Effect Of Suppression Of Natural Enemies On Wheat Aphids And Cotton Aphids And Cold Tolerance Of P. Japonica And H. Axyridis

Aimed to reasonable use the biological control function of natural enemies, in agroecosystem of Northern China, its biological control to wheat aphids and cotton aphids was evaluated by caged method and systemic investigation under field conditions, and the seasonal fluctuation of cold tolerance of the two ladybirds were also measured.(1) P. japonica and H. axyridis were the most important predators in wheat fields and cotton fields either. The natural enemies could effectively suppress wheat aphid populations, and the bio-control services indices were 35%, 42%, and 32% at the steady, peak, and decline period of wheat aphid populations, respectively. The densities of cotton seedling aphids were always below the economic threshold at the most sampling dates, and the bio-control services exceeded 91%. However, since the populations of natural enemies were too low to control summer cotton aphids and the bio-control services indices were always lower than 20%. We suggested that protection measures to natural enemies should be conducted in the occurrence of wheat aphids and cotton seedling aphids, while measures of enhancing natural enemies’ populations by habitant management should be conducted before the occurrence of cotton summer aphids.(2) We also observed that color patterns and body weight had significant effects on the cold tolerance of H. axyridis, while gender had not significant effects. Further analysis showed that there were significant differences of cold tolerance among the ladybirds with different bottom color, and no significant differences among those with same bottom color while different spot numbers were detected. Body weight was positively correlated with cold tolerance, and the individual with more weight had more cold tolerance.(3) Cold tolerance of H. axyridis and P. japonica were fluctuant seasonally. The maximum cold tolerance of H. axyridis was detected during the overwintering period. Its water content of body, enzyme activities of SOD and CAT increased at first and then reduced. However, protein content was continuously decreased across the season. Similar to H. axyridis, body water content and the enzyme activities of SOD and CAT of P. japonica increased at first and then decreased. Contrary to H. axyridis, protein content of P. japonica was continuously increased across the season. These results indicated there might be different chemical adaption to the temperature change between the two ladybirds.