Selection of an efficient, simple and accurate screening method is extremely important in identifying a resistant or tolerant variety.  The screening method selected should give distinctly different reactions for plants of susceptible, moderately resistant and resistant cultivars.  Therefore, to establish an efficient screening method, proper understanding of insect biology, its potential plant damage, number of insects necessary to infest each plant, number of insect releases required, site of insect release, growth stage of the plant at which release should be made and the most appropriate time interval between infestation and evaluation is needed. 

1. TOPIC:
Grading of resistance for screening of major pest of Cash crop
ADVANCED HOST PLANT RESISTANCE
ENT-608

2. Introduction:
• Selection of an efficient, simple and accurate screening method is extremely
important in identifying a resistant or tolerant variety.
• The screening method selected should give distinctly different reactions for
plants of susceptible, moderately resistant and resistant cultivars.
• Therefore, to establish an efficient screening method, proper understanding of
insect biology, its potential plant damage, number of insects necessary to infest
each plant, number of insect releases required, site of insect release, growth
stage of the plant at which release should be made and the most appropriate
time interval between infestation and evaluation is needed.

3. Different screening techniques:
1) Field Cage Screening:
• In this screening type, insects are caged on test plants.
• Cages limit the emigration of the test insect and also protects these
insects from predation and parasitism.
• Whole plants can be placed in cages constructed of wood, plexiglass
or metal frames supporting screened aluminum panels of nylon. Cage
size and shape are determined by the type, age and number of test
plants under evaluation.
• Dimensions vary from small field cages to large field cages placed over
galvanized metal frames to cover entire experimental plot. Plant parts
can be enclosed in sleeve cages made of dialysis tubing, polyester
organdy or nylon cloth.

4. • 2) Green House Screening:
• Green house screening permits greater control in selecting
resistant plants but restricts the amount of material that can be
screened over a given period of time.
• Green house screening is a time saving tool for the crops having a
long growing cycle.

5. • Standard seed box screening test:
• Test cultivars are sown in wooden or metal flats filled with soil.
• A susceptible check and a resistant variety are also sown in random rows in
each seed box.
• Test insects, in numbers sufficient to kill the susceptible check cultivars are
uniformly distributed onto the seedlings.
• The damage is graded upon the death of the susceptible check cultivar. This
procedure is replicated four or five times.
• This screening test can be applied to rice leaf and plant hoppers.

6. • Modified seed box screening test:
• Because the seed box screening test is mostly qualitative, cultivars with
moderate levels of resistance are usually rated as susceptible.
• So the seed box test has been modified to detect cultivars with moderate
resistance to rice brown planthopper (BPH), Nilaparvata lugens.
• In the modified test, plants are older at the time of infestation and the
infestation rate is lowered. In addition, the progeny rather than the initial
source of infestation are the insects that cause the plant damage. With
these exceptions, all the other aspects are similar to that of conventional
type. This method can be used to all hoppers with slight modifications.

7. • No-choice screening test:
• This method is relatively simple, inexpensive and is extremely useful in the preliminary
evaluation of resistance.
• Uneven distribution of insects causes an unbalanced infestation resulting in the escape
of some cultivars from insect damage.
• In order to avoid this shortcoming, no-choice screening test is conducted where each row
of test cultivar seedlings was excluded from the other by an intervening vertical Mylar
plastic partition and exposed to equal insect infestation.
• This method ensures an even distribution of test insects on all test cultivars from the
beginning to the termination of the experiment.
• No choice technique has been widely used to complement free choice procedures to
identify and confirm the presence or absence of insect resistance in wide variety of
plants. To maximize the identification and measurement of insect resistance, use of both
free choice and no choice screening methods is suggested to provide reliable results.

8. • Laboratory Screening:
• Techniques for evaluating resistance under more controlled laboratory
conditions are often necessary, since field and greenhouse tests are affected
by a number of environmental factors that cannot always be controlled.
• Laboratory screening methods should only be viewed as a reliable and rapid
method for confirming insect resistance before or after field or greenhouse
testing. Leaf discs or plant tissues are commonly used in insect feeding
bioassays of chewing insects.
• Choice tests and no choice forced feeding tests are generally used to
evaluate insect resistance. Plant damage by insects is measured based on
the basis of area fed, dry weight of control and damaged tissue etc reduced
leaf area, loss in dry weight index, and poor chlorophyll concentration.
• Damage is estimated by visually scoring the amount of uneaten material on
a damage scale.

9. • Damage Rating:
• Host reaction scales are generally developed to accurately describe
insect damage levels.
• These scales are capable of differentiating among small differences in
plant damage and of clearly defining resistant, intermediate and
susceptible plants.
• Rating techniques associated with the scales should be fast and easy to
execute because thousands of plants must normally be evaluated during
the screening process. Basically, insect damage rating scales are used for
the measurement of damage caused.
• This is usually based on a 0 to 5, 1 to 5 or 1 to 6 scale. A rating of 0 to 2
or 1 to 2 suggests some resistance and need for further testing and a
rating of 4 to 5 or 5 to 6 represents highly susceptible cultivars.

