Lethal Effects of Fungi Beauveria bassiana (Bals.) and Nanosilica on Cabbage Aphid Brevicoryne brassicae (L.) and Its Parasitoid Diaeretiella rapae (McIntosh) in Laboratory Conditions

Cabbage aphid, Brevicoryne brassicae (L.), is one of the most important and harmful pests of crucifers, which is active in many parts of Iran and causes quantitative and qualitative damage in the cultivation of these plants during the winter. Some natural enemies such as the parasitoid wasp Diaeretiella rapae (McIntosh) are active on this pest. The indiscriminate use of insecticides for controlling pests caused evolution of pest resistance, environmental pollution and negative effects on natural enemies and beneficial insects. In recent years, environmentally compatible factors have received more attention due to less side effects in pest control. This study was carried out to determine the effects of the entomopathogenic fungus Beauveria bassiana (Bals.), and nanosilica separately and in combination on survival of the cabbage aphid and its parasitoid treated by different concentrations of fungus (103, 104, 105, 106 and 107spore/ml) and nanosilica (125, 250, 500, 1000 and 2000 ppm).

Cabbages (Brassica olearaceae var. capitata) were grown from seed in plastic flower pots. Aphids were collected from Brassicaceae plants in fields of Nazlo Campus of Urmia University near Urmia city. This colony was used as a source for all aphids used in all laboratory experiments. Also, a colony of D. rapae was reared on B. brassicae under greenhouse conditions. Wasps were fed a 10% honey solution and used to parasitize aphids, or for use in experiments, 24 hours later.Fungi was cultured on Sabouraud Dextrose Agar (SDA) in petri dishes (6 cm in diameter) and incubated for two weeks to induce sporulation. Spores were harvested from the surface of 14-day old culture and transferred to a test tube containing sterilized distilled water with 0.05% Tween®-80. The conidial concentration of final suspension was determined by direct count using a neubauer hemocytometer and serial dilutions were made to obtain different conidial concentrations. Sio2 nanoparticles of pashgamannano® company was used in bioassay tests. Bioassay test was carried out on aphid and its parasitoid. Aphids were treated by conidial concentrations using dipping method. Control adults were treated with distilled water containing 0.05% Tween®-80. In order to evaluate the effect of nanosilica on aphids, leaf discs were dipped in to different concentrations of nanosilica and after drying, adult cabbage aphids were transferred on treated leaf discs in Petri dishes. The mortality was counted 24, 48 and 72 hours after treatment. Experiments were carried out at 25±2 Co temperature, 65±5% RH and a photoperiod of 16: 8 h. (L: D) under laboratory conditions. LC25, LC50 and LT50 values of fungi and nanosilica were estimated. Also, combination effects of fungi and nanosilica were evaluated in laboratory. A preliminary test was evaluated to determine side effects of different concentrations of nanosilica on viability and germination of fungi spore. Germination rate for fungi was calculated after 24 hours. The LC25, LC50 and LT50 values (with 95% confidence limits) were calculated using the Probit analysis method. Bioassay data were subjected to one-way analysis of variance (ANOVA) after checking for normality and means were compared by Tukey’s test.

The LC50 values obtained from the analysis of probit as a result of the effect of different concentrations of B. bassiana 7 days after treatment and nanosilica 24, 48 and 72 hours after treatment on cabbage aphid were 2.2×105 spore/ml and 4356.14, 1164.42, 487.19 ppm, respectively. Furthermore the LC50 values of B. bassiana 4 days after treatment and nanosilica 24, 48 and 72 hours after treatment on D. rapae were 1.2×106 spore/ml and 3639.79, 619.51, 289.77 ppm, respectively. Results showed that the adults of the cabbage aphid were completely sensitive to the fungal isolate. The shortest time needed for the mortality of 50% of the population of cabbage aphid was 5.02 days at 107 conidia/ml concentration. The mortality rate of nanosilica at highest concentration, 72 hours after treatment was 90 and 96.6% respectively for cabbage aphid and parasitoid wasp. Also, combination of these factors against adults of cabbage aphid had significant difference with control.

According to the results of this study, it can be concluded that B. bassiana and nanosilica are effective on cabbage aphid and can be used as effective factors in the integrated pest management program of this pest.