Contents - Faperta

62 Biotechnological Approaches for Pest Management and Ecological Sustainability
Phenotyping Mapping Populations for Resistance to Insects
Phenotyping for identifi cation of quantitative trait loci (QTL) or genes associated with
insect resistance can be carried out under fi eld, greenhouse, or laboratory conditions.
Nearly 250 to 500 F 6-10 recombinant inbred lines (RILs) derived from a single cross, double
haploids, backcross progenies, or F 2s should be tested under fi eld conditions in a lattice
square or balanced alpha design with three to fi ve replications. A plot size of two rows,
2 to 4 m long, should be maintained. The plot size and number of replications to be used
will depend on the resources available and fi eld variability. Evaluation for resistance to
insects can be carried out under natural or artifi cial infestation. Data should be recorded
on insect survival and development, plant damage, and yield, with utmost precision. The
precision of the phenotypic data determines the quality of the genetic linkage map and the
robustness of the QTL or markers associated with resistance to insects. The mapping populations
should be tested across seasons and locations to assess the genotype-environment
interactions. The mapping populations can also be evaluated using cage screening in the
fi eld or in the greenhouse, excised plant parts, diet impregnation assay, or monitoring of
physicochemical traits associated with resistance to insects. The information generated on
insect survival, development, and plant damage is used to develop the genetic linkage
maps and identify the QTLs or genes associated with resistance to insects.
Measurement of Host Plant Resistance to Insects
Precise assessment of insect damage or insect numbers at the initial stage is most important
to screen for resistance to insect pests. Data should be recorded at the appropriate
stage on extent of leaf, pod, boll, or plant damage, depending on the nature of the damage
(Sharma, Singh, and Ortiz, 2001). Some polyphagous insect pests, such as pod borers and
bollworms (Heliothis, Helicoverpa, and Maruca), feed on several plant parts, such as leaves,
fl ower buds, fl owers, pods, bolls, and the seeds, whereas the cereal stem borers (C. partellus,
Busseola fusca Fuller, and Sesamia inferens Wlk.) feed on leaves, leaf sheaths, stems, and the
peduncle, leading to leaf scarifi cation, stem tunnelling, peduncle breakage, or partial seed
set. Damage to vegetative plant parts results in indirect loss, and generally gets compensated
as a result of regrowth in the affected plants or plant parts. However, damage to the
reproductive parts, particularly to fl owers and developing seeds, results in direct loss and
there is little chance to recover from insect damage. Hence, the level of infestation during
the fl owering and fruiting phases is quite important for assessing insect damage to quantify
genotypic resistance to insects feeding on reproductive structures. Numbers of fruiting
bodies and the extent of damage is the fi nal outcome of complex interactions involving
the insects and their host plants. Percentage damage to plants and plant parts, such as
leaves, bolls, fruits, and pods, are commonly used for determining the genotypic susceptibility
to pod borers and bollworms. However, this criterion often leads to unreliable results
due to variations in insect population, damage to the foliage (which is not refl ected in the
damage to the fruiting bodies), damage to fl owers, shedding of the reproductive plant
parts as a result of insect infestation, and the genotypic ability to produce a second fl ush
in case the fi rst fl ush is lost due to insect damage. At times, the second fl ush may escape
insect damage, and thus give an erroneous estimate of genotypic susceptibility to insect