by K.L. Heong
Insect Ecologist
International Rice Research Institute, Los Baños, Philippines
Hopperburn caused by planthopper outbreak in Malaysia. Photo credit: Mohamad Norowi Hamid.
Ecologists have been fascinated with the diversity of life strategies of organisms. MacArthur and Wilson (1967) first used the terms, r K selection to discuss different species adaptations under short and long term environments. Pianka ((1970) pointed out that r and K do not represent a true dichotomy but merely the endpoints of a spectrum and used the term r – K continuum. Along this continuum, insects and annual crops are more r selected while vertebrates and perennials are more K selected. Southwood and Comins (1976) used a synoptic model relating strategies and habitat stability showed that r strategists can develop into outbreak proportions when they escape natural enemy control.
Synoptic model of Southwood and Comins (1976) illustrating the importance of the “natural enemy ravine” in r pests.
I find the r –K continuum a simple way to illustrate why some rice insect pests are hard to manage using control measures that focus at the farm scale. Many rice pests, such as the planthoppers and leaf folders are typically r strategists with short life cycles, rapid reproduction capacities, highly mobile and migrate long distances. These “opportunistic” species in rice are also monophagous and move from rice fields to rice fields, often after one of two generations in each patch. They also tend to possess high capacities to overcome local stresses in new environments they migrate to, like host plant resistance, elevated temperature conditions and places with high insecticide use. Thus we see the rapid development of populations that have high insecticide resistance, high tolerances to elevated temperatures and frequent breakdown of resistance rice varieties.
To explore pest strategies and management options, we can also group rice insect pests into “endogenous”, those that tend to develop within farms and “exogenous” those that tend to move into the farm from outside (see figure). The range of management options may be examined across these two groups. The exogenous pests tend to be extremely r, have higher capacities in reproduction, mobility and adaptation. Farm scale management options, such as chemicals, perhaps more useful for endogenous pests, might be less effective against exogenous pests. Landscape scale approaches, such as ecological engineering (Gurr et al 2004) to strengthen ecosystem services at the regional level, might provide more sustainable controls.
Framework to consider type of pests, scale they occur and strategic options.
While farm scale management techniques may be implemented by individual farmers, landscape level management options require multi stakeholder community participation. Planthopper outbreaks are due to breakdown of ecosystem services and thus management techniques implemented by individual farms may not suffice. For sustainable management of such pests we need to explore the use of frameworks used in managing environmental problems, such as DPSIR (Driving force, Pressure, State, Impact and Response). Details in http://maps.grida.no/go/graphic/the_dpsir_framework.
References
Gurr, G.M., Wratten, S.D. and Altieri, M.A. 2004. Ecological Engineering for Pest Management. CSIRO Publishing, Collingwood, Australia.
MacArthur, R.H. and Wilson, E.O. 1967. The theory of island biogeography. Princeton Univ. Press, Princeton, NJ.
Pianka, E.R. 1970. On r and K selection. American Naturalist, 104, 592 – 597.
Southwood, T.R.E. and Comins, H. 1976. A synoptic population model. J.Anim Ecol, 45, 949-965.
Based on the synoptic model, I can say that BPH is the K-selected species because they are living in the tropical area and suffering a high tress of natural enemies. However, a difference is that BPH is also proliferous in reproduction because its is monopgagous and its only host plant, rice, is cultivating all around and intensively in the area. Therefore, the r characteristics of BPH can overcome the natural enemies’ stress and it becomes a superb insect pest that we need to develop a deliberate strategy to overcome its damage.
Dr Huynh
Comparative analysis done by Japanese entomologists, like Kiritani clearly show that planthoppers a re extreme r strategists, relative to say insect pests, like the GLH. The synoptic model suggests that r pests are managed by natural enemies most of the time and when they escape from this, they become outbreak pests. Any strategy to manage such pests has to be directed at strengthening biological control. Since rice is ephemeral thus the biological control services may be enhanced through enriching floral biodiversity, which is one approach in ecological engineering
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