Ecological Basis for Low-Toxicity Integrated Pest Management 183control in tropical irrigated rice ecosystems are far more stable and robust than
purely terrestrial agroecosystems. This stability, however, was found to be lower in
rice landscapes that are subject to long (more than three month) dry seasons and
where rice is planted in large-scale synchronous monocultures, as well as in areas
where farmers use pesticides intensively. Increased amounts of organic matter in
the soil of irrigated rice fields, by itself a highly valuable practice for sustainable
nutrient management, has the additional advantage of boosting both populations
of detritus-feeding insects and insect predators, and thereby improving natural
levels of pest control (Settle et al, 1996).
A second consideration for rice IPM is the ability of most rice varieties to com-
pensate for damage. The rice plant rapidly develops new leaves and tillers early in
the season replacing damaged leaves quickly. The number of tillers produced is
always greater than the number of reproductive tillers allowing for some damage
of vegetative tillers without affecting reproductive tiller number. The flag leaf con-
tributes to grain filling but the second leaf provides photosynthates as well, while
lower leaves are actually a sink that compete with the panicle. Finally, photosyn-
thates appear to move from damaged reproductive tillers to neighbouring tillers so
that total hill yield is not as severely impacted as expected when a panicle is dam-
aged by stemborers.
Thus, early season defoliators (such as whorl maggot, case worms and army-
worms) cause no yield loss up to approximately 50 per cent defoliation during the
first weeks after transplanting (Shepard et al, 1990; Way and Heong, 1994)
although higher damage occurs when water control is difficult. As early tillering is
also higher than what the plant can ultimately support reproductively, up to 25 per
cent vegetative tiller damage by stemborers (‘deadhearts’) (caused by Scirpophaga
spp., Chilo spp. and Sesamia spp.) can be tolerated without significant yield loss
(Rubia et al, 1996). Significant damage (above 50 per cent) to the flag leaf by leaf-
folders (Cnaphalocrocis mdeinalis and Marasmia spp.) during panicle development
and grain filling can cause significant yield loss, although this level of damage is
uncommon where natural enemies have been conserved (Graf et al, 1992). Late
season stemborer damage (white heads) also causes less damage than previously
expected such that up to 5 per cent white heads in most varieties does not cause
significant yield loss (Way and Heong, 1994; Rubia et al, 1996).
The conspicuous rice bug (Leptocorisa oratorius) is another major target for
insecticide applications. However, in a recent study involving farmers and field
trainers at 167 locations, van den Berg and Soehardi (2000) have demonstrated
that the actual yield loss in the field is much lower than previously assumed. The
rice panicle normally leaves part of its grain unfilled as if to anticipate some level
of loss (Morrill, 1997). Numerous parasitoids, predators and pathogens present in
most rice ecosystems tend to keep these potential pests at low densities (Shepard
and Ooi, 1991; Barrion and Litsinger, 1994; Loevinsohn, 1994; Ooi and Shepard,
1994; Matteson, 2000).
Thus, under most situations where natural enemies are conserved, little yield
loss is expected from typical levels of insect pests. Up until recently, insecticide