182 Agroecology and Sustainability
Insects
Studies by Settle and farmer research groups in Indonesia (Settle et al, 1996) show
that flooding of fields triggers a process of decomposition and development of an
aquatic foodweb, which results in large populations of detritus-feeding insects
(especially Chironomid and ephydrid flies). These insects emerge onto the water
surface and into the rice canopy in large numbers, very early in the growing season,
providing critical resources to generalist predator populations long before ‘pest’
populations have developed (Figure 9.1).
This is quite different from the usual predator–prey models taught in most
basic IPM courses and provides a mechanism to suggest that natural levels of pest
Organic matter drives the development of high early-season populations of predators through
parallel pathways: (1) microorganisms (zooplankton and phytoplankton) are fed on by
filter-feeders (mosquitoes and midges), and (2) organic matter directly feeds detritus-feeding
insects (Diptera larvae, Collembola, and some Coleoptera larvae). Each of three pathways
dominates at different times of the season: microorganism/filter-feeders early-season; plant/
herbivore mid-season; and detritivores post-harvest. The pattern of interaction leads to
consistently high populations of generalist predators early in the growing season, and low and
stable populations of herbivores later in the season.
Source: After Settle and Whitten, 2000
Figure 9.1 Hypothesized flow of energy in tropical rice ecosystems