Plutella xylostella diamond back moth

Several wasp predators reportedly attack the larvae of P. xylostella. The larvae are generally susceptible to the biological control agent B. thuringiensis, but this may cost more than using insecticides (Talekar et al., 1985; Talekar, 1986; Talekar and Shelton, 1993). Genes from two strains of B. thuringiensis have been transferred into broccoli by Agrobacterium tumefaciens-mediated transformation, which has effectively controlled the diamond back moth (Cao et al., 1999). Plants with high levels of Cry1c protein cause rapid and complete mortality of three types of diamond back moth larvae, with no defoliation. Because of the rapid ability of diamond back moth to mutate and overcome B. thuringiensis, Shelton et al. (2000) have shown that it is

Fig. 7.10. Line diagram of the diamond back moth (Plutella xylostella).

necessary to provide refuges to conserve susceptible larvae and thus reduce the numbers of homozygous offspring. At the same time, refuges with high levels of B. thuringiensis toxic plants resulted in a reduction of predators (Riggins and Gould, 1997).

Considerable effort has been invested in developing resistance to the diamond back moth and small cabbage white butterfly in B. oleracea. Some glossy-leafed lines have shown good consistent resistance (Lin et al., 1983) in many parts of the world. The glossy leaf is, however, unacceptable horticulturally and reduces yield and delays maturity, and so far no adequate resistance has been found in non-glossy lines. Resistance was found in cauliflower PI 234599, cabbage G8329 and lines of Chinese cabbage.

Some normal leaf lines developed by selection had moderate resistance, but this was probably insufficient to be economically successful. The mechanism of host resistance appears to operate by preventing penetration of the first instar into the leaf tissue. There are, however, reports of good resistance from a commercial breeding programme in South Korea. Predictive control based on counts of the numbers of plants infested allowed the application rate of insecticides to be reduced.

Pheromone traps may aid chemical treatment timing. Single spray applications timed correctly may be sufficient to prevent yield losses from this pest in northern regions; in more southerly locations where there is intense population pressure and frequent reproduction, sprays at weekly intervals or even more often are necessary. Some strains of this pest, particularly in Asia, have resistance to many chemical pesticides because of overuse. There are no methods of cultural control.

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