L. Fabellar1, P Garcia1, Zhongxian Lu2, P.V. Tuong3, Wantana S4. and M.S.Maisarah5
1International Rice Research Institute, Los Baños, Philippines,
2Zhejiang Academy of Agricultural Sciences, Hangzhou, PR China
3Plant Protection Department, Ho Chi Minh City, Vietnam
4Rice Department, Bangkhen, Bangkok, Thailand
5Malaysian Agricultural Research and Development Institute, Bertam, Malaysia
In the early 1990s a new group of insecticides, the neonicotinoids, was introduced into the market. These are neuro active compounds modeled after nicotine and are effective against sucking insects, like the planthoppers. Imidacloprid is one of the insecticides used extensively in rice in some countries. Sold in numerous tradenames (> 500 in China), used as seed treatments as well as sprays, the insecticide had developed resistance very rapidly (See Matsumura et al 2009 (Read: Current status of insecticide resistance in rice planthoppers). Some labeled this novel insecticide “From hero to zero” to describe the rapid decline of a product due to the lack of control over its uses which led to rampant misuses. In BPH the resistance might have been developed from a nicotinic acetylcholine receptor (nAChR) mutation at the target site in resistant populations developed in the laboratory but this apparently had not be observed in the field (Matsumura et al 2009).
Using the same standardized insecticide resistance monitoring procedure (Read: Monitoring protocol) we compared toxicological data from 17 locations in 5 countries between July 2009 and July 2010. The experiments were conducted in the respective country laboratories by the authors who underwent training at IRRI (Read: Toxicology training). Since the insect rearing methods, preparation of test materials and the dosing equipment and technique were standardized, the data collected is highly comparable.
The data from the 17 locations could be grouped into two according to the slopes of the probit lines. The first group of 12 locations had slopes ranging from 1.14 to 2.05 while the second group of 5 locations had slopes ranging from 2.11 to 3.27. Of all the locations the most susceptible population was that from Davao, Philippines and we used this as the most susceptible population for comparison. Probit analyses were performed using the software PoloPlus available from LeOra Software which computes the LD50 values and parallelism tests. For valid comparisons the probit lines should be parallel. Table 1 shows the data from the group of 12 sites where slopes were parallel (chi-square = 19.45 p= 0.053). The two data sets from Thailand had relatively high heterogeneity but regression was significant (>8) which might possibly be due to variability in the experiments. The other data sets were more homogeneous and had heterogeneity factors of less than 1.0.
Table 1: Toxicities and relative potencies of imidacloprid to BPH from 12 different locations in Asia
|Location||LD50 in mg/g insect
(95% fiducial limits)
|Davao, Phil||0.011 (0.008-0.015)||1.79 (0.18)||0.43||1.0|
|San Pablo, Phil||0.058 (0.038-0.078)||1.35 (0.20)||0.32||5.27|
|Muda, Malaysia||0.086 (0.0043-0.135)||1.14 (0.58)||0.58||7.82|
|Nueva Ecija, Phil||0.121 (0.077-0.170)||1.14 (0.19)||0.44||11.00|
|Chainat, Thailand||0.162 (0.061-0.284)||1.66 (0.19)||3.04||14.73|
|Nakon Ratchasima, Th||0.169 (0.102-0.265)||1.45 (0.13)||2.15||15.36|
|Pila, Philippines||0.232 (0.187-0.285)||2.05 (0.25)||0.76||21.09|
|IRRI, Philippines||0.245 (0.175-0.327)||1.68 (0.26)||0.78||22.27|
|Tien Giang, Viet||2.891 (2.225-4.196)||1.78 (0.32)||0.06||262.8|
|An Giang, Viet||3.035 (2.295-4.404)||1.84 (0.35)||0.20||275.9|
|Long An, Viet||3.077 (2.372-4.430)||1.79 (0.32)||0.22||279.7|
|Guilin, China||6.800 (5.192-8.421)||1.59 (0.18)||0.12||618.0|
The highest LD50 we found was from Guilin, China which is 618 times higher than that of the most susceptible population in Davao. The relative potencies of the various populations are shown in the map above. BPH populations from the Mekong Delta were between 260 to 280 times that of Davao. Populations from locations in the Philippines, Malaysia and Thailand were much lower ranging from 5 to 23 times that of Davao or about 10 times lower than Vietnam
The second group of 5 data sets from 3 countries with parallel slopes (chi-square = 9.10 p= 0.059) is shown in Table 2 below. The Jinhua population had the highest LD50 about 1.7 times that of populations from Guilin, which implied that resistance of BPH to imidacloprid could be as high as 1,053 times that of the most susceptible population. Populations from Changsa and Guilin had similar LD50 values but have significantly different slopes.
Table 2: Toxicities and relative potencies of imidacloprid to BPH from 5 different locations in Asia
|Location||LD50 in mg/g insect
(95% fiducial limits)
|Bicol, Philippines||0.179 (0.144-0.215)||2.70 (0.33)||0.61||1|
|Isabela, Philippines||0.207 (0.167-0.254)||2.11 (0.20)||0.96||1.16|
|Angthong, Thailand||0.509 (0.266-0.760)||3.27 (0.24)||2.38||2.84|
|Changsa, China||5.526 (3.573-7.722)||2.45 (0.20)||1.43||30.87|
|Jinhua, China||11.596 (8.588-14.887)||2.23 (0.20)||1.25||64.78|
The comparison of toxicological tests done in 5 countries showed that multiple fold resistance to imidacloprid had undoubtedly developed in China and Vietnam. Resistance factor varies from 260 – 280 folds in Vietnam and 600 – 1000 folds in China. In Thailand, Philippines and Malaysia resistance might be developing as relative potencies varied between 2 to 25 folds, although the Ang Thong population seems to have higher resistance of about 3.4 times more that of Chainat but still 5 and 10 times lower than Vietnam and China respectively. The resistance gradient in the 17 locations reflected the intensities of imidacloprid use in rice in the respective areas. It is likely that in Davao imidacloprid use in rice is very low while in Vietnam and China it is intensively applied.