Contents - Faperta

104 Biotechnological Approaches for Pest Management and Ecological Sustainability
changes in the micro-environment of the target insect. Consequently, insect survival may
be lower in polycultures than in the monocultures (Baliddawa, 1985). The population density
of melon worm, Diaphania hyalinata (L.) is lower in polyculture (maize-cowpea-squash)
than in monoculture (squash alone) (Letourneau, 1986). The total crop yield in polycultures
is greater when estimated as a land equivalent ratio. However, the role of polycultures or
plant diversity should be carefully assessed for its effects on insect damage, and the activity
and abundance of natural enemies. Also, the effects of plant diversity on pest damage
can change over time and locations, depending on the herbivore diversity and interactions
among the harmful and benefi cial insects.
Multilines/Synthetics
The feeding preference of polyphagous insects can be altered by including genetically
different genotypes with similar maturity and height. Biological control of the cereal leaf
beetle, Oulema melanopus (L.) has been achieved with mixed cropping of beetle-resistant
and beetle-susceptible wheat varieties on an area-wide basis (Casagrande and Haynes,
1976). Simulated growth of aphid predators on the susceptible plants in variety mixtures
also slows down the rate of development of virulent aphid biotypes (Wilhoit, 1991). The combined
effect of varietal mixtures and natural enemies is quite effective in suppressing
populations of insect pests.
Trap Crops
Trap crops attract insect pests or other organisms so that pest incidence on the target crop
is minimized. Reduction in pest damage is achieved either by preventing the insect pests
from infesting the target crop or by concentrating them in a certain part of the fi eld where
they can be easily destroyed (Hokkanen, 1991). The principle is similar to associational
resistance, in which the insect pests show a distinct preference for certain plant species,
cultivars, or a crop stage. Crop stands can be manipulated in time and space so that attractive
host plants are offered to the insect pests at a critical stage of insect development. The
insects concentrate at the desired site on the trap crop, and as a result, the main crop
seldom needs to be treated with insecticides and thus the natural control of insect pests
remains operational in most of the fi eld. Trap cropping is particularly important in subsistence
farming in the developing countries, and its application in cotton and soybean has
been very successful (Newsom et al., 1980). In cotton/sesame intercrop, row strips of sesame
(constituting 5% of the total area) can be used as a trap crop to attract Heliothis/Helicoverpa
species from the main crop of cotton. Sesame, which is highly attractive to Heliothis species
(from the seedling stage to senescence), attracts large numbers of insects away from the
cotton (Sharma, 2005). It also attracts the parasitoid, C. sonorensis, which parasitizes large
numbers of H. virescens larvae (Pair, Laster, and Martin, 1982).
Nutrient Application and Plant Resistance
Crop susceptibility to different insect pests changes with the amount and type of fertilizers
applied. Therefore, care should be taken to apply appropriate combinations of nutrients to
minimize pest damage and realize maximum crop yield in combination with insect-
resistant cultivars. Expression of resistance to insects changes with the availability of
nutrients. Sorghum shoot fl y incidence decreases with an increase in application of nitrogenous
fertilizers (Reddy and Narashima Rao, 1975; Sharma, 1985).