rotary Nets Rotary nets are devices used for the absolute assessment of insect populations. The devices consist of two or more revolving nets in which flying insects get caught. The method yields good results independent of the wind speed.
- 43. Sticky traps and greasy traps Sticky traps and greasy traps are used for the relative assessment of diurnal insects. The device is coated with a very sticky adhesive or grease suitable for immobilising insects. Insects are attracted to the trap either by its bright colour like yellow or white or by an attractant like a sex pheromone. The advantage of a greasy trap is that the immobilised insect can be removed from the trap in one piece.
- 45. Pheromone traps Pheromone traps are used for the relative assessment of insect populations. The attracted insects either fall into a container with preservative or are immobilised on adhesive if the trap is used in combination with a sticky trap. Pheromone traps containing sex attractants usually attract only one particular sex, eg. the males. The traps are very simple, cheap and effective.