Laboratory evaluation of different insecticides against Brevicoryne brassicae and its parasitoid, Diaeretiella rapae (Hymenoptera: Braconidae: Aphidiinae)

The toxicity of six insecticides (Advantage, Pyramids, Polo, Imidacloprid, Match and Mospilon) on Brevicoryne brassicae (Hemiptera: Aphididae) and its parasitoid, Diaeretiella rapae (Hymenoptera: Braconidae) were evaluated under constant laboratory conditions (25±2 oC; with 65-70% R.H.). All the tested insecticides were lethal against B. brassicae with the passage of time at highest concentration. Maximum mortality was observed in Polo and minimum in Mospilon followed by Advantage, Pyramid, Match and Imidacloprid. All the LC50s were lethal against its parasitoid with maximum mortality at 48 HAT (Hours after treatment) and minimum in all insecticides at 12 hours after treatment. The maximum mortality (100%) were observed at 48 HAT in Polo and Advantage followed by Imidacloprid and Pyramid (93.33%), Match (92.33%) and Mospilon (86.67%). The minimum mortality of D. rapae was recorded at 12 HAT in Mospilon (36.67%) as compared to other insecticides. observed harmful (class IV) but only Mospilon was moderately harmful (class III) to D. rapae. Maximum survival ship of D. rapae was observed in Pyramid after 48 HAT and minimum in Polo. In general, D. rapae exhibited the highest sensitivity to all evaluated insecticides. The insecticide Polo showed maximum and minimum mortality of aphid was observed in Mospilon application. In case of parasitoid mortality, the maximum mortality was observed at LC50 of Polo ® and minimum at Mospilon and trend of parasitoid mortality was that, Polo > Advantage > Match > Pyramid > Imidacloprid > Mospilon.

prefers feeding on young leaves and flowers and feeding deep into the heads of brussels shoots and cabbage [6-8]. Use of insecticides to control the insect pests in crops is common all over world. Managing population of cabbage aphid is not an easy practice although synthetic insecticides are speciously accessible for usage. However for the control these insect pests, the use of insecticides is fashioned with several disadvantages such as development of insecticide resistance and hazards to nontarget organisms including human [9, 10]. Application of synthetic pesticides and botanicals are harmful to natural enemies present in field. Due to use of pesticides, the parasitoid survival range, reproductive rate and suitability of host is reduced. These are great threat to natural enemies as well as for pollinators [11]. For the protection of agricultural and surrounding habitats, evaluation of side effects of insecticides on natural enemies is necessary for pesticide registration [12]. Some  In agriculture crops, parasitoids and predators generally contribute the reduction in pest population. Keeping in view the importance of beneficial insects and the ill effects of pesticides to these bio-control agents the present studies were carried out. The aim of present study was to evaluate the side effects of few insecticides used for pest control in canola fields on canola aphid and its parasitoid. The more basic aims were to obtain more data regarding survival ship of parasitoid on the side effects of selected insecticides against parasitoid and prove to investigate the suitability of these insecticides for use in integrated pest management (IPM) programs.

Materials and methods Aphid parasitoid (Diaeretiella rapae)
Population of parasitoids was maintained at 25±2 °C temperature and 65-70% relative humidity in plastics cages. The culture was fed by 10% honey solution and aphid population was offered for maintenance of culture. The reared parasitoids were used for the investigations.

Canola Aphid (Brevicoryne brassicae)
Colonies of B. brassicae were collected from canola fields and screened out for the presence of parasitoid(s). Aphids were transferred to glass cages containing 10 plants at least. The colony was maintained at 24±2 °C with photoperiod of 16:8 (L: D). Plants were replaced at weekly interval [21]. Insecticides Different concentrations of insecticides were used against B. brassicae under laboratory conditions to check their efficacy. Detail of insecticides used is given in (Table 1).

Bioassays
To check the effectiveness of insecticides leaf dip bioassay method was used. Leaves were dipped in each concentration for 10 seconds and were completely dried for 15 minutes at room temperature. A slightly moistened filter paper was also placed in petri dishes to keep the leaf material turgid. Twenty apterous aphids were released in each petri-dish on these leaflets using fine camel hair brush. Experiment was repeated three times.

Corrected mortality = 1 -n in T after treatment x 100 n in Co after treatment
Whereas, T= treated and Co= control

Statistical analysis
Data for percent corrected mortality were analyzed using three factor factorial design under CRD to check the significance of insecticides. LC50 value for each insecticide was calculated by Probit analysis.

Impact of insecticides on Diaeretiella rapae
Newly emerged adult parasitoids were tested against LC50 of each insecticides selected from previous experiment. Detail of LC50 of each insecticide is given in (Table 2).

Procedure for bioassay
To check the effect of above mentioned pesticides on the aphid parasitoid, bioassay were carried out into petri dishes. Untreated filter papers were cut into half of the diameter of the petri dish with the help of a scissor.
Half of petri dish left without filter paper to provide insecticide free environment to the parasitoids. Systemic insecticidal dilutions (LC50) were sprayed through injector on these filter papers. Five pairs of newly emerged adult parasitoids were released in the petri dishes. Artificial food as cotton soaked with 10 % honey solution was also provided. The % corrected mortality of aphid parasitoids were calculated after 12, 24 and 48 hours using Abbott's formula;  (Fig.1). The results showed that all the concentrations of all insecticides had lethal impact to B. brassicae. Concentration one of all pesticides showed maximum mortality of B. brassicae. Maximum mortality was observed in Polo ® and was observed in Mospilon ® at first concentration followed by Advantage ® , Pyramid ® , Match ® and Imidacloprid ® . In all tested insecticides the minimum mortality were observed at fifth concentration. It is evident from the results that all the insecticides showed maximum mortality by increasing the concentration of the insecticides.  (Table 3). These concentrations showed that the toxicological observations under laboratory conditions. The values for Imidacloprid (70 WS), Lufenuron (0.50 EC), Carbosulfan (20 EC), Difenthiuron (500 SC), Nitanpyram (10 SL) and Acetamiprid (20 SP) were 0.037 g, 0.062 ml, 1.07 ml, 0.033 ml, 0.069 ml and 0.08 g respectively. LC50 values of Carbosulfan (20 EC), Difenthiuron (500 SC), Nitanpyram (10 SL) and Acetamiprid (20 SP) were observed less toxic to B. brassicae. The observation of all insecticides varied significantly as the fiducial limits for tested pesticides overlap each tothers. The regression parameters showed that contribution of Imidacloprid (70 WS), Lufenuron (0.50 EC) to the mortality of B. brassicae was highly significant and positive.

Effect of different insecticides on the mortality of D. rapae at different time intervals
All the insecticides showed maximum mortality at 48 HAT (Hours after treatment) and minimum mortality at 12 hours after treatment. Maximum mortality (100%) were observed at 48 HAT in Polo ® and Advantage ® followed by Imidacloprid ® and Pyramid ® (93.33%), Match ® (92.33%) and Mospilon ® (86.67 %) (Fig. 2).The minimum mortality of D. rapae was recorded at 12 HAT in Mospilon ® (36.67 %) as compared to other insecticides. Our findings showed that parasitoids are more sensitive to chemical insecticides than aphids.