Contents - Faperta

Pest Management and the Environment 5
would help realize the potential of biocontrol agents in pest management. A few studies
have focused on insect population dynamics and key mortality factors under fi eld conditions
(Zalucki et al., 1986; Fitt, 1989). Early-stage mortality is invariably the most severe,
although its causes and extent vary greatly, and comparable data sets are too few to
identify the factors responsible for population regulation across regions. Varieties with
moderate levels of resistance that allow the pest densities to remain below ETLs are best
suited for use in IPM in combination with natural enemies. Restless behavior and prolonged
developmental period of the immature stages increases the susceptibility of the target pest
to the natural enemies. However, plant morphological characteristics and secondary plant
substances sometimes reduce the effectiveness of natural enemies for pest management.
The use of insect-pest-resistant varieties and biological control brings together unrelated
mortality factors, which reduce the pest population’s genetic response to selection pressure
from plant resistance and natural enemies. Acting in concert, they provide a density-
independent mortality at times of low pest density, and density-dependent mortality at
times of high pest density (Bergman and Tingey, 1979).
Parasitoids
A diverse range of parasitoids lay their eggs on or in the body of an insect host, which is
then used as a food by the developing larvae. The most important parasitoid groups are
trichogrammatids, ichneumonids, braconids, chalcids, and tachinids. The trichogrammatids
parasitize the eggs of several insect species, and have been used extensively in biological
control. The ichneumonids and braconids prey mainly on the larvae of butterfl ies and
moths. The chalcid wasps parasitize the eggs and larvae of insects. The most important
and widely used parasitoids for biological control of insects are the egg parasitoids such as
Trichogramma, Chelonus, and Telenomus. The larval parasitoids such as Cotesia, Encarsia,
Gonatocerus, Campoletis, Bracon, Enicospilus, Palexorista, Carcelia, Sturmiopsis, etc., have also
been used in several countries for biological control of insects.
Cropping systems can be altered successfully to augment and enhance the effectiveness
of natural enemies (Andow and Risch, 1985; Altieri, Wilson, and Schmidt, 1985). Optimal
microclimatic conditions, nectar sources, and alternate hosts may exist in some cropping
systems, but not others. Physicochemical characteristics of the host plants also play an
important role in host specifi city of both the insect hosts and their parasitoids (Sharma,
Pampapathy, and Sullivan, 2003). Host-plant-mediated differences in the activity and
abundance of natural enemies have been recorded in the case of Helicoverpa armigera
(Hubner) (Pawar, Bhatnagar, and Jadhav, 1986; Zalucki et al., 1986; Manjunath et al., 1989).
The average rates of parasitism of the eggs of H. armigera (mainly by Trichogramma spp.)
have been found to be 33% on sorghum, 15% on groundnut, and 0.3% on pigeonpea, and
little or no parasitism was observed on chickpea (Pawar, Bhatnagar, and Jadhav, 1986).
Manjunath et al. (1989) observed up to 98% parasitism of H. armigera eggs by Trichogramma
chilonis Ishii on tomato, potato, and lucerne, but no egg parasitism was recorded on chickpea,
probably because of the acid exudates secreted by the leaves. Therefore, due consideration
should be given to the host plant and the species of the parasitoid involved while
planning for biological control of insect pests.
Predators
In general, predators have received much less attention than parasites as natural control
agents. They exercise greater control on pest populations in a diverse array of crops