By
K. L. Heong, International Rice Research Institute, Los Baños, Philippines
H. V. Chien, Southern Regional Plant Protection Center, Long Dinh, Vietnam
T. T. M. Phoung, Sub Plant Protection Department, Long An, Vietnam
M. M. Escalada, Visayas State University, Philippines
Leaf damages by leaf folders in the first 40 days after sowing that trigger farmers to start their spray routines. Photo credit: KL Heong
In our recent visit to Tan Tru district in Long An province, Vietnam, we conducted a focus group discussion with 50 farmers from a village which had severe hopperburn and virus infestations last season. Farmers described how they started spraying in the early crop stages for leaf folders (sau cuon la) and continued spraying multiple times for planthoppers. Several farmers even sprayed the neighboring habitats thinking that these were planthopper refuges. The most common insecticide used was “Dragon” which contains chlorpyrifos ethyl and cypermethrin.
Fig. 1. Farmers’ insecticide use in Long An province from 1994 to 2008
In 1994 the “no early spray” campaign to reduce farmers’ sprays for leaf folders in the first 40 days after sowing launched in Long An province, reduced insecticide use by 53% (Escalada et al 1999). However none of the farmers we met could recall the campaign or knew that leaf folders in the early crop stages need not be sprayed. This is probably to be expected as the campaign was conducted 14 years ago and over this period, farmers insecticide sprays had remained low for about 8 years and lately has risen substantially (Figure 1). Average yields in the province has remained quite stable. Rice plants have high compensatory abilities and can easily recover from leaf damages at early crop stages. The early season insecticides instead do more harm to the ecosystem services and increase the vulnerability of the crop to invading planthoppers. In this post, we would like to explain the mechanisms involved.
Fig. 2. Food chain lengths – Systems with chain length 3 have strong ecosystem services. Outbreaks occur when chain length approaches 2.
Farmers often respond to visual cues, like leaf folder damages (see picture 1), by spraying insecticides or in some cases follow a prophylactic routine. Common insecticides used include pyrethroids, organophsopates and organochlorines with high toxicity. Such compounds not only kill leaf folders, but predators, parasitoids and aquatic detritivores. In fact the aquatic fauna is most severely affected because all the spray drips are collected in the water. The sprayers farmers commonly use have poor spray nozzles that deliver large spray droplets that will rapidly slip off the leaves into the water. Detritivores are important components of the ecosystem as they provide food for predators (detritivore shunt). When insecticide pressure in IRRI farm was reduced by 95%, species biodiversity of detritivores increased more than 5 folds (Heong et al 2007). These sprays, usually applied in the first 40 days after sowing, also have detrimental effects on predators and parasitoids, thus markedly reducing their biodiversity and abundance and the ecosystem services they provide. The two most important regulatory services affected are pest invasion resistance and pest regulation. Both predator and parasitoid biodiversity doubled in IRRI farm when insecticide pressure was reduced (Heong et al 2007). Besides interfering with the functional biodiversity, these early season sprays reduce the food web structure and disorganize the food chain lengths reducing this from 3, a healthy condition, to 2, an outbreak condition (Heong and Schoenly 1998).
Fig. 3. Ecosystem services are in place at the beginning and remain strong throughout the crop.
Fig. 4. Early sprays then to destroy ecosystem services making the crop vulnerable to invading pests.
Fig. 5. Pests that invade crops with reduced ecosystem services tend to multiply exponentially to outbreak proportions.
Thus rice ecosystems that have received spray disruptions in the early crop stages are placed in a vulnerable condition, lacking abilities to resist invading planthoppers or regulate their population. Should planthoppers invade these crops, there is a high tendency for the planthoppers to develop exponentially leading to an outbreak (illustrated in Figures 3 to 5). Planthopper invasions are random and we often see hopper damages in pockets and not uniformly distributed. In most cases, fields sprayed early may be vulnerable but “escape” hopper damages because hopper invasions may be low of none at all. Spraying for leaf folders in the early crop stages with highly toxic pesticides put the crop at risk to invading hoppers and thus do more harm than good.
Similarly such early season sprays can also cause heavy late season leaf folder attacks. Again leaf folders are also invading species, like planthoppers and will do very well in crops where ecosystem services are compromised (i.e. when food chain length is closer to 2). Farmers who continue to adopt the “no early spray” heuristic will be much better off and there is need to conduct motivational campaigns more frequently to continuously remind farmers about the bad effects of early season sprays.
References
Escalada, M.M., Heong, K.L., Huan, N.H. and Mai, V. 1999. Communication and behavior change in rice farmers’ pest management: The case of using mass media in Vietnam. Journal of Applied Communications, 83, 7 -26.
Heong, K.L. and Schoenly, K.G. 1998. Impact of insecticides on herbivore-natural enemy communities in tropical rice ecosystems. Pp 381-403 ( P. T. Haskell and P. McEwen Eds.) Ecotoxicology: Pesticides and Beneficial Organisms. Chapman and Hall, London.
Heong, K. L., Manza, A., Catindig, J., Villareal, S. and Jacobsen, T. 2007. Changes in pesticide use and arthropod biodiversity in the IRRI research farm. Outlooks in Pest Management 18, 229 – 233.
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