Insect rearing
The cotton mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae), was collected from infested cotton plants (Gossypium barbadense var. Giza 86) at the field of Aga district, Dakahalia governorate, Egypt. The mealybug was transferred to the laboratory, and sprouting potato tubers were used as a host plant for its rearing. Gravid females of P. solenopsis were inserted in sprouting potatoes. Each sprouted potato was infested with an adult female and observed daily (Attia and Ebrahim 2015). From the reared culture, newly hatched crawlers of P. solenopsis were placed on each sprouted potato before being confined in a carton cylindrical box of 8-cm long and 12-cm diameter. The carton boxes were kept at 30 °C and 60 ± 5% R.H. Daily examination for the morphological changes were recorded and monitored until adult emergence (Attia and Ebrahim 2015).
Natural enemy culture maintenance
Larvae of C. carnea were obtained from the Bio-Control Laboratory of Plant Protection Research Institute, Dokki, Egypt. They were maintained on P. solenopsis at 27 ± 2 °C, 65 ± 5% RH, and 16:8 L:D.
Natural products
Cinnamon oil and its active ingredient, cinnamaldehyde, were used in this study and were bought from Essential oil Extracts Center, National Research Center. Jun-Ran et al. (2015) proved that cinnamaldehyde is the active ingredient of cinnamon oil.
Cinnamaldehyde formula (Vogt 2010)
Preparing the stock solution of the tested materials
The stock concentrations of each tested material (cinnamon oil or cinnamaldehyde powder) were prepared on basis of weight and the volume of the distilled water (w/v) in the presence of tween 80 (0.1%) as emulsifier. Four diluted concentrations for each material were used to draw the LC-P lines. Three replicates were used for each concentration.
Toxicity test
Direct experiment
Toxicity of cinnamon oil and cinnamaldehyde powder was evaluated against adult of P. solenopsis. Thirty newly emerged adults, 10 individuals in each replicate, were placed on okra leaves in each Petri dish. Each material had four concentrations, 500, 1000, 5000, and 10,000 ppm, which were sprayed on the individuals. Mortality was recorded for 7 days after treatment. The mortality percentage was estimated and corrected according to Abbott (1925). LC50 values were determined using probit analysis statistical method of Finney (1971).
Equation: Sun (1950) (to determine LC50 index)
$$ \mathrm{Toxicity}\ \mathrm{index}\ \mathrm{for}\ {\mathrm{LC}}_{50}=\left(\frac{{\mathrm{LC}}_{50}\ \mathrm{of}\ \mathrm{the}\ \mathrm{most}\ \mathrm{effective}\ \mathrm{compound}}{{\mathrm{LC}}_{50}\ \mathrm{of}\ \mathrm{the}\ \mathrm{least}\ \mathrm{effective}\ \mathrm{compound}}\right)\times 100 $$
Indirect experiment
After calculating LC50 for each material, each LC50 was sprayed on 30 adults of P. solenopsis, 10 individuals for each replicate; these individuals were introduced to one larva of the predator, C. carnea, after 24 h of direct spraying. Four replicates were used, one prey in each. The results were corrected by control, and the mortality percentage was estimated and corrected according to the analysis of variance (ANOVA) (Analytical software 2005).