Comparison of the efficacy of two insecticides for the management of Dactylopius opuntiae on prickly pear cactus in Lebanon and monitoring of the insecticides residues dissipation rates in fruits and cladodes

Abstract

Environmental conditions and availability of new cultivated areas of prickly pear cactus in Lebanon appeared lately to be favourable for the survival and development of D. opuntiae which has become highly noxious. To thwart use of random chemicals and broad spectrum pesticides by local farmers, efficacy of sulfoxaflor (Closer™ 240 SC, rate 0.179 kg a.i/ha) and diflubenzuron (Dimilin® 25 W, rate 0.329 kg a.i/ha) was compared. Individual insecticidal solutions of each applied alone or in mixing with mineral oil (spray oil 7E, rate 1.010 kg a.i/ha) on potted cladodes in laboratory conditions (T = 23 ± 4 °C, RH = 58–63%) showed a sharp decrease in the number of individuals within colonies 48 h after treatment especially among first- and second-instar larvae. Similarly, the same insecticidal solutions applied on separate plots in field conditions (T = 20–35 °C, RH = 55–75%) according to the Good Agricultural Practices showed efficacious control of the pest development. D. opuntiae mortality mean percentage reached 80% and 72%, respectively, for sulfoxaflor and diflubenzuron 96 h after treatment. The added mineral oil acted as a synergist and enhanced efficacy of both insecticides. Mortality mean percentages increased to 92% and 88%, respectively. Insecticides residues content over time in fruits and cladodes were afterwards monitored. Samples were harvested at 3 days interval and residues extracted by the QuEChERS method and quantified by LC–MS. Dissipation rates after 30 days of treatment attained 70.8% and 80.6% in fruits and 98.5% and 74.0% in cladodes, respectively, for sulfoxaflor and diflubenzuron. Half-life time varied accordingly and was higher for fruits treated with sulfoxaflor (14.20 days) than for those treated with diflubenzuron (11.18 days). On contrary, for cladodes, half-life time of the former (4.18 days) was lower than that of the latter (15.47 days). The addition of mineral oil into the spray solution of the insecticides increased their persistency in the crop. Consequently, half-life times increased and dissipation rates decreased. Findings of this study should enlarge the spectrum of chemical means for D. opuntiae management and promote resilience of the crop. Analytical method applied for residues analysis should be useful for registration of the insecticides on the crop and MRLs setting.