Contents - Faperta

Development of Resistance to Transgenic Plants 391
moth, P. xylostella as a model system to examine resistance management strategies. The
higher number of larvae on refuge plants in fi eld tests indicated that a “separate refuge” is
more effective in conserving susceptible larvae than a “mixed refuge,” and would thereby
reduce the number of homozygous resistant (RR) offspring. Care must be exercised to
ensure that refuges, particularly those sprayed with effi cacious insecticides, produce
adequate numbers of susceptible insects.
A strip confi guration is the least costly method of planting a 20% non-Bt maize refuge
(Hyde et al., 2001). The non-Bt plants produce the susceptible insects, which have a probability
of mating with insects emerging from the Bt crops nearby, and thus dilute the frequency
of the resistant individuals. Separate refuges are superior to seed mixtures for
delaying resistance. Movement of H. zea larvae from nontransgenic to transgenic plants
may result in an increase in damage and reduce the yield in mixed stands of Bt and non-Bt
plants (DuRant et al., 1996). In cotton, the numbers of eggs do not differ between mixed
and pure stands of transgenic or nontransgenic plants (Halcomb et al., 1996; Lambert,
Bradley, and Duyn, 1996). Planting two-row strips may be as good as separate refuges in
delaying resistance, but their adoption carries greater risk because of the uncertainty surrounding
the movement and survival of neonates (Onstad and Gould, 1998). Transgenic
and nontransgenic plants could be grown in separate rows with a wider row spacing (strip
planting) to minimize the rate of resistance development (Ramachandran et al., 1998a). As
the proportion of nontransgenic plants increases, the number of larvae and amount of
injury also increase.
Role of Alternate Hosts as Refugia
The ability of noncrops to support complete development of insect pests is an important
factor in determining their impact on resistance management in transgenic crops. For
polyphagous pests that feed on several fi eld crops and alternate hosts, there may not be
any need to maintain refugia under subsistence farming conditions in the tropics, where
several collateral and alternate hosts are available over time. There may not be any need to
plant refuge crops for cereal stem borers and Heliothis/Helicoverpa in the tropics where they
feed on several crop and weed hosts. The probability of European corn borer, O. nubilalis
to complete development on a plant other than corn is relatively low, and the smallerstemmed
noncorn plants may not make a substantial contribution to the pool of susceptible
adults (Losey, Carter, and Silverman, 2002), although some individuals that survive could
serve as a source of refuge. Cameron et al. (1997) studied the host range and occupancy of
diamondback moth, P. xylostella, and potato tuber moth, P. operculella to determine what
proportion of their populations occur in refugia and would therefore not be exposed to
Bt-transformed Brassica or potato. Although P. operculella was found on Solanaceae other
than potato in the fi eld, the alternative hosts were rare in areas where vegetables were
growing and, therefore, were not a signifi cant source of refugia for this pest. Diamondback
moth, P. xylostella, was common on Brassicaceae other than vegetable brassicas, particularly
on white mustard, Sinapis alba L., which was the main refuge. Wild radish, Raphanus raphanistrum
L., was less preferred, but was the most common alternative host plant. The area
covered by wild radish was approximately 6% of the area planted in commercial brassicas
and therefore formed a minor refuge for P. xylostella. The results suggested that to limit the
development of resistance, insect-resistant transgenic potatoes should be less widely used
than insect-resistant transgenic brassicas.
A low proportion of potato tuber moths, P. operculella, foraged beyond 100 to 250 m to
infest tubers or plants (Cameron et al., 2002). Light traps indicated that the number of