Diamondback moth , resistance

Resistance genes can be dominant, recessive, incompletely dominant, or incompletely recessive. Resistance to carbamates and organophosphates is usually dominant or incompletely dominant. Resistance to DDT, Bt, and spinosyns is usually recessive. Resistance to dieldrin is usually incompletely dominant. Resistance to pyrethroids is usually incompletely recessive. As shown in Figure 10.1 and 10.2, diamondback moth resistance to per-methrin was inherited as an incompletely recessive, autosomal factor, whereas resistance to methomyl was inherited as an incompletely dominant, autosomal factor. In cases of monofactorial inheritance of resistance to insecticides, the degree of dominance (D) in the progeny can be calculated, as described by Stone (1968), as follows ... 

Tabashnik, B.E. and N.L. Cushing. 1989. Quantitative genetic analysis of insecticide resistance variation in fenvalerate tolerance in a diamondback moth (Lepidoptera Plutellidae) population. Jour. Econ. Entomol. 82 5-10. 

Insects have evolved resistance to Bt toxins in the laboratory, yet only one crop pest, the diamondback moth (Plutella xylostella), has evolved resistance to Bt toxins under open field conditions (Tabashnik et al. 2003). But this resistance was not caused by Bt crops, rather it occurred in response to repeated foliar sprays of Bt toxins to control this pest on conventional (non-GE) vegetable crops (Tabashnik 1994). Based partly on the experience with diamondback moth and because Bt crops cause season-long exposure of target insects to Bt toxins, some scientists predicted that pest resistance to Bt crops would occur in a few years. 

Figure 10.1 Probit regression lines for permethrin against susceptible (S) and resistant (R) diamondback moths and their F, progeny. (From Yu, S.J., /. Econ. Entomol, 86, 680, 1993. With permission.)...

Reduced binding of toxin is a primary mechanism of insect resistance to Cry proteins (Ferre and Van Rie, 2002). Resistance of the Indian meal moth to Bt was due to a 50-fold reduction in affinity of the membrane receptor (from brush borer membrane vesicles of larval midguts) for 5-endotoxin. In pinkbollworms, Bt resistance is associated with reduced binding of CrylAb to the brush borer membrane vesicle. Similarly, CrylAc resistance in diamondback moths was demonstrated to be due to dramatically reduced target binding. 

Yu and Nguyen (1996) showed that selection of a strain of diamondback moth (Plu-tella xylostella) with permethrin for 21 generations resulted in over 600-fold resistance to permethrin in this strain. The resistant strain was also cross-resistant to all pyrethroids tested, including bifenthrin, fenvalerate, esfenvalerate, A.-cyhalothrin, fluvalinate, and tral-omethrin. However, it remained susceptible to organophosphate, carbamate, cyclodiene, neonicotinoid, avermectin, and microbial insecticides tested. Biochemical studies indicated that pyrethroid resistance observed in this strain was most likely due to decreased target site sensitivity. 

Although it was once thought that insects would not develop resistance to microbial insecticides, scientists have confirmed cases of diamondback moths in commercial farms that showed resistance. This is another argument for using BT with care, and only after trying cultural or physical control methods for which resistance is not a factor. 

Studies on the metabolism of ary toxicant often involve techniques of comparison between susceptible and resistant strains. Although there are reports of resistance to topically applied PBO alone, e.g. in the house fly (Sawicki, 1974) and diamondback moth (Stir et /., 1985), there appear to be no reports of relevant studies tin either the mode of action or the tnechanism(s) of resistance to l3BO and how they might have evolved and been selected. 

Other cases where resistance has reached crisis proportions include Colorado potato beetle on Long Island, New York (3.) and diamondback moth in Southeast Asia (24). 

Unique Mode of Action. As described above, the Bt delta endotoxin is a stomach poison which acts on the midgut epithelial cells of susceptible insects. This is in contrast to the typical mode of action for synthetic chemical insecticides, which act as nerve poisons. Because the Bt site of action is so different from that of synthetic insecticides, Bt has been successfully utilized for control of otherwise resistant insects such as mosquitoes, the diamondback moth and the Colorado potato beetle. 

Lin, J.G., Hung, C.F. and Sun, C.N. (1989) Teflubenzuron resistance and microsomal mono-oxygenases in larvae of the diamondback moth. Pesticide Biochemistry and Physiology 35, 20-25. 

Table II shows the insecticidal activity of flubendiamide against major insect and acarina species. Flubendiamide provided high activity on all lepidopterous insect pests, and its ECjq values were between 0.004 and 0.58 mg a.i./L. However, flubendiamide did not show activity against other insect species. Thus, the insecticidal spectrum of flubendiamide is expected to be broad among lepidoptera pests in agriculture. Against the resistant strain of diamondback moth, flubendiamide provided the same level of activity as against the susceptible strain. This result indicates that flubendiamide will be useful for insecticide resistance management (IRM) programs.

Tabashnik, B.E., Cashing, N.L., Finson, N. and Johnson, M.W. (1990). Field development of resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera PyralUdae). J. Econ. Entomol. 83,1671-1676. 

Perng FS, Sun CN. Susceptibility of diamondback moths (Lepidoptera Plutellidae) resistant to conventional insecticides to chitin synthesis inhibitors. J. Econ. Entomol. 1987, 80(1), 29-31. 

While dieldrin target-site resistance (Section 29.5.2.2) can confer some level of resistance to fipronil, this cross-resistance has not led to failure of fipronil in the field. However, resistance due to other mechanisms was seen in southeast Asia as early as 1996 in diamondback moth Plutella xylostella), within three years of the introduction of the product. The high intrinsic activity of fipronil and lack of alternatives in the mid-1990s led growers in countries such as Thailand to use it up to 40 times per year on cruciferous crops. By early 1997, many populations were resistant and field failures were widespread. At the same time, use of the... 

Pyridalyl controls a wide variety of lepidopterous pest strains such as the diamondback moth (Plutella xylostella) which is resistant to various existing insecticides (8). This insecticide is also effective against thysanopterous pests. Moreover, it does not exert an adverse effect on various beneficial arthropods such as natural predator insects (e.g., Oristar-A ) and honeybees. Therefore, this novel insecticide is expected to be a usefiil material for controlling lepidopterous and thysanopterous pests in IPM and insecticide resistant management programs. 

Eigenbrode, S.D., K.E. Espelie A.M. Shelton. 1991. Behavior of neonate diamondback moth larvae [Plutella xylostella (L.)] on leaves and on extracted leaf waxes of resistant and susceptible cabbages. J. Chem. Ecol. 17 1691-1704. 

Tabashnik, B. E. Liu, Y. -B. Finson, N. Masson, N. Heckel, D. G. One gene in diamondback moth confers resistance to foirr Bacillus thirringiensis toxins. Proc. Natl. Acad. Sci. U.SA. 1997,94, 1640-1644. 

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