Contents - Faperta

64 Biotechnological Approaches for Pest Management and Ecological Sustainability
yield reduction indicate direct insect feeding injury to plants. Plant damage can also be
determined by measuring the number of plants with insect damage, incidence of tissue
necrosis, fruit abscission, and stem damage. The quality of produce can also be used to
measure the effect of insect damage. Leaf defoliation or damage to fruiting bodies is
usually determined by rating scales that make use of visual estimates of plant damage
based on percentages or numerical ratings as described above. Several such rating scales
have been developed to assess insect damage in crop plants (Sharma et al., 1992; Smith,
Khan, and Pathak, 1994). Direct measurements of leaf area are also used to measure insect
damage. Indirect feeding injury measurements, such as plant growth, photosynthetic
rates, transpiration rates, ethylene production, and respiratory rates can also be used to
assess genotypic resistance to insects. In some cases, the test genotypes can be fi rst evaluated
visually for insect damage on a scale of 1 to 9, and then percentage damage to the
fruiting bodies or seeds can be recorded by counting the total number of fruiting bodies
with and without insect damage in fi ve plants selected at random in the center of each plot.
Additional information on susceptibility to other insects and diseases should also be
recorded at the appropriate stage of crop growth, as and when feasible.
Simulated Feeding Injury
Insect feeding injury can be simulated by mechanical defoliation. This is particularly
useful in the case of leaf-feeding insects such as grasshoppers, beetles, and leaf-feeding
caterpillars. However, plants respond differently to artifi cial defoliation than to actual
insect feeding. Therefore, the relationship between artifi cial defoliation and natural insect
feeding should be determined before results on artifi cial defoliation are accepted as a
measure of insect damage-yield-loss relationship. Insect injury can also be measured by
injection of toxic insect secretions into plant tissues, for example, the application of crude
extract of greenbug in sorghum.
Association of Physico-Chemical Characteristics and Molecular Markers
with Insect Resistance
Plant resistance can also be assessed by measuring the concentrations of allelochemicals
or the density, size, and distribution of morphological structures, and by using molecular
markers associated with resistance to insects (Smith, Khan, and Pathak, 1994; Sharma and
Nwanze, 1997). This permits a rapid identifi cation of plants potentially resistant to the
target insect species. Use of biochemical (Table 3.6) and morphological markers (Table 3.7)
to measure host plant resistance to insects also overcomes the variation associated with
insect infestation, and the effect of environmental factors on the expression of resistance to
insects. Molecular markers associated with insect resistance can also be used to monitor
the presence of QTLs associated with resistance to insects in the test material using association
mapping (Dhillon et al., 2006). Molecular markers can be used to track genes from
both conventional sources and transgenic plants for gene pyramiding.
Sampling Insect Populations
Plant resistance to insects can be measured in terms of insect numbers on the test genotypes.
Insect numbers can be estimated by sampling at the plant site where damage has taken
place, and at the appropriate phenological stage and time. The population of small-sized
immobile insects can be measured visually, but this method is subjected to variations in