Comparative Toxicity of Selected Insecticides

Published: 2021-07-15 11:55:04
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A range of insecticides are used extensively to control D. opuntiae and others scale pests around the world which can also indirectly affect beneficial organisms (Vermeulen et al. , 1990; Campiche et al. , 2006; Dawar et al. , 2016). Deltamethrin, methidathion and carbaryl have been registered for the control of this scale pest (Vermeulen et al. , 1990). Also because of the environmental and health problems associated with the heavy use of synthetic insecticides in the developing countries, it is necessary to find alternative options for the control of pests.
The present study tested several synthetic insecticides and biopesticides for their efficacy against D. opuntiae and their lethal effect on the predator C. montrouzieri. Two of the biopesticides tested (d-limonene, and mineral oil) were effective against the cochineal, and at the same time had a minimum impact on adults and larvae of the predator ladybird. Bouharroud et al. (2018) reported also that d-limonene caused high mortality rate to D. opuntiae adults females under field conditions. The highest mortality rates caused by this biopesticide were 98% and 99% at 120 and 150 ppm, respectively 144h after treatment. D-Limonene ((R)-4-isopropenyl-1-methylcyclohexene) is a monoterpene, referred to 10-carbon terpenoids, obtained from citrus juice (Bouharroud et al. , 2018; Kim et al. , 2013). It is the major component of the oil extracted from citrus fruits peels (Kim et al. , 2013). Its molecular formula is C10H16 and its molecular weight is 136. 238 g mol-1 (Pubchem database, 2017).It was first isolated and purified in 1870 (Dev et al. , 1982). D-limonene, and mineral oil are biorational insecticides that acts by physical contact on many insect pests of crops and surely both had a spectrum of actions very broad and not very selective, but the lack of studies does not allow to conclude as to this assumption. The physical effect of these biorational insecticides affect the mechanical barrier provided by the waxy cottony covered D. opuntiae females, and help the predator to access directly to their prey. Since the two botanical pesticides used in this study have contact effects, and because the larvae of the predator (C. montrouzieri) have waxy strings, they are prevented from the direct contact with the toxins (Toorani et al. , 2017), which explained the high percent mortality associated with predator adult stage (11. 76%- 21. 18% mortality for d-limonene (60 g/l), and 15. 29% – 22. 35% mortality for mineral oil (780 g/l) 120 h after treatment) compared to larvae stage (10. 71% – 20. 24% mortality for d-limonene (60 g/l), and 11. 90%-20. 24% mortality for mineral oil (780 g/l) after 120 h after treatment).
The insecticides tested (malathion, alpha-cypermethrin, and chlorpyriphos-methyl) had significant insecticidal activity against the nymphs and adult females of the cochineal, but had negative impact on the predator ladybird. Previous study on lethal and sublethal effects of different insecticides on larvae and adults of C. montrouzieri revealed that treatments of monocrotofos (0. 04%), malathion (0. 1%), endosulfan (0. 075%), prophenofos (0. 075%) and acephate (0. 1%) proved toxic to the larvae and adults of C. montrouzieri. Acetamiprid (0. 004%) was moderately toxic to the larvae and adults, while thiamethoxam (0. 005%) was moderately toxic to the larvae. Chlorpyriphos-ethyl (50%) + cypermethrin (5%) (0. 055% of mix) and imidacloprid (0. 005%) were found to be the least toxic to the larvae, while Chlorpyriphos-ethyl (50%) + cypermethrin (5%) (0. 055% of mix), thiamethoxam (0. 005%) and imidacloprid (0. 005%) were found to be the least toxic to the adults of C. montrouzieri (Dumaniya et al. , 2015). Planes et al. (2013) also reported that thiamethoxam and pyriproxyfen are harmful, and chlorpyriphos and spirotetramat are harmless to C. montrouzieri. Imidacloprid, Carbaryl (Halappa et al. , 2013) and Acetamiprid (Anjitha et al. , 2013) are also reported as harmful to C. montrouzieri.
Also lethal and sublethal effects of many insecticides have been reported on several beneficial insects including predators for example in the coleopterans Schymnus coccivora (Sahito et al. , 2011; Anjitha et al. , 2013), Schymnus Saturalus (Sahito et al. , 2011), Coccinella septempunctata (Xiao et al. , 2016), Harmonia axyridis (Nawaz et al. , 2017), Hippodamia convergens and Coleomegilla maculate (Lanka et al. , 2013). The results of this study showed that the two biopesticides (limocide, and insecticide 101) can be used in an IPM program for the control the cactus mealybug, Dactylopius opuntiae; they do provide a very good level of control of the pest and at the same time are not harmful to the predator Cryptolaemus montrouzieri.

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