10. Screening for insect resistance in cash
crops :
• 1) Cotton :
• A special procedure for determining the resistance to pink bollworm in cotton cultivars based on
carryover population in leftover green bolls has been given by Agarwal et al. (1973) and Sukhija
et al. (1983).
• The screening of cotton varieties against jassid is being done on the basis of injury grades. Four
leaf injury grades have been recognized. Under natural conditions, the screening of cotton
varieties against jassid is also being done by growing an infestor row of an okra between the
two cotton rows (Batra and Gupta, 1970).
• Free choice test for screening the germplasm against cotton whitefly under greenhouse
conditions has been suggested by Butter and Vir (1989) and several genotypes were screened
using this method. The sampling of whitefly adults and eggs from the lower surface of the three
fully opened leaves of the upper canopy and 4th instar red eye nymphs of the middle canopy
leaves has been advocated for correct population estimates by Butter and Vir (1990).
• Another criterion of leaf injury index based on plant damage which can be used for screening
the germplasm has been suggested by Sukhija et al. (1986) and Butter and Kular (1986).
Further studies are needed to refine these techniques of resistance for quick identification of
whitefly resistant cultivars.

11. 2. Sugarcane:
• i. Top borer:
• Evaluation of varieties for top borer infestation is generally done by
randomly selected stalks of each of healthy and damaged canes which
are examined for length, number of internodes, girth, weight and sugar
concentration. Yadav (1985) categorized sugarcane varieties against top
borer on the basis of percent incidence which are detailed below :
• ii. Shoot borer:
• Categorizing varieties for reaction to shoot borer infestation based on the
attacked plants ha-1 is claimed to be the most appropriate method.
However, based on the economic threshold level of 15% incidence, the
varieties may be graded as being less susceptible (0-15%), moderately
susceptible (15.1-30%) and highly susceptible (above 30%)

12. Incidence (%) Category
00.0-10 Highly Resistant
10.1-15 Resistant
15.1-20 Moderately Resistant
20.1-25 Moderately Susceptible
25.1-50 Susceptible
50.1 and above Highly Susceptible

13. • iii. Scale insect:
• Evaluation of sugarcane varieties against scale insect has been done
on the basis of its infestation, namely, heavy, moderate and light,
depending upon the percent incidence or percent intensity of the pest
under field conditions (Agarwal, 1960).
• A quick technique for screening of sugarcane cultivars against
infestation of scale insect has been reported by Singh and Nigam
(1985) in which mature cane stalks of test varieties of sugarcane were
cut from a nursery plot and one end was sealed with molted wax.
• One hundred freshly emerged nymphs of M. glomerata were released
on stalks and kept in a dark place for 48 h. The number of nymphs
found settled 25 days after release was recorded and cultivars were
graded

14. Sl No. Particular Susceptibility
1
When only a few insects are seen
on any of the internode without a
well established colony (very
light) or when the incrustation of
the pest covers only about ¼ of
an internode (light)
Less susceptible
2
When the pest incrustation covers
nearly ½ of an internode
(moderate)
Moderately susceptible
3
When the incrustation covers ¾ of
an internode (severe) or more
than ¾ of an internode (very
severe) or when the canes show
drying due to the pest attack
Highly susceptible

15. 3.Groundnut :
Finally the infestation index is calculated as :
Infestation index = (Ixa+IIxb+ IIIxc+IVxd+Vxe)/( a+b+c+d+e)
Where I, II, III, IV and V are the injury graded
a, b, c, d and e are the number of plants falling in each grade.
i. Leaf miner screening method:
The resistance in groundnut leaf miner was evaluated on the basis of
the leaf miner injury. In the 12th All India Workshop of AICORPO (1978),
the following procedure was suggested by restricting observations on 10
leaflets plant-1 on 5 randomly selected plants suing 1-5 scale injury
grade .

16. • Conclusion:
• The majority of screening that is carried out in pant breeding is
not in the laboratory but in the field, where conditions are more
variable and where screening and selection techniques need
to be simple and economical of time and effort.
• Hence visual assessment of plant using scales or indices are
the commonest form of resistance evaluation.
• The need for screening plants under natural conditions of
infestation is a cause supported by few breeders but, on its
own, the use of insecticides in this way plays a
major contribution to the problems of crop susceptibility

17. References:
• 1.Mukhopadhyay A, Mandal A. Screening of brinjal (Solanum melongena) for
resistance to major insect pests. Indian J. Agric. Sci. 1994;64:798-803.
• 2. Mote UN. Studies on the varietal resistance of brinjal, Solanum melongena to
jassid, Amrasca biguttula biguttula under field conditions. J. Veg. Sci.
1978;5(2):107-110.
• 3. Uthamasamy S. Studies on the resistance in okra. Abelmoschus esculentus (L)
Moench to the leafhopper, Amrasca devastans (Dist). Trop. Pest Mgt. 1986;32(2):146-
147.
• 4. Uthamasamy S. Studies on host resistance in certain okra varieties to the leaf
hopper Amrasca devastans. Ph.D thesis. Tamil Nadu Agricultural Univerity,
Coimbatore; 1979.
• 5. Sandhu GS, Sharma BR, Singh B, Bhalla JS. Sources of resistance to jassid and
whitefly in okra germplasm. Crop Improv. 1974;1(1-2):77-81.