3. ASSESSMENT OF PEST POPULATION AND CROP LOSS

3. assessment of Pest Population and Crop Loss

Assessment of pest population or crop loss (Survey) is a planned activity to collect data.  When assessment or survey is carried out at regular intervals in the same locality to record data, it is called “Surveillance”.

It is not possible to count all the insects in a habitat.  Therefore, to estimate the population density of a pest or intensity of crop loss one has to resort to sampling.

SAMPLING:

Sampling is a tool for reliable estimates of pest-incidence that are representative of the range of insect abundance.

Sampling unit: A sampling unit is a proportion of the habitat, from which an estimate is made.  The sampling unit may be a plant, branch of a plant, leaves, a fruit, a hill, a clump, a micro-plot or number of plants in one meter of row length, etc. A sampler can also determine the size of the sampling unit.

Sample:  A group of sampling units is called the sample.

Population intensity: It is the number of insects per habitat unit and it relates closely to crop injury.  Estimation of population intensity helps in establishing economic injury level (EIL).

 OBJECTIVES of SAMPLING:

1.  To determine the insect number or damage at a given time.

2.  To know the presence of natural enemies.

3.  To decide the implementation of management.

Common Sampling Techniques:

Sampling technique involves how and from where the samples are to be taken.  Several sampling methods are adapted in the assessment of pest population or crop loss.  Each technique has certain advantages and disadvantages in terms of precision, efficiency, cost, ease etc.  The selection of technique also depends on the Pest’s biology and ecology.

DIRECT COUNT:

This technique is useful for counting large and conspicuous insects usually on a part of the plant (Eg. leaves (or) whole plant). However, for smaller insects like aphids, thrips and mites, help of a hand lens may be taken in making the counts.  Further, when these insects are in large number, 1cm² windows may be cut out in a thick paper sheet and placed on the leaf surface for counting the insects visible through windows. A quadrate (1 m²) or a square metal frame can also be used to take observations from the plants following in the frame.

1.      Knockdown: Either causing the insects to fall into a tray or on a sheet or piece of cloth and are subsequently counted.  The method of dislodgement may be jarring, application of insecticides etc.

a)      Jarring:  It is probably the most common method of knocking down insects from the plants. A branch is given jerks several times. E.g. White grub adult beetles on neem tree, gram pod borer larvae in chickpea plants.

b)      Application of insecticide: In certain cases, the plants may be treated with a quick-knockdown insecticide such as pyrethrum and insects collected on a cloth spread under it.

2.      Netting: Netting of insects is one of the most widely used techniques, which is relatively simple and inexpensive. Netting may be of several types:

a)      Sweep netting: Here, muslin net is swept through the crop canopy and the plant is jarred so that the insects present on the plant may fall into the net.  After 20-25 swings or sweeps, plant debris is removed and the insects are counted. 

b)      vacuum netting:  In this technique, engine power is used to create strong vacuum which sucks the insects from plant canopy. Such nets have been used for sampling insects like leafhoppers.

3.      Trapping: Insects attracted to different visual, chemical or olfactory cues are trapped in this method. Traps are useful for detecting the initial appearance of a pest. Seasonal activity of various pests can also be monitored using traps.  Different types of traps are used to sample insects. 

a)      Light traps: It is the most widely used visual trap employed for sampling agricultural pests, particularly moths, hopers and beetles, etc.

b)      Pheromone traps: Pheromone traps have been widely used in detecting or sampling fruit flies, spotted bollworms, cotton bollworm, codling moth, pink bollworm etc.  Since pheromone traps are specifically attractive to the target species, there is no sorting or identification problem.  Further, no power is required is required as in the case of light traps.

c)      Bait traps: Bait traps rely on an insect olfaction, or sense of smell for attraction. E.g. A mixture of yeast and molasses in a cone trap is used to attract corn maggot, Fish meal trap for sorghum shoot fly.

d)     Malaise trap: The malaise trap is a tent made of cotton or nylon mesh with one side open that intercepts flying insects.

e)      Water trap: It consists of a shallow open pan mounted on a wooden post which is filled with water having some detergent or soap or an oil film to aid in wetting or drowning the insects.

f)       Sticky trap: These traps are installed on a wooden stack or a bamboo stick at various heights above a plant canopy.  The insects are stuck in the adhesive applied to the trap’s surface.

g)      Pitfall trap: It captures ground-moving insects.  The pitfall trap is a bottle sunk in the ground with a funnel at the soil surface that empties into the container.

