C. (Kees) J. Stigter, Agromet Vision, Bondowoso/Depok, Indonesia
Planthopper fluctuation in Indonesia between 1991 and 2010 that Mr. Budyanto presented (Graph 1) showed that the peaks of the two most serious outbreaks were related with the occurrence of La Ni?a. The graph shows the peaks of BPH outbreaks in 1998, 2005, and 2010 with its highest damages of up to 140,000 ha. Damage was close to 120,000 ha in 1998.
From the Climate Charts posted by Kelly O’Day on January 25, 2011 (Graph 2), the 2010—2011 cycle seems to be following a similar path to four previous cycles shown in the chart: 1970—1971; 1973—1974; 1983—1984; and 1998—1999. The current La Ni?a is close to the La Ni?a lows in 1998—99 and 1973—74. The year of 1998—99 mentioned by Budiyanto also had high BPH outbreaks similar to those in 2010—2011.
Climate parameters such as temperature, rainfall and relative humidity routinely observed have been used for analyses to relate climatic factors with BPH in rice fields. However BPH inhabit at the base of rice plants and thus live in a microenvironment of the crop canopy. Rainfall may affect ambient relative humidity but the close distances between hills are important for survival and growth of the nymphs.
Crops planted using SRI (System of Rice Intensification) tend to have wider between hill distances and may favor BPH development less. Similarly the temperatures experienced by hoppers in their habitat may also be affected. Higher temperatures promoted by dense canopies may shorten BPH generation times. “Smart” microclimate manipulations can thus help to reduce population growth.
The existing literature shows that rainfall, temperature and relative humidity are positively related to BPH outbreaks when they are high. Only statistical relationships between climate and BPH outbreaks are available, whereas the cause and effect connections of these relationships are little known. However in different seasons (rainy and dry seasons), our analyses reveal that the continuous rain periods throughout the 2009/2010 rainy season and the 2010 dry season in Java had greater impact on BPH outbreaks across diverse regencies, but also in adjacent fields in the same regency.
In the low flat areas with abundant water, the heavy continuous rainfall of the La Ni?a of 2010/2011 seems to lead to high relative humidities in densely planted fields, making them conducive for BPH. In rainfed areas however, the conditions were different as water shortage would reduce the relative humidity. In the La Ni?a of 2010/2011, farmers in rainfed areas would thus have higher benefits from rice planting in the dry season. Organizing water conditions in the fields “smartly” can therefore play a significant role in BPH management.
Climate is not the only factor
Climate elements are not the sole factors affecting the microenvironment of rice fields that favor outbreaks of a pest like BPH. Water management by farmers and the state is one example of human activities which play a role in increasing or reducing the relative humidity under the condition of water availability in particular places. Using SRI is another example.
In addition other crop management practices also play vital roles. Among the “not so smart” practices are :
1) using highly susceptible varieties and plant rice continuously;
2) using hybrid varieties with narrow gene base and high susceptibility to pests and diseases;
3) asynchronous cropping which provides the constant availability of food for the pest;
4) misuse and overuse of fertilizers and insecticides that can lead to more serious outbreaks by increasing BPH fitness and reducing biological control ecosystem services.
In summary, the rice ecosystem sustainability is under jeopardy. The increasing vulnerability to BPH outbreaks is a crucial factor. Examining the origin of the recent outbreaks and the contributing factors in different places in Java can help us better understand the mechanisms. Since these outbreaks happened 25 years after the 1985-1986 severe infestation, it is interesting to know the similarities and differences between the two BPH resurgences. The same applies to the 1998/1999 observed BPH outbreaks. In particular the fourth factor above, i.e. pesticide and fertilizer misuse and overuse very often may be a triggering cause of the BPH epidemics.