by
Manit Luecha, Director Chainat Seed Center
Rice Department, Thailand

A large variety of insecticides are sold for rice pest control. Farmers rely on salesmen’s recommendations when selecting what pesticide to use.

Insecticides are known to cause secondary developments and resurgences in brown planthopper (BPH) populations. They disrupt the food web structures, disorganize predator-prey relationships and favor rapid population developments of invading BPH resulting in hopperburn (Heong and Schoenly, 1998). Broad spectrum insecticides that have strong effects on natural enemies tend to make rice fields more vulnerable to hopperburn. We surveyed 341 farmers’ insecticide use in Chainat in August 2009 just prior to the massive BPH outbreaks in Central Thailand.

Farmers were growing rice varieties Phatum Thani 1 (61%), Suphan Buri 3 (14%), Chainat 1 (12%) , Suphan Buri 1 (8%), Suphan Buri 60 (3%) and 3 other varieties.  On average they sprayed insecticides 3. 1 times with some as many as 12 times and some none at all. We looked at the types of insecticides that farmers used at the various crop stages (Table 1).  The insecticides Abamectin (37%), Cypermethrin (12%), Chlorpyrifos (13%) were used in all crop stages although they were only recommended for treating leaf folders. These three insecticide accounted for 62% of all farmers using insecticides. In focus group discussions, we found that farmers’ insecticide selections had been based on recommendations from the pesticide salesmen.  Pesticides are called “ya” which mean medicine and they are good for all insects.

Farmers often spray their crops at very early stages using mist blowers. Such practices are detrimental to aquatic and biological control fauna.

Table 1. Percent of farmers using each insecticide at different crops stages

The three most popular insecticides that Chainat farmers had used had poor ecological properties (Table 2). They have high toxicities to fish and aquatic fauna as well as bees.  Toxicities to key natural enemies were not available but their high toxicities to bees would mean that they are likely to be toxic to hymenopterans, including parasitoids.  Spiders tend to be susceptible to similar ecological properties and were probably badly affected as well.

Large areas in Chainat province were damaged by hopperburn

Table 2.  Ecological effects – Toxicities of selected insecticides on some non target organisms

Source: http://extoxnet.orst.edu/pips/

The three insecticides are known to have detrimental effects on biological control elements and cause resurgences. Their frequent usage in Chainat have probably disrupted the biological control services and increased the vulnerability of rice fields to hopperburn.

Reference

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.