Contents - Faperta

42 Biotechnological Approaches for Pest Management and Ecological Sustainability
develop uniform and standardized procedures for assessing insect populations and
damage. The following techniques can be used to evaluate transgenic plants and mapping
populations for resistance to insect pests in different crops.
Techniques to Screen for Resistance to Insects Under Natural Infestation
Rarely is a researcher able to grow a set of genotypes and evaluate insect damage accurately
under natural infestation. Either there are insuffi cient insect numbers to cause
adequate damage or insects occur at an inappropriate phenological stage of crop growth
(Sharma, Singh, and Ortiz, 2001; Sharma, 2005). Field infestations are normally used to
evaluate a large number of genotypes at an early stage of the resistance-breeding program.
However, fi eld evaluations are infl uenced by nontarget insects, which may interfere with
the damage caused by the target insect, or the insect populations are kept under check by
the natural enemies. As a result, it is diffi cult to achieve dependable screening of the test
material under fi eld conditions. Managed or augmented insect density ensures uniform
distribution of insects under fi eld conditions, but the insects may be infl uenced by biotic
and abiotic population regulation factors. Major advantages of screening under natural
infestation are the convenience and low cost. However, seasonality, unpredictability, interference
by nontarget insects, and uneven distribution makes screening under natural
conditions unreliable, time consuming, and less effective.
Knowledge concerning the periods of maximum insect abundance and hot spots is the
fi rst step to initiate work on screening and breeding for resistance to insect pests. Delayed
plantings of the crop and use of infester rows of a susceptible cultivar of the same or of a
different species have been used to increase insect infestation under natural conditions
(Sharma et al., 1992; Smith, Khan, and Pathak, 1994; Sharma, 2005). Sowing time should be
adjusted such that the most susceptible stage of the crop is exposed to maximum insect
infestation. Generally, no insecticide should be applied in the resistance screening nursery,
but plant protection measures may be adopted if necessary to control other insects that
interfere with screening for resistance to the target insect species. Several procedures have
been employed to obtain adequate insect pressure for resistance screening under fi eld
conditions (Table 3.1). The objective of all these approaches is to have an optimum insect
density-to-damage ratio that allows the researcher to observe maximum differences
between the resistant and susceptible genotypes. Screening for resistance to insects under
natural conditions is a long-term process because of the variations in insect populations
across seasons and locations. In addition, there are large differences in the fl owering times
of different genotypes. Genotypes fl owering at the beginning and the end of the season
are exposed to low insect infestation, whereas those fl owering during the mid-season
suffer heavy damage. As a result, it is diffi cult to identify reliable and stable sources of
resistance under natural infestation.
Use of Hot-Spot Locations
Hot spots are the locations where the insects are known to occur regularly in optimum
numbers across seasons. Hot-spot locations can be used for large-scale screening of the
germplasm, segregating breeding material, and multilocational testing of the transgenic
plants. Several insect species are known to occur in high numbers every year at several