Behavioural Response Of Bird Cherry-Oat Aphid, Rhopalosiphum Padi, To Temperature Gradients

Insects, as free-moving living organisms, can search for favourite temperatures by active crawling or flying under natural conditions. However, the tested insects were forced to live under uniform temperature conditions in most experiments for studying effects of constant or alternative temperatures on insects. Results from those experiments cannot demonstrate what are the favourable temperatures for those insects in nature. Investigations on thermotactic behaviours of insects in temperature gradients can show preference of insects to the optimal temperature, and it is useful for understanding spacial dynamics, improving prediction techniques and innovating new control methods of those insects. Investigation on the behavioural activity of heat-avoidance and favourable temperature of Rhopalosiphum padi were conducted with temperature gradient apparatus, and subsequently on the avoidance limit-temperature of cereal aphids Rhopalosiphum padi, Sitobion avenea and Schizaphis graminum during crawlling and feeding on the wheat leaves with temperature gradient and increasing temperature, respectively. The main results displayed as follows:1 The behaviour of heat-avoidance of R. padi was confirmed. When aphids moved near to the heated wall of the temperature gradient fields, they turned back to the unheated wall suddenly.2 The upper avoidance limit-temperatures (uALT) for crawlling and feeding of the cereal aphids R. padi, S. avenea and 5. graminum were revealed. Temperatures at which the aphids turned back were defined as the avoidance limit-temperatures for crawling, when the aphids moving along the temperature gradient from the cool side to the hot side. Similarly, temperatures at which the aphids began to struggled were defined as the avoidance limit-temperatures for feeding, when the aphids feeding on the increasingly heated plant leaves. The uALT of R. padi, S. avenea and 5. graminum were 42.0℃, 39.1T;and 38.5℃, respectively, for crawlling;and 39.3℃, 40.2℃ and 39.0℃ for feeding.3 The upper inhabitation limit-temperatures (uILT) of R. padi were demonstrated. uILT were defined as the temperature at which the accumulative percentage of aphids distributed in the temperature gradient was P_accumulative In leaf temperature gradient (26-43℃), the uILT of 50%, 75% and 95% were 27.7℃, 29.7℃ and 31.9℃, respectively.4 The activity of heat-avoidance of R. padi was impacted significantly by temperature gradient and acclimation temperature. The heat-avoidance percentage (HAP) of R, padi was increased significantly along with the increasing temperature of the heated wall of the temperature gradient fields, synchronously. The HAP of R. padi reached to 52.5%, 72.5%, 92.1% and 100% when the temperature gradient were setted as 21-22.7℃, 32²3.8℃, 37²5.3℃ and 42²7.7℃, respectively. Acclimating adults at 15℃ for 1.5h had a higher HAP (77.50%) than those at 35℃ for 1.5h (64.17%), suggesting the ability of R. padi to adapt to high temperature.The results from this article may be useful to not only improving the survey method of sampling the cereal aphids in the fields, but also enhancing the accuracy of the forecast of the infestation of cereal aphids. Furthermore, the method of this study may be valuable for investigating on other insects. Exceptfor temperatures, the activities of cereal aphids in the nature are also impacted by foods, sunlight, humidity and density etc. Therefore, what the relationship between the percentage of individuals of cereal aphids that moving from one place to another and the fluctuated temperatures in the fields is? And what the key factors which affect the relationship mentioned before are? These questions should be taken into account in the future.