Variation in carboxylesterase frequency and insecticide resistance of Plutella xylostella (L.) as a response to environmental gradients

A simple observational approach for studying the response of the diamondback moth (DBM), Plutella xylostella (L.), to environmental gradients including latitude, insecticide application and climate change, was developed in this study. Insecticide resistance and carboxylesterase frequency in DBM collected in 1979/1980 and 1986/1987 were selected as two ecological variables for analysing and defining the response. Variation in the correlation between larval carboxylesterase 9b and malathion resistance of this insect was also investigated during the period of 1987 to 1997. A clustering dendrogram based on frequencies of seven carboxylesterase isozymes in 16 Taiwan populations showed a decreasingly latitude-dependent distribution of three major clades orientated from north to south. A multidimensional scaling configuration based on the same data had a stress value of 0.18 indicating that these three clades are barely separated. Data on insecticide resistance of 18 populations to five insecticides in a study by Cheng (1981) were used for clustering and multidimensional scaling analysis. The cluster of insecticide resistance in DBM also showed a latitude-dependent distribution of three clades. Multidimensional scaling configuration of insecticide resistance had a stress value of 0.03 indicating that the whole operative taxonomic unit was unable to be separated except for the Tou-Cheng population. Nevertheless, these two clustering dendrograms were similar to each other in three aspects: 1. susceptible populations of Tou-Cheng and Jeo-Feng diamondback moth in northern Taiwan stood as an outgroup from the two different operative taxonomic units; 2. both clusters and both configurations showed that the Sheh-Tzu population in northern Taiwan was grouped together with three southern populations, indicating autopomorphy; 3. geographically related populations adjacent to one another were grouped together as three latitude-dependent clades. However, discordance of the morphometric gradient of the isozyme of DBM populations among adjacent regions was observed. Of seven carboxylesterase isozymes, frequency of EST 9b in DBM was found to be positively correlated with resistance to mevinphos, malathion, fenvalerate and permethrin in this insect. These findings suggest that: 1. the increasing frequency of EST 9b in these populations is roughly associated with decreasing latitude and the distribution of EST 9b in these populations is not random; 2. populations of higher insecticide-resistance have a higher frequency of EST 9b in the zymogram of DBM; 3. both frequencies of EST 9b and insecticide-resistance in DBM are temporarily sustainable and may have varied little during the period of 1981-1997. However, a series of studies on the correlation between EST 9b frequency in DBM and the susceptibility of DBM to a discriminating dose of malathion (0.12 mg per larva) during 1989-1997 revealed that the slopes of the linear regression lines increased in a tangent angle from 16.7, 20.4 to 29.0 . All three slopes intercepted approximately at zero frequency of EST 9b. This suggested that the association between the mechanism of malathion-resistance and EST 9b in 1989, 1991 and 1997 increased during this period of time. Frequency of EST 9b in 2001 DBM reached its highest titre of 78% suggesting that the association between EST 9b and the resistance mechanism may reach its maximum. Two factors may be involved in the rapid spread of EST 9b in Taiwan populations: 1. dispersal of EST 9b increased in susceptible populations due to migration of the gene coded for EST 9b; 2. a warm winter season due to El Niño in 1998/1999 enhanced the dispersal of EST 9b in all populations. Significance of variation in the correlation between frequency of EST 9b and malathion resistance of DBM is discussed.