A comparative study on product index (PI) of lady beetles, Hippodamia variegata and Coccinella septempunctata feeding on wheat green aphid, cabbage aphid and melon aphid under laboratory conditions

A useful parameter to assess the combined influence of diet and temperature is product index (PI). The objective of this study was to determine which prey species and temperature provide favorable conditions for optimal laboratory propagation of Hippodamia. variegata and Coccinella. septempunctata. For this purpose, product index of above mentioned lady beetles was studied on wheat aphid (Schizaphis graminum), cabbage aphid (Brevicoryne brassicae) and melon aphid (Aphis gossypii) under laboratory conditions. Each diet treatment was monitored at 26°C and 30°C, photoperiod 16L:8D and a relative humidity of 65±5%. The initial number of first instars larvae in each treatment (two temperatures and three preys) was 20. Each beetle larvae was placed in a 60×10 mm plastic Petri-dish and supplied daily with fresh third nymphal instars of aphid (nymphs taken directly hosts foliages) and larval development was observed every 24 h. Adult predators after eclosion in each treatment were weighed using an electronic balance with a resolution to 0.001g. Product index was determined using Pl_dt = [(log _e WT) / T ] × S  where PI_dt is the index for production according to diet (d) and temperature (t), WT represents live weight (mg) of adult, T represents larval developmental time (days) and S represents survival percentage. Larval period of mentioned treatments showed significant differences in H. variegata. In the seven spotted ladybeetle, prey treatments were significantly different and in treatment of temperature, only in treatment of the melon aphids, the difference was significant. The weight of adults H. variegata except for one case (the cabbage aphid treatment) showed significant differences and in C. septempunctata all treatments significantly were different. The shortest larval period in both ladybeetles obtained at 30°C when fed on melon aphid and the longest larval period occurred when the cabbage aphid was used for feeding at the temperature of 26°C. The highest production indices, 41.3 and 43.12 were calculated for H. variegata and C. septempunctata fed on A. gossypii at 30°C, respectively. The lowest product index was obtained for H. variegata (12.01) and C. septempunctata (17.31) when beetles larvae were fed on B. brassicae at 26°C.