Risk assessment of insecticides used in rice on miridbug, Cyrtorhinus lividipennis Reuter, the important predator of brown planthopper, Nilaparvata lugens (Stal.)
Introduction
Green revolution initiated in the mid 1960s and characterized by the successful breeding and widespread adoption of new high yielding varieties, pesticides and nitrogen fertilizers, has doubled the production of many crops, such as rice, wheat and maize. Meanwhile, the inputs of pesticides and fertilizers have resulted in some negative effects, ‘unwelcome harvest’, on environments and resources, as well as the considerable disturbances to plant and animal communities (Conway and Pretty, 1991, Conway, 1997). Crop losses caused by insect pests gradually increased in spite of the effective technological development in insecticide synthesis and application for pest management (Scriber, 1984).
Brown planthopper (BPH), Nilaparvata lugens (Stal.) is one of the most economically important insect pests attacking rice crop (Krishnaiah et al., 2006). The insect damages the plant through the removal of plant sap and as a vector of rice viruses. As a result “hopper burn” and various virus diseases grassy stunt, ragged stunt and wilted stunt occur, respectively in rice field (Hibino, 1979, Chen and Chiu, 1981). The most commonly used method of controlling brown planthopper is the application of insecticides. Many insecticides have been identified for control of rice planthoppers under green house and field conditions (Krishnaiah and Kalode, 1993, Sarupa et al., 1998, Krishnaiah et al., 2002).
Chemical control remains a major strategy in the integrated pest management (IPM) system as it is quick, efficient, easy to use and cost-effective against the insect (Zhao, 2000, Endo and Tsurumachi, 2001). However, lethal and sublethal effects of broad-spectrum and non-selective pesticides are a high risk to beneficial species (Croft, 1990, Ruberson et al., 1998). Misuse of chemical insecticides can cause outbreaks of the pest because extensive and intensive use of insecticides and development of resistance (Kilin et al., 1981, Hirai, 1993) as well as indiscriminately killing a wide range of natural enemies (Way and Heong, 1994, Tanaka et al., 2000). Dyck and Orlido (1977) reported that reduction in the population of mirid predator, Cyrtorhinus lividipennis Reuter after regular spraying with methyl parathion causes BPH resurgence.
In rice ecosystem, the action of predators is more conspicuous and perceptible than parasitoids of plant and leafhoppers. Among the several predators reported on hoppers, the green miridbug, C. lividipennis is widely distributed in rice fields and is a promising biocontrol agent against both leaf and planthoppers. They search the host randomly (Heong et al., 1990) and the rice volatiles also play an important role in the foraging behaviour of C. lividipennis (Lou and Cheng, 2003). Cyrtorhinus lividipennis feeding on both eggs and nymphs of hoppers (Katti et al., 2007) is the dominant predator in irrigated rice (Sigsgaard, 2007). A predator nymph consumes an average of 7.5 eggs or 1.4 hoppers per day for a period of 14 days. Adults consume about 10.2 eggs or 4.7 nymphs or 2.4 adults per day for a period of 10 days (Reyes and Gabriel, 1975). Thus a single bug can consume 66 BPH nymphs in its lifetime of 24 days.
Chemical and biological control are the two important strategies used in an IPM program (Zhao, 2000). Integration of chemical and biological control systems is a key for the success of any IPM program (Wright and Verkert, 1995) especially in rice fields which have a number of biocontrol agents. Chemical control should be used only when it is necessary and is least disruptive to biological control. Knowledge of compatibility and impact of pesticides (lethal and sublethal) on beneficial species is essential for active integration of chemical and biological control (Greathead, 1995).
Lethal or adverse effects of insecticides on beneficial arthropods are often expressed as acute or chronic mortality resulting from contact with or ingestion of insecticides (Haseeb et al., 2004). Desneux et al. (2007) pointed out that the determination of acute toxicity of pesticides to beneficial arthropods had traditionally and largely relied on the measurement of an acute median lethal dose or concentration and the estimated lethal dose or concentration. Chemical insecticides need to be correctly and selectively used to ensure sustainable crop protection and environmental stability (Jepson, 1989, Greathead, 1995, Haseeb et al., 2000, Haseeb, 2001).
Currently, many selective toxic organophosphates, pyrethroids and other novel insecticides are being investigated as potential alternatives to replace highly toxic and broad-spectrum insecticides. In addition to evaluating their toxicological effect against target insects, these insecticides must be assessed for their adverse impact on natural enemies, but there is little information and knowledge about the toxic and adverse effects of currently popular insecticides on C. lividipennis. Keeping in mind the idea of agroecosystem the research program was undertaken to assess the risk of eleven insecticides having different modes of action on rice brown planthopper, N. lugens and its mirid predator, C. lividipennis.
Section snippets
Insects
TN 1 rice plants (35 days old after transplanting into earthen pots [10 × 10 cm]) were used as host plant for mass culturing the predator as well as its host. Brown planthopper (BPH) and the miridbug were collected from rice fields unexposed to insecticides at the Paddy Breeding Station, Tamil Nadu Agricultural University, Coimbatore. Uniform sized insects (BPH) were selected and reared on rice plants kept in nylon mesh cages (75 × 60 × 90 cm). TN 1 rice plants pre-oviposited by brown planthopper were
Contact toxicity (LC50) – miridbug
Data of contact toxicity of the insecticides to C. lividipennis nymphs are summarized in Table 1, Table 2. Based on LC50 values (mg a.i L−1), the order of toxicity of the insecticides were as follows: At 24 HAT, BPMC (0.02) > ethofenprox (0.04) > clothianidin (0.08) > imidacloprid (1.39) > pymetrozine (3.29) > deltamethrin (4.22) > chlorantraniliprole (5.95) > acephate (32.07) > chlorpyriphos (36.59) > methyl parathion (45.25) > endosulfan (212.59) (Table 1). At 48 HAT also, BPMC was found to be the highly toxic
Discussion
This study indicated that insecticides present significantly different risks to C. lividipennis, and this can provide more choices for integration of chemical control with biological control. Neonicotinoids/chloronicotinyls were introduced into the market in the early 1990s and are currently used to control sucking insects (Nauen et al., 2001). However, the use of neonicotinoid insecticides should be evaluated carefully in IPM programs (Poletti et al., 2007) and these results have shown that
Acknowledgements
The authors are grateful for the financial support received from Syngenta India Ltd., for this project. We also wish to thank Tamil Nadu Agricultural University (TNAU) for providing the facilities to conduct the research successfully.
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