K.L. Heong, International Rice Research Institute, Los Baños, Philippines
Planthopper outbreaks are becoming ever so common in many rice areas in Asia and in some cases farmers experience devastating losses. (Read Planthopper outbreaks in 2009). In Thailand many farmers who had lost 3 consecutive rice crops were left in debt from loans they obtained from the bank. The nation cut the 2009/10 production forecast by 16%. It is becoming well known that planthopper outbreaks are induced by insecticides as the prophylactic applications. (Read: Insecticide use make fields vulnerable). The insecticides destroy ecosystem services and make rice crops vulnerable to invading planthoppers increasing to outbreak proportions (Read: Fields sprayed early more vulnerable). All insecticides are biocides and have killing effects to all arthropods. Because of size and mobility of most parasitoids and predators, they tend to be more at risk than the BPH since they often remain hidden at the bases and in between tillers of rice.
In fact the BPH eggs are inserted into the plant issues and only accessible to the egg parasitoids and the egg predator, Cyrtorhinus lividipennis.
There are variations in pesticide hazard to ecosystem services. A procedure to assess risks of some commonly used insecticides based on their properties has been developed. The model is after that developed by Kovach et al (1992) and the environmental impact quotient (EIQ) values are obtained from an updated chemical list found on the New York State IPM Program web site at:
The scoring scheme for 4 main categories of ecological effects of insecticide properties that make fields vulnerable to BPH outbreaks are in Table 1 below. The environmental toxicology data and information were obtained from EXTOXNET.
Table 1: Scoring scheme for risk variables
|Key to risk categories||Ranges||Scores|
|Bee toxicity||> 100 mu gr 2.0 – 100 mu gr
< 2.0 mu gr
|Beneficial toxicity||Low impact Moderate impact
|Systemicity||Non systemic Systemic||1 3|
|Residual action based onLeaf surface half life (days)||0 – 14 15 – 28
Bee toxicity is of particular importance as an insecticide that has high bee toxicity will likely to have high toxicity to hymenopteran parasitoids. Effects on other beneficial arthropods, like spiders, crickets, beetles and generalists are incorporated in the beneficial toxicity. The longer residual effect the insecticide has would mean it will pose more risks to natural enemies since predators and parasitoids move around in search of prey.
The assessment showed that fipronil posed the highest risk to hopper outbreaks followed by diazinon, cyfluthrin, imidacloprid and abamectin. Sprays of soap and pymetrozine posed the least risk, while cypermethrin deltamethrin and chlorpyrifos posed immediate risks. While these values provide indications of the potential risks they have, the actual risk of a field to have a BPH outbreak will depend on the crop stage, amount and frequency of sprays applied to the field.
Kovach, J., C. Petzoldt, J. Degni and J. Tette. 1992. A method to measure the environmental impact of pesticides. New York’s Food and Life Sciences Bulletin. NYS Agricul. Exp. Sta. Cornell University, Geneva, NY.