Laboratory and semi-field experiments were carried out to investigate the joint action of oleic, linoleic and stearic acids
with each of emamectin benzoate, lufenuron and indoxacarb against the 4th instar larvae of cotton leafworm, Spodoptera
littoralis (Boisd.) (Lepidoptera: Noctuidae). The sub-lethal effects of the selected insecticides and fatty acids on some
biological parameters of S. littoralis were also evaluated. Results showed that, toxicity of emamectin benzoate (LC50 = 0.001
mg L-1
) was 80 times more toxic than indoxacarb (LC50 = 0.08 mg L-1
), 1400 times more toxic than lufenuron (LC50 = 1.4 mg
L
-1
). Linoleic acid was the most toxic fatty acid with LC50 = 2435.8 mg L-1
compared to oleic acid (LC50 = 3654.6 mg L-1
) and
stearic acid (LC50 = 4645.2 mg L-1
). Potentiating effect was obtained when linoleic acid at concentration equivalent to LC10
was mixed with emamectin benzoate or indoxacarb or lufenuron at concentration equivalent to LC50 after 48, 72 and 96 hrs of
exposure with co-toxicity factors (CTFs) ranged between 27.78 and 58.33. Mixtures of oleic acid with each of tested
insecticides resulted in additive effects. Whereas, mixtures of each tested insecticides with stearic acid led to antagonistic
effects. All treatments with tested insecticides and fatty acids were significantly reduced pupal mean weight (mg/pupae),
percentage of pupation, percentage of adult emergence, fecundity (no. eggs laid/female), fertility (% egg hatch) and adult
longevity (days) compared in control treatment. Semi-field data revealed that mixture of indoxacarb (0.5 FR) with the LC10
of oleic or linoleic acids had the highest residual toxicity compared to mixtures of the other insecticides against the 4th instar
larvae during the cotton seasons of 2017 and 2018. Each of oleic and linoleic acids extended the residual toxicities of the
tested insecticides at 0.5 FRs compared to 0.5 FRs of each insecticide alone