Effect of Lethal and Sub-lethal Concentrations of Three Insecticides on Some Growth Parameters of Parasitoid Wasp, Habrobracon hebetor by Contact and Poisonous-host Method

Introduction: Population growth and the high food demand have led to more efforts to increase agricultural production. With making chemical pesticides, farmers were encouraged to use much chemicals, but their destructive effects on the environment, human and other organisms have been later revealed. Parasitoids are important natural enemies of crop pests. Most of them belong to order hymenoptera and superfamily Ichneumonoidea. The Braconidae is a family of parasitoid wasps and one of the richest families of insects. Nowadays, parasitoid wasp, Habrobracon hebetor Say has been widely used against lepidopteran larvae. Habrobracon hebetor is a well-known gregarious, idiobiont, ectoparasitoid of the larvae of a wide range of economically important moths infesting stored grains, nuts, and fruits as well as field crops worldwide. Due to overuse of chemical pesticides for larvae controlling, determining the side effects of insecticides on the biocontrol agents such as parasitoids is required.Materials and Methods: In the present study, we investigated the side effects of three insecticides on H. hebetor by direct (contact) and indirect (poisonous-host) methods. After rearing this parasitoid on the laboratory host, Ephestia kuhniella Zeller (Lep.; Pyralidae), the lethal and sub-lethal concentrations of insecticides were evaluated. Bioassay experiments were carried out under laboratory condition (25±2°C, 60±5% RH, 16L: 8D photoperiods) in five replicates and each replication included 30 parasitoids in contact method and 15 flour moth larvae in poisonous-host method. The lethal concentrations of these insecticides in contact method on the parasitoid were determined to be 1.38, 0.037 and 6.621 ml/L and on flour moth larvae were 0.490, 2.155 and 0.138 ml/L, respectively. For contact method, different concentrations of insecticides were applied on all inner sides of transparent plastic cup (4.5×8 cm) and air-dried. 15 pairs of 24-hours old parasitoids were introduced inside each cup. After 24 hours, one pair of alive parasitoids was introduced into cup with four last instar host larvae to oviposit. The host larvae were replaced daily. The parasitoid characteristics such as longevity and survival rate of different stages, TPOP and Ovi-day, the number of deposited eggs and sex ratio were then recorded. As to poisonous-host method, whole wheat flour (10 g) was mixed with 3.5 ml of each tested concentration of insecticides per each experimental set. After 72 hours, alive larvae were transferred in a plastic cup with one pair of H. hebetor wasp for 24 hours. Then, each larva was transferred separately in a petri dish with untreated flour. Different growth factors of parasitoids such as longevity and survival rate, TPOP and Ovi-day, the number of deposited eggs and sex ratio were recorded. Estimated LC30 and LC50 were considered as sub-lethal and lethal concentrations in all experiments, respectively.Results and Discussion: In contact method, the highest survival rate of pre-adult stages and adult female were observed in Sirinol treatment at lethal concentration, after control. In poisonous-host method, the highest survival ratio of the pre-adult stages belongs to Sirinol and in adult female the highest survival ratio was observed in lethal concentration of Sirinol. The lethal concentration of Proteus reduced pre-adult longevity of the parasitoid significantly, followed by lethal and sub-lethal concentration of Sirinol. Lethal and sub-lethal concentrations of Proteus also exhibited a significant reduction of total longevity. In poisonous-host method, all concentrations of all insecticides generally induced highly significant differences for ovi-day compared with that of control (17.91 days), as the shortest value was recorded for sub-lethal dose of Abamection (7.46 days) and the longest period was found for sub-lethal dose of Sirinol (13.2 days). In poisonous-host method, the longest TPOP was observed in sub-lethal concentration of Sirinol (16.02 days), followed by sub-lethal concentration of Abamection (14.63 days), and the shortest period was recorded in sub-lethal concentration of Proteus (12.3 days), after control (12.19 days). On the other hand, in contact method, the longest ovi-day and TPOP were observed in sub-lethal concentration of Sirinol (18.19 days) and lethal concentration of Abamection (13.26 days), respectively. The shortest adult female longevity was observed in lethal concentration of Proteus (27.55 days) in contact method and lethal concentration of Sirinol (22.85 days) in poisonous-host method. In all treatments, the number of deposited eggs was significantly reduced compared with control. The largest reduction in egg deposition was recorded for sub-lethal concentration of Proteus (71.05 eggs) in contact method and sub- lethal concentration of Abamection (36.14 eggs) in poisonous-host method, compared with control (227.47 eggs). It seems that Sirinol is suited to be used as a component of IPM alongside with H. hebetor.