Possible resistance to insecticides, imidacloprid and fipronil developing in Guilin, China

by
Zhongxian Lu, Deputy Director, Institute of  Plant Protection and Microbiology, Zhejiang Academy of Agricultural Sciences (ZAAS), Hangzhou, China
Yan  Ling, Liping Long,  Institute of  Plant Protection, Guangxi Academy of Agricultural Sciences (GXAAS), Nanning, China, and
Patrick Garcia, Research Assistant, International Rice Research Institute, Los Banos, Philippines and
K.L. Heong, Insect Ecologist, International Rice Research Institute, Los Banos, Philippines

The brown planthopper has two forms, short winged (A) and long-winged (B) which migrates. Photo credit: S. Villareal (IRRI)

Insects generally develop resistance to insecticides when their populations are subjected to high selection pressure from any biocide in the field. Insecticides that have long residual effects, either by design or by formulation, usually provide much longer duration of high selection pressure on an insect population. We used a standardized procedure developed by the Rice Planthopper Project to obtain toxicological responses of the brown planthopper (BPH) collected in Guilin, Guangxi province to imidacloprid and fipronil.  In Guangxi, rice production is not as intensive as in other provinces along the east coast of China, like Zhejiang, Guangdong and Fujian; and thus, insecticide usage is generally lower. However, since the BPH can be dispersed by wind, populations from areas with high insecticide use could have dispersed to Guangxi.

We compared the toxicological responses of BPH with those collected in IRRI, Philippines where imidacloprid has never been used while fipronil was introduced in the year 1999. We used 1 to 3 day old macropterous females reared from a susceptible rice variety and applied 0.24 micro liter (?L) using a hand operated microsyringe and recorded mortality 24 hours after dosing. The experiments were conducted in the laboratory with average temperature and relative humidity of 26^o C and 85%, respectively; and probit analyses were conducted using Polo Plus.  All 4 cases had significantly low heterogeneity factor  < 1.0. This indicates that each of the respective test populations was homogenous.

Figure 2. Mortality probit lines of BPH populations from IRRI and Pila Philippines and Guilin, China in after imidacloprid treatments.

Imidacloprid

The probit lines in Figure 2 show that the BPH populations in IRRI and Pila, in Laguna, are more susceptible to imidacloprid with an LD50 of 0.023 (Pila) and 0.051 (IRRI) microgram per gram (µg/g) insect compared with 6.8 mcg/g in Guilin (Table 1). Although imidacloprid has never been used in IRRI farm, its population was about 2.2 times more tolerant than that in Pila. The slopes of the probit lines in the Philippines were not significantly different, 3.62 for Pila and 3.39 for IRRI, while they were significantly different from that in Guilin, 1.59. Thus, the hypothesis of parallelism was accepted for the 2 Philippines populations and rejected between Philippines and Guilin. This shows that the BPH population in Guilin is at least 130 more tolerant to imidacloprid than the Philippine population. The data shows clearly that the BPH population in Guilin has acquired or developed resistance to imidacloprid and are most probably enhanced through immigration of resistant strain(s) from other areas.

Table 1:  Statistical details of brown planthopper responses to imidacloprid in IRRI, Philippines and Guilin, China.

Figure 3. Mortality probit lines of BPH populations from IRRI and Pila Philippines and Guilin, China after fipronil treatment

Fipronil

Figure 3 and Table 2 show that the BPH population in Guilin is about 3.8 to 6.5 times more tolerant to fipronil than populations in the Philippines. The slopes of the probit lines are not significantly different and, in addition, the parallelism test shows that the lines are parallel. This implies that the Guilin BPH population, although slightly more tolerant than the Philippine populations, has similar response to fipronil.

Table 2:  Probit analysis data of brown planthopper response to fipronil in IRRI, Philippines and Guilin, China.

We have used the IRRI BPH population where imidacloprid has never been used. There is clearly a marked tolerance development in the Guilin population where imidacloprid has been used frequently. Here imidacloprid is often sold under numerous trade names and farmers often use several insecticides with the same ingredients. While the Philippines populations seem to have remained susceptible and homogenous in response as shown by the steep slope of 3.8, the Guilin BPH populations have clearly acquired tolerance of more than 130 folds. See http://ricehoppers.net/2009/03/05/imidacloprid-from-hero-to-zero-in-the-control-of-brown-planthopper/.

For fipronil, IRRI farm started using this insecticide in 1999; and Fig 4 shows the annual amount used up till 2006. The two populations have similar responses to fipronil and their probit line slopes are low at < 2.0, indicating that a certain degree of tolerance to fipronil has begun in both locations. The Guilin population is about 3.8 and 6.5 folds more tolerant to fipronil than the IRRI and Pila populations.

Fig. 4. Total volume (liters) of fipronil purchased from 1998-2006 by IRRI, Philippines.

Tagged as: Insecticide resistance, insecticide resistance in Guilin, resistance to fipronil, resistance to imidacloprid