The Intraguild Predation Among Propylea Japonica (Thunberg), Coccinella Septempunctata L.and Harmonia Axyridis (Pallas)

Along with the further study of coccinellidae predatory behavior, it is universally believed that there is intraguild predation (IGP) among ladybeetles. The phenomenon has imposed a complicated and enormous influence on the population dynamics of species in the agro-ecosystem as well as the effect on the pest bio-control. There are of a great variety of species in coccinellidae and various methods have been adopted to identify their preys. Regular methods, such as midgut dissection, behavior observation and other traditional research methods cannot evaluate the intraguild predation of the coccinellidae accurately. The technology of polymerase chain reaction (PCR) with its merits of high specificity, sensitivity, efficiency and convenience has gained an important position in the field of species identification.Propylea japonica (Thunberg), Coccinella septempunctata L. and Harmonia axyridis (Pallas) are three common predatory enemies in the agro-ecosystem of North China. In this research, we studied the intraguild predation through laboratory and field experiment and also accessed the DS50values of the three ladybeetles in different predators by developing a fast and accurate molecular detection method based on PCR technology. The research offers a method for accurate identification of coccinellidae species and theoretical evidence for scientific evaluation. The major results are listed as follows.1. Intraguild predation measurement in laboratoryThe intraguild predation among P. japonica, C. septempunctata and H. axyridis measurement was conducted in non-choice, choice and different development stages predation tests. In the non-choice test, the4th instar larvae of the three tested coccinellid species showed same predation pattern. All of them had their largest predation amount on P. japonica eggs, C. septempunctata eggs in the second place and least on H. axyridis eggs. C. septempunctata and H. axyridis4th instar larvae had similar predation capacity on coccinellidae eggs and P. japonica4th instar larvae had the poorest capacity. In the choice test, all larvae preferred P. japonica eggs to H. axyridis eggs except the H. axyridis larvae. The size of the4th instar larvae and eggs go in same order H. axyridis>C. septempunctata>P. japonica from the largest to the smallest. The larger the predator is, the stronger capacity it owns. The smaller the prey is, the larger possibility of being eaten it has. It may in some extent explain the results of the tests above. The1st instar larvae predated the least amount of coccinellidae eggs. The predatory capacity increased with the development of the ladybeetles and reached their largest predation around2-day-old adult stage, then declined. 2. Detection of intraguild predation in fieldsThe specific primers of P. japonica, C. septempunctata and H. axyridis were designed according to the CO I gene. The specificity of the primers were detected against the species appearing in the habitats of target ladybeetles. Through the specificity and sensitivity detection, we finally got one appropriate pair of specific primers for each ladybeetle. In the DS50evaluation, we set6predator x prey combinations, namely C. septempunctata larvae x H. axyridis eggs, C. septempunctata larvae×P. japonica eggs, H. axyridis larvae×C septempunctata eggs, H. axyridis larvae×P. japonica eggs, P. japonica larvae xC. septempunctata eggs and P. japonica larvae×H. axyridis eggs. The larvae were frozen with liquid nitrogen after feeding at Oh,4h,8h,12h,16h,20h and24h and sampled for further detection. The DS50values were calculated according to the prey DNA detectability and the results varied from0.9h to11.8h. With the species-specific primers and the DS50values, we detected the field-collected samples and revised the data. The detected rates were different among different predator x prey combinations,9.1%in C. septempunctata×P.japonica,26.7%in H. axyridis×P. japonica,0.0%in P. japonica×C. septempunctata,0.6%in H. axyridis×C septempunctata,53.2%in P. japonica×H. axyridis and72.7%in C. septempunctata x H. axyridis. And the reviesed data were1.09%、3.16%、0.00%、0.21%、4.06%and7.52%respectively. It proved that there was indeed intraguild predation among ladybeetles in the field. The detected rates showed the similar trend with the eggs amounts in the field, especially in the H. axyridis×P. japonica and P. japonica×H. axyridis combinations.