Abstract
Transgenic
Bacillus thuringiensis
(
Bt
) cotton is engineered to express delta endotoxin (Cry toxin) proteins in lepidopteran midgut when ingested by lepidopteran larvae such as the beet armyworm,
Spodoptera exigua
(Hübner). The efficacy of
Bt
technology depends on stage, age, and susceptibility of the tested population to Cry proteins. The baseline susceptibility bioassay test for first, second, and third larval instars of
S. exigua
, collected from Faisalabad, Multan, and Bahawalpur, compared with susceptible laboratory population was carried out in 2015 and 2016. The LC
50
ranged from 0.45 to 2.52 μg ml
−1
, 1.08 to 5.74 μg ml
−1
, and 2.01 to 7.85 μg ml
−1
for first, second, and third larval instars, respectively. The Bahawalpur population was highly resistant and showed 5.63, 5.30, and 3.89 variations than the susceptible population, followed by Multan 3.01, 3.71, 3.10, and Faisalabad 1.93, 2.41, 2.31 population for first, second, and third larval instars, respectively. The molt inhibitory concentration (MIC
50
) ranged from 0.04 to 0.56 μg ml
−1
,
0.08 to 0.99 μg ml
−1
, and 0.10 to 1.42 μg ml
−1
for the three instars, respectively. The trend in lethal concentration and its respective resistance level was higher in 2016 than in 2015.