Ecological Foundations to Tropical Rice IPM

Key process #2: Aquatic plankton provide food for filter-feeding insects.

The small to intermediate-sized zooplankton and the phytoplankton found in every flooded rice field provides food for filter feeders (midges and mosquitoes).  Densities of chironomid midge larvae have been observed in irrigated rice fields in the thousands/m2 range.  Again, abundant midges and mosquito populations are a general characteristic of rice systems.

Key process #3: larvae and adult plankton-feeders and detritus-feeders provide a consistent and abundant source of alternative food for generalist predators from early in the season, until after harvest.

Alternative prey for predators, in the form of filter-feeders and detritivores, show a consistent peak at about 30 days after transplanting (e.g., Fig. 3a).  The existence of abundant alternative prey effectively "decouples" predator populations from a dependence on the pest populations, giving predators the opportunity to develop well in advance of the normal pest populations.  As a result, mortality of pest populations due to predation is high —beginning with early-season pest migrants and carrying forward throughout the season.  This process minimises the likelihood that pest populations can "escape" control by natural enemies and reach outbreak levels. Furthermore, high populations of detritivores found in harvested stubble fields assure a continued food source for generalist predators some time after harvest (Settle, unpublished). 

Figure 3.  Trophic-level population dynamics in a field in West Java, 1992. 

(A.) Untreated - No insecticides: note the rise of a “neutral” peak at 30 DAT, followed by the subsequent rise of predator populations. As a result of the early-season dynamics in which predator populations build on neutrals, pest populations have little chance to develop later in the season. 

(B.)  Typical early-season applications of insecticides by farmers lead to a suppression of early-season predator populations and subsequent “resurgence” of pest populations (mostly BPH).  This is a classic example of “pesticide-induced pest resurgence”, a phenomena that is responsible for the problems with BPH over the past 30 years.

This hypothesis was tested experimentally by manipulating organic matter in test plots compared with low organic matter control plots, and were able to show significant positive responses by both the detritivore, filter-feeder, and predator populations (Fig. 4).  A somewhat different experimental approach conducted in China reached the same conclusion (Wu et al. 1994) .  Wu et al. eliminated midge and mosquito larvae by using  highly selective Bacillus sphaericus insecticide.  In response, predator populations in early-season irrigated rice were diminished for lack of food, and pest populations were subsequently higher compared with controls. 

Figure 4.  Soil organic matter (OM) experiment.  In order to test the hypothesis of whether energy flows from OM to predators in tropical rice systems (Fig. 1&2), plots were set up with low and high OM, and examined during the first 40 days.  High OM resulted in significantly more neutrals and predators (a) below the water and, (b) on the surface of the water and in the plant.

Over all, results of six Indonesian surveys show that some 19% of the arthropod species are detritivores or plankton feeders (Fig. 5).  Except for pioneering work done in China (e.g., Wu et al. 1994) previous recognition of these functional groups has been largely ignored or misunderstood.  For example, chironomids were all considered to be root feeders (phytophagous), and therefore thought to be harmful, but this is not generally true (see Settle et al. 1996 for details) .  Recent studies in southern China using ELISA techniques have shown that chironomids represent up to 80% of the diet of spiders early in the season (Zhang Wenqing, Institute of Entomology and State Key Laboratory for Biological  Control, Zhongshan University, Guangzhou, pers. comm. to W.H. Settle).

Figure 5.  Trophic-level distributions from vacuum samples over six sites in West and Central Java:  A. Number of species (fraction of total in parenthesis), and B. Fraction of total abundance

Implications for Rice IPM Practice.  The existence of diverse populations of natural enemies, supported by abundant alternative food species, assures that populations of pests are consistently maintained at low levels. In effect, the structured biodiversity of arthropods in tropical irrigated rice functions to consistently suppress pest populations by denying pests refuge in time or space.  All the key variables can be found in any rice ecosystem—only when the process is disrupted do pest populations explode, causing serious damage. 

Natural enemy populations can be suppressed by insecticides.  This is by far the most common cause of pest outbreaks, especially for pests such as the rice brown planthopper, and is generally referred to as “pesticide-induced resurgence” (Fig. 3b).  Eggs of many pests, such as BPH, are not susceptible to chemical sprays.  By reducing the abundance of natural enemies, chemical sprays create a refuge for the development of pests, whose migratory abilities are generally better and generation times faster than for natural enemies.