Contents - Faperta

46 Biotechnological Approaches for Pest Management and Ecological Sustainability
differences in the fl owering times of different genotypes within a crop. Because of fl uctuations
in insect populations over the crop-growing season, it becomes diffi cult to obtain
uniform insect pressure on genotypes fl owering at different times. Genotypes fl owering at
the beginning and end of the cropping season escape insect damage, while those fl owering
in the mid-season are exposed to heavy insect damage. As a result, it becomes diffi cult to
make meaningful comparisons between the resistant and susceptible genotypes. To overcome
this problem, it is important to group the test material according to maturity and
height. It is equally important to include resistant and susceptible checks of appropriate
maturity in different trials for proper comparison (Sharma et al., 1999, 2005a; B. Singh and
Yadav, 1999a, 1999b).
Sequential Plantings
Even with the best of insect forecasting models, it is not possible to pinpoint the periods
of maximum abundance of an insect under fi eld conditions. At the same time, it is not
possible to take up timely planting of the test material because of the variation in the onset
of rainfall in different seasons and locations. As a result, it becomes diffi cult to coincide
the susceptible stage of the crop with maximum insect abundance because of changes
in weather conditions, and the effect of environment on both the insect and the crop.
Therefore, the test material should be planted two to three times at an interval of 15 to
20 days so that one of the plantings is exposed to adequate insect damage. Such an approach
also helps to reduce the chances of escape from insect damage as the late-fl owering genotypes
in the fi rst planting and the early-fl owering genotypes in the second planting are most
likely to fl ower during the mid-season, and are exposed to maximum insect abundance
(Sharma, Singh, and Nwanze, 1997).
Selective Control of Nontarget Insects
Resistance screening trials are quite often rendered useless because of interference by the
nontarget insect pests. Selective insecticides can be used either to control other insects infesting
the crop at the same or at different times or to kill the natural enemies of the target insect
species to allow population build up of the target insect (Sharma, Vidyasagar, and Leuschner,
1988a). It is important to protect the cotton crop from jassid damage during the seedling
stage to evaluate the test material for resistance to bollworms. Sorghum at the seedling stage
(5 to 10 days old) can be sprayed with insecticides to control the shoot fl y to avoid interference
with screening for resistance to spotted stem borer. Similarly, sorghum and pigeonpea
can be sprayed at the milk stage to control the sucking pests to avoid interference in screening
for resistance to sorghum midge and pod borer, respectively. Insecticide application at
times also leads to resurgence of some insect species, for example, brown planthopper,
Nilaparvata lugens (Stal), in rice (Chelliah and Heinrichs, 1980; Heinrichs et al., 1982). Pesticide
application kills the natural enemies and the insects then multiply at a faster rate in the
absence of natural enemies. Selective insecticides can also be used as powerful tool in conserving
and enhancing insect populations (Shepard, Carner, and Turnipseed, 1977).
Augmentation of Insect Populations
Insect infestation in the screening nursery can be increased by collecting indigenous insect
populations from the surrounding areas and releasing them in the test plots. Insect abundance
in the screening nursery can also be augmented by placing nondestructive light,