Comparative Toxicity Of Selected Pesticides

Abstract: The present experiments were carried out to determined contact toxicity of various biological and chemical pesticides on the first, second instar nymph and adult of Opuntia cochineal scale, Dactylopius opuntiae, and the predator Cryptolaemus montrouzieri Mulsant under field conditions. The results indicated that the highest mortality rates (99 to 100%) was caused by d-limonene (60 g/l) at 100 and 150 cc/hl, mineral oil (780 g/l) at 2400 cc/hl and malathion (500 g/l) at 300 cc/hl on the first instar nymph, 24 h after treatment. However, only d-limonene (60 g/l) at 150 cc/hl was the most toxic (99%) on the second instar nymph. The highest mortality (99%) on adult female of D. opuntiae was observed 120 h after treatment for d-limonene (60 g/l) at 150 cc/hl and mineral oil (780 g/l) at 2400 cc/hl. Highest mortality was (92 to 97%) on C. montrouzieri adults 24 h after treatment for malathion (500 g/l) at 100, 200 and 300 cc/hl and alpha-cypermethrin (100g/l) at 75, 150 and 225 cc/hl.

The least harmful on C. montrouzieri adults and larvae were d-limonene (60 g/l) at 50 cc/hl and mineral oil (780 g/l) at 1000 cc/hl with mortality of 11 and 15%, 120 h after treatment, respectively. The most harmful pesticides on C. montrouzieri larvae 24 h after treatment were malathion and alpha-cypermethrin with mortality rates of 89 to 95%. In addition to malathion and alpha-cypermethrin, mortality ranged from 87 to 100% 120 h after treatment for chlorpyriphos-methyl (480 g/l) at 75, 150 and 225 cc/hl and spinosad (480 g/l) 100, 200 and 300 cc/hl on C. montrouzieri larvae. Based on the results d-limonene (60 g/l) and mineral oil (780 g/l) can be a viable alternative to other high-risk chemical pesticides. These two biological insecticides are effective in controlling the Opuntia cochineal scale and at the same time have no adverse impact on the predator C. montrouzieri.

Keywords: Dactylopius opuntiae; Cryptolaemus montrouzieri; Toxicity; Pesticides; Cactus; d-limonene; mineral oil

Introduction

The Opuntia cochineal scale Dactylopius opuntiae (Cockerell) (Hemiptera: Dactylopiidae) causes severe damage to prickly pear (Opuntia ficus-indica (L. ) Miller) in many countries around the world. This scale insect was first recorded in Morocco in 2015 (Bouharroud et al. , 2016). D. opuntiae feeds directly on the plant causing chlorosis and premature dropping of cladodes and fruits. Severe infestations (≥75 % of the cladode surface) can result in death of the plant (Vanegas-Rico et al. , 2010, 2015). In Brazil, the damage caused by D. opuntiae on O. ficus indica used as forage resulted in the loss of 100,000 ha, valued at 25 million US dollars (Lopes et al. , 2009). In Mexico, damage to fruit and nopalitos (edible young Opuntia cladodes) resulted in lower yields and higher production costs (Badii and Flores, 2001; Portillo and Vigueras, 2006).

According to Vanegas Rico et al. , (2010), natural enemies associated to D. opuntiae and other Dactylopiidae were only predators belonging to coleoptera, dipetra and lipedoptera. The predator, Cryptolaemus montrouzieri (Coleoptera: Coccinellidae) is native to Australia and commonly known as ‘mealybug destroyer’ with both adults and larvae prey on pests completely (Clausen, 1978). Because of its efficiency, this predator has been introduced in many countries for biological control of many mealybug species (Moore, 1988; Solangi et al. , 2012). The predator was reported to prey on citrus mealybug, Planococcus citri (Singh 1978), pink mealybug, Maconellicoccus hirsutus (Mani and Thontadarya, 1988), and the cochineal, Dactylopius tomentosus infesting the prickly pear Opuntia dilenii (Murali Baskaran et al. , 1999).

The lady beetle was introduced into Brazil for biological control of D. opuntiae, and primarily cochineal attacking cassava and citrus (Sanches and Carvalho, 2010). In Israel 100,000 C. montrouzieri were successfully released in infested areas in the north of the country (Protasov et al. , 2017). Recently, the mealybug destroyer was tested in Morocco and results stated on the biocontrol efficiency of D. opuntiae (Bouharroud et al. , 2018)As a chemical control of D. opuntiae the insecticides deltamethrin, methidathion and carbaryl have been registered for the control of this pest (Vermeulen et al. , 1990). However, the increasing resistance of D. opuntiae to insecticides, in addition to the other problems associated with the adverse effects of pesticides on humans, the environment and non-target organisms including natural enemies (Laznik and Trdan, 2014), requires the development of other safe options for the control of D. opuntiae. The aim of this study was to compare the biological insecticides to conventional insecticides for the control of D. opuntiae and to evaluate their side effect on the predator, C. montrouzieri.

Materials and methods

The study was conducted in the experimental field of the National Institute of Agronomic Research in 2017-2018. This area is located in Zemamra region, El Jadida (latitude N 32° 21, longitude W8° 22, and altitude 168m).

Insecticides toxicity on D. opuntiae

D. opuntiae was introduced from unsprayed stock colony to individual potted plants (10 month-old) of susceptible Opuntia ficus-indica in fine mesh cages. In this experiment, 11 insecticides were sprayed at 3 doses (X, 0. 5X, and 1. 5X, with X is recommended concentration) including water as a control (Table 1). To homogenate treatments and replicates, only 60 individuals of each of the first, second instar nymphs and adult females of D. opuntiae were kept alive on the cladodes using hand loupe and needle to remove extra nymphs and adults (Toorani et al. , 2017). For each cladode, a spray volume of 30 ml was sprayed using a laboratory sprayer to ensure full coverage. A completely randomized design with six replicates for each doses was adopted for this trial and the experiment was replicated five times (11 insecticides x 3 doses x 6 replicates x 5 experiments). Mortality of D. opuntiae females was evaluated at 24-hour time intervals and continued until 120 h after treatment, and the mortality of nymphs was evaluated at 24-hour after treatment (Damavandian, 2010).

Side effect on the predator C. montrouzieri

The predator C. montrouzieri colony was established from adults imported by the entomology laboratory of INRA, Agadir and was maintained for more than 30 generations. Adults were placed in entomological cages (80-80-80 cm) made of a wooden frame covered by a mesh fabric to allow ventilation. Access to water was provided via a cotton wick inserted into a 25 ml glass vial and feed based on D. opuntiae. For each treatment, 10 adults or larvae were introduced in netted D. opuntiae infested cladodes that were previously sprayed with 30 ml of each 3 doses of insecticides. Mortality of adults and larvae were evaluated at 24-hour time intervals and continued until 120 h after treatment. The experimental design adopted was completely randomized with three replicates of each doses and the experiment was replicated five times (11 insecticides x 3 doses x 3 replicates x 5 experiments).