Three planks in ecological engineering for rice pest management

The three planks for ecological engineering to enhance natural biodiversity and their services.

The rice ecosystem, especially in the tropics, is a usually richly endowed with a great diversity of generalist predators and parasitoids, which tend to be more species specific. As the rice habitat is an ephemeral habitat, most pest species that can cause significant damages and yield losses are generally immigrants. 

Many of these pest species are also specific rice feeders, monophagous or oligophagous (limited host range). Such specialization may be constrained by local host availability.

But the more successful pest species overcome this by migrating, remaining in the rice habitat for only two or three generations, possessing high genetic diversity to overcome changes in the host plants through adaptation and evolution and reproductive capacities.  

The rice planthoppers are such pests. Under normal undisturbed situations, the high reproductive capacities are kept in check by the huge biodiversity in the natural enemy community. Smart and sustainable pest management strategies would thus focus on maximizing these natural enemy services.

Herbivores should thus not be treated as pests unless “proven guilty” that they are indeed causing economic losses (Way and Heong 1994). Prophylactic spraying is thus not smart and not sustainable and should be completely avoided as they do more harm.

In the new book “Biodiversity and Insect Pests”, the chapter we wrote presents three planks for ecological engineering that can focus on reducing the ecological fitness of many rice pests.

Plank 1:  Moderated insecticide use – especially early in the season

Early season insecticide applications in the first 40 days after sowing are known to have little or no effects on yields. They are generally applied as a prophylactic or to control leaf folders and pyrethroids and insecticides that have high toxicities to hymenopterans are typically used. 

Such sprays do more harm than good, by destroying natural enemies and the aquatic detritivore prey fauna. It has been shown that early sprays significantly decrease food chain lengths and disorganize predator-prey relationships (Heong and Schoenly 1998).

The campaigns in Vietnam to motivate farmers to reduce early spraying contributed significantly to the successes in managing planthopper outbreaks in the Mekong Delta

Plank 2:  Enhancement of generalist natural enemies – predator build up on detritivore prey.

Chinese scientists call the abundance of arthropod species that live in the paddy water “neutral insects”. Settle et al (1996) first suggested the idea that these insects are in fact not neutrals, as they can be alternative prey for predators and many of them predators themselves. 

Thus at the early crop stages, these neutrals are vitally important and need to be conserved.  Insecticide spays at this period of the crop however are detrimental to the aquatic neutrals.

In addition most sprayers farmers use do not have adequate spray droplet control and thus most of the active ingredients converge into the paddy water.  In IRRI farm when insecticide use decreased by 95%, the aquatic detritivore specuies diversity increased 5 folds (Heong et al 2007).

Trophic relationships in a rice ecosystem showing the importance of detritivores and non crop vegetation components.

Plank 3: Enhancement of specialist natural enemies by habitat manipulation – nectar plants on bunds

The third plank targets at providing resources, shelter and food, for natural enemies, especially the specialists. This may be achieved by populating bund and non rice habitats with nectar rich flowering plants.  In China when nectar-rich sesame plants are grown on bunds, parasitoid abundance increased.

Parasitoid searching efficiency was enhanced by sesame flowers. In Vietnam flowers on the bunds increased egg parasitism of planthopper eggs. In Thailand parasitoid species richness increased when fields were surrounded by flowering plants. The use of herbicides on the bunds to ensure clean cultivation practices may be counter productive.  In Hainan, fields with clean bunds had lower spider biodiversity.

Pic-EE-in-Vietnam

Ecological engineering - Flowers on the bunds in Vietnam (Photo credit Dr H.V. Chien)

Most parasitoids are hymenopterans and thus insecticide toxic to bees and hymenoptera in general will need to be avoided.  These will include the neonicotinoids and pyrethroids.  Scientists have recently established direct links between neonicotinoids, bees and pollination services. Similar effects on biological control services provided by the parasitoids are to be expected.

The three planks elucidated in the chapter are based on an amalgam of sound ecology, successes in other crop systems and most importantly empirical evidences available from research.  

Reducing the unnecessary insecticide uses, promoting natural biological control ecosystem services through enhancing organic matter and enriching bunds and other surrounding areas with nectar rich plants can significantly prevent planthopper outbreaks.  

Such practices that focus on biodiversity conservation, enhancing ecosystem services and avoiding pollution will build resilience in production systems.

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