transgenic crops producing insecticidal proteins of bacillus thuringiensis ( bt ) covered 14 million ha in 2002 ( james 2002 , shelton et al . 2002 ) , but evolution of resistance by insect pests could limit their efficacy ( gould 1998 ) . although field - evolved resistance to bt crops has not been documented yet ( carrire et al . 2003 , tabashnik et al . 2003 ) , many pests have been selected for resistance to bt toxins in the laboratory , and diamondback moth ( plutella xylostella ) populations have evolved resistance to bt sprays in the field ( tabashnik 1994 , ferr & van rie 2002 ) . in a preliminary experiment in which diamondback moth larvae ate leaf discs in petri dishes , sayyed et al . ( 2003 ) found that treating the discs externally with bt toxin cry1ac increased pupal weight of a laboratory - selected resistant strain from malaysia ( serd4 ) . they also raised the question of whether bt transgenic crops would therefore have unanticipated nutritionally favorable effects , increasing the fitness of resistant insects . as summarized below , the relevant published data show that bt toxins in transgenic crops do not improve the performance of resistant insects . the most directly relevant studies compared the performance of highly resistant strains of diamondback moth on cry1ac - producing transgenic crucifers and their untransformed non - bt counterparts . tests of a resistant strain from hawaii revealed no differences between bt and non - bt canola ( brassica napus ) in survival and head capsule width of five - day - old larvae , pupal weight , percentage pupation , and percentage adult emergence ( ramachandran et al . likewise , experiments with a resistant strain from florida detected no differences between bt and non - bt broccoli ( brassica oleracea ) in larval survival or weight gain ( tang et al . in contrast to the two aforementioned studies , in which performance of resistant insects did not differ between bt plants and their non - bt counterparts , at least eight other reports show adverse effects of bt plants on resistant insects . for example , tests of a cry1c - resistant strain of diamondback moth ( cry1c - sel ) , showed negative effects of cry1c - producing broccoli ( zhao et al . performance of resistant strains of pink bollworm ( pectinophora gossypiella ) was generally impaired on bt cotton compared with non - bt cotton ( liu et al . 1999 , 2001 ; 2003 ) , with the exception of one case in which survival of resistant pink bollworm did not differ between bt and non - bt cotton ( tabashnik et al . whereas resistant insects successfully completed development on bt plants in the cases described above , other studies found that bt plants prevented completion of development by resistant insects . for example , bt - resistant european corn borer ( ostrinia nubilalis ) did not survive on transgenic bt corn ( andow et al . 2000 , huang et al . 2002 ) and bt - resistant colorado potato beetle ( leptinotarsa decemlineata ) did not survive on bt potato ( wierenga et al . the generally adverse effects of bt plants on bt - resistant insects are likely caused by high concentrations of bt toxins in the transgenic plants . other factors that could negatively affect performance of resistant insects on bt plants include prolonged exposure to bt toxins and interactions between plant chemistry and bt toxins ( tabashnik et al . insect strains are deemed resistant if bioassays show that they are significantly less susceptible to bt toxins than are unselected conspecific strains ( tabashnik 1994 ) . however , the reduction in susceptibility may not be sufficient to completely overcome the negative effects of bt toxins in transgenic plants ( tabashnik et al . 2000b , 2003 ) . the disadvantage suffered by resistant insects on bt plants relative to non - bt plants is called  incomplete resistance  ( carrire & tabashnik 2001 ) . in contrast , fitness costs on non - bt plants are caused by negative pleiotropic effects of genes that confer resistance ( groeters et al . if fitness costs occur , resistant individuals are less fit than susceptible individuals when toxin is not present . refuges of non - bt crops , recessive inheritance of resistance to bt crops , low initial frequencies of alleles conferring resistance , fitness costs , and incomplete resistance may be key factors delaying resistance to bt crops in the field ( carrire & tabashnik 2001 , carrire et al .
sayyed et al . ( ecology letters ( 2003 ) 6 : 167169 ) hypothesized that insecticidal bacillus thuringiensis ( bt ) toxins produced by transgenic crops could have nutritionally favorable effects that increase the fitness of resistant insects eating such crops . 
 this idea was based on increased pupal weight of resistant larvae of diamondback moth , plutella xylostella ( l. ) , fed leaf discs treated externally with a bt toxin . 
 we summarize evidence from diamondback moth and other pests showing that the bt toxins in transgenic crops do not enhance performance of resistant insects . aside from a few notable exceptions in which performance of resistant insects did not differ between bt and non - bt crops , bt crops had adverse affects on resistant insects.abbreviation:btbacillus thuringiensis