A.   INDIRECT TECHNIQUES:

In indirect techniques, insect population is estimated either by measuring the insect injury on crop plants or with the help of insect products. This is called as population indices.

a)      Insect injury: e.g. number of leaves mined by leaf miners, percentage defoliation by armyworms, percentage of plant attacked, percentage of bolls damaged, leafhopper injury grade in cotton, number of wilted or dead plants by termites, plants with “dead hearts” caused by borers, etc. 

b)      Insect products: Sometimes, pest populations are assessed with the help of insect  products which include larval and pupal skins, frass, honey dew (e.g. secreted by aphid, whitefly) and nests of colonial insects. The measurement of frass drop in a collecting tray has been used to assess population size of several forest pests.

c)      Remote Sensing: Remote sensing techniques such as radar can automatically monitor the height, horizontal speed, direction, orientation, body mass and the shape of arthropods intercepting the radar beam.  It can provide information of aerial migration of pest and natural enemies.

     Parameters of sampling:

1.   Insect stage to be sampled:  The insect stage most often sampled is the one causing damage. However, a stage prior to the damaging stage may be selected so that early predictions can be made. 

2.   Number of sampling units:  The number of sampling units depends on the degree of precision needed and the cost.

3.   Time to sample:  Sampling time depends on pest as well as crop phenology.  It is most important for the sampling to coincide with the time of peak numbers of the stage.

4.   Pattern of sampling:  Depending on the uniformity of the habitat and population dispersion, the sampling pattern may vary.  The most common sampling patterns are the X, W, U, diagonal, zig-zag diagonal, micro-plots, a fixed row length, etc. 

5.   Types of sampling:

a)      Random sampling:  It involves selecting a number of samples from a population such that every sample has an equal chance of selection.  For this, random number tables can be used.

b)      Stratified random sampling:  In this case, the population is divided into different strata from which random samples are then taken.  The strata are sub-divisions of the samples based on knowledge of the distribution of the population.

c)      Systematic sampling:  It involves taking samples at fixed interval.  The first sample is taken at the reference point and subsequent sample at successive intervals. 

d)     Trap sampling:  It involves the use of various types of traps in collecting insect populations.

e)      Sequential sampling; It is based on insect dispersion pattern and economic decision levels.

4. ASSESSMENT OF PEST POPULATIONS AND THEIR DAMAGE IN RICE AND VEGETABLES

1.      RICE: Nursery:

S.No	Pest	Sampling method (10 cents area)	ETL (Economic threshold level)
1.	GLH	v  Net sweeping and counting the adults  (25 sweeps) v  Select 5 micro plots of 1 m2 each. Count the nymphs and adults in 20 seedlings in each micro plot and workout the mean.	60 insects/25 sweeping 5/hill - vegetative phase, 10/hill - flowering phase, 2/hill in tungro endemic areas
2.	Thrips	v  Passing a wet table tennis bat painted white, over the foliage at 5 places and counting the population v  working out the percentage of affected seedlings	25 insects/5 sweeps or Terminal rolling of top leaves in 10% seedlings
3.	Stem borer	v  Count the egg masses in 2 m2 nursery area (1 m2 each) in two places.	2 egg mass/m2

Main field:

S.No	Pests	Sampling method (one acre)	ETL (Economic threshold level)
1.	BPH	Tap vigorously  the selected hills at random and count the insects  fallen on the water surface, work out the mean	1/ tiller , 2/tiller (If 1 spider/hill present)
2.	Stem  borer	Dead heart and white ear: Count the total and affected tillers per hill and work out the percentage. Record symptoms in 10 randomly selected plants.	5-10% plants with dead heart or 2% white ears.
3.	Leaf folder	Count 10 randomly selected plants for infested leaves and workout the percent damage of leaves	10% damage  (Vegetative stage) 5% damage (Boot leaf stage)

COTTON

Sl.no.	Pests	Sampling method
1.	Boll worm	Count the number of damaged bolls with circular bore hole and healthy bolls in randomly selected ten plants and work out the percentage
2.	Whitefly	Select ten plants randomly and select three leaves form each plant (upper,middle & lower) and count the number of insects in each leaf and work out the mean

2.      VEGETABLES

S.No	Pests	Sampling method (one acre)	ETL (Economic threshold level)
1.	Fruit borer in Bhendi	Count larvae in one meter row length from 10 random sites in a field. Workout the average.	1 larva/meter row length
2.	Fruit borer in tomato	Count the larvae in 1m2 micro plot from random sites in a field.	1 larva/ m2
3.	Leafhopper in bhendi	Count the hoppers in three leaves in the upper canopy of each 20 randomly selected plants.	2 -5 Nymphs/ leaf

Workdone

Green leaf hopper

Plant no.	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10	Mean
No.of insects

Brown plant hopper

Plant no.	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10	Mean
No.of insects

Rice stem borer (vegetative stage)

Plant no.	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10	Mean
No.of dead heart
Total tillers

Rice stem borer(reproductive stage)

Plant no.	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10	Mean
No.of white ear
Total productive tillers

Cotton boll worm

Plant no.	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10	Mean
No.of damaged bolls
Total no.of bolls

Cotton whitefly

Plant no.	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10
Top leaf
Middle leaf
Lower leaf
Mean