159tactics. The development of resistance can reduce the effectiveness of insecticidal
Bt proteins in transgenic plants. Therefore, the main threat to the continued success
of Bt crops is the evolution of resistance in pests. Some reviews have addressed
insect pest resistance in GM crops (Carrière et al. 2010 ; Pardo-Lopez et al. 2013 ;
Tabashnik 1994 ). Several studies have compared the field outcomes for resistance to
Bt crops but with limited sample sizes for both field outcomes and other factors
predicted to affect resistance (Carrière et al. 2010 ; Tabashnik 2008 ; Tabashnik et al.
2009 ). A 2012 review summarized pest resistance to Bt crops by analyzing field
monitoring data for resistance from 77 different studies, and reported resistance for
some populations of 5 of 13 major pest species (lepidopteran and coleopteran)
examined when compared with resistant populations of only one pest species in
- The number of major pest species with field-evolved resistance and reduced
transgenic crop efficacy increased from one to five (three Bt corn and two Bt cotton)
from 2005–2010 (Tabashnik et al. 2013 ). Another study revealed that the insect
resistance percentage to GM crops increased from 0.93 % in 2010 to 5.5 % in 2013.
Therefore, pest resistance management is necessary to attain the full benefit of GM
crops to minimize food security issues. The factors responsible for delaying resis-
tance include the use of abundant non-Bt refuge crops, recessive inheritance of
resistance, the low initial frequency of resistant alleles, and separate deployment of
two-toxin Bt crops from one-toxin Bt crops. It was predicted that without natural
refuge crops, the percentage of resistant insects to GM crops would exceed 98 % in
2013 but would increase by only 1.1 % if natural refuges were as effective as non-Bt
cotton refuges. Similarly, the percentage of resistant insects increased from 37 % in
2010 to 84 % in 2013 with the non-recessive inheritance of resistance (Jin et al.
2015 ; Tabashnik et al. 2013 ). The integration of two or more toxic genes with other
control tactics may further slow increases in resistance.
4.6.2 Impacts on Non-target Insects
Insects play an important role in the ecosystem and human economy as crop polli-
nators, natural enemies (predators and parasitoids) and detritivores. Honey bees are
the best-known pollinators and often improve fruit and seed yields (Crane and
Walker 1984 ) with the additional advantage of honey production. Cotton nectar is
attractive to bees, but they are not required for pollination. It is considered that GM
crops may have hazardous effects on bees as well as other insect pollinators. Most
studies showed no deleterious effects on pollinators, but facts indicate dose-
dependent hazardous effects to bees (Malone and Burgess 2009 ). Similarly, natural
enemies (predators and parasitoids) are significant regulators of insect pests. It is
evident that the survival of natural enemies depends on the supply of host insects.
So, the reduction in insects feeding on GM crops ultimately affects the natural ene-
mies’ population. GM crops could have both direct and indirect effects. Many labo-
ratory studies on Bt cotton demonstrated negative tritrophic impacts on predators
(Orius tristicolor W. and Geocoris punctipes S.) (Ponsard et al. 2002 ) and parasit-
oids (Cotesia marginiventris C. and Copidosoma floridanum A.) (Baur and Boethel
Insect-Pests in Dryland Agriculture and their Integrated Management