Effects Of Heat Stress On Survival,Host Selection And Leaf Rolling Behavior Of The Rice Leaf Folder

The rice leaf folder Cnaphalocrocis medinalis is a serious insect pest of rice and widely distributed in the rice growing regions of the world.In China,rice leaf folder generally occurs in the warmer part of a year.There have been many studies on the thermal impacts on the rice leaf folder population development,occurrence,survival,and reproduction.It was observed that development or survival of egg,larvae,pupae and adult were confined to a specific range of the high temperature.Favourable rearing temperature for the rice leaf folder was between 26-27℃.The copulation frequency during mating behavior was decreased and moth produced less or no eggs when experienced higher temperature stress.Moreover,summer temperature in the Eastern and Southern China can be more than 36℃which could be adverse for behavioral decisions of the rice leaf folder.Every year,high temperature triggers the migratory behavior of the rice leaf folder during summer.But severe outbreaks of the rice leaf folder have been observed.Therefore,we presumed that the rice leaf folder would have ecological strategies to cope with the high temperature.In this study,larval feeding behavior and microhabitat selection under heat stress were studied.The results will provide us with vital information on behavioral responses of the rice leaf folder to high temperature.1.Effects of heat stress on pupation rate and reproductionThe larval pupation rate was tested by exposing the third instar larvae till pupation under 27℃,30℃,and 34℃.Larval pupation rate was significantly decreased when larvae were reared at high temperatures 30 and 34℃.The survival rate of larvae was the lowest at 34℃.One day old male and female adults were exposed to 34,36 and 40 ℃ for 4h per day for 1,2 and 3 successive days,and their eggs produced in 48h after heat stress exposure was significantly decreased when adults were exposed to 40℃.2.Effects of heat stress on host plant choice of larvaThe larvae were heat stressed at 34,37 and 400℃ in climate chambers for 4 h with constant light and 60-70%RH.After the heat stress,three choice tests were performed by arranging the leaf cuts of rice,maize and wheat in a petri dish.Host plant choice and settling choice of larvae were significantly depended on heat stress.The rice leaf folder larvae preferred maize leaves to rice and wheat at the normal temperature 27℃,but the preference for rice leaves became stronger after exposure to high temperature.Larvae preferred to settle on the lower leaf surface and frequency not to find a leaf was increased with the rise of temperature.3.Effect of heat stress on building shelter of larvaLeaf selection time,shelter-making time,the length of the leaf shelter,number of linked leaves and number of silk binds in a shelter were observed when larvae were in or after heat stress at 34,37，and 40℃.Leaf shelter making activity of the rice leaf folder was decreased with the increase of temperature.Leaf selection time of the third and fifth instar larvae was increased with the increase of heat stress.Larvae would link more leaves to build a shelter under heat stress and built more random shelters.The leaf rolls made by larvae under heat stress at 34,37 and 40℃ were shorter than those made at 27℃ without heat stress.The number of silk binds in a shelter made by the third instar larvae was significantly decreased when they experienced heat stress at 34,37,40℃.The thermal acclimation for the larval was performed before observation of the leaf folding behavior at high temperatures.The short-term heat acclimation could not improve the capacity of the 3rd instar larvae to make leaf folds at 40 ℃,and the numbers of leaf fold were still significantly less at 40℃ than that at 27,30,32,34 and 370C.4.Effects of heat stress on microhabitat choiceThe 3rd and 5th instar larvae were released on maize or wheat seedlings and placed at 27℃ till the completion of leaf folds.After leaf fold completion,larvae in the leaf rolls were exposed to 34,37,and 40℃ for 4,5,or 6h intervals.After each hours heat stress,larvae dropping or staying in or out of the leaf roll were counted.Larvae in the wheat leaf rolls would move out of leaf rolls when they were exposed to 34,37,and 40℃ for 4,5,and 6h,whereas the larvae settling in the maize leaf rolls would not move under these heat stress.Larvae from the third instar were reared on rice leaves under heat stress.The results showed that larvae clearly preferred for the top young leaves(L1-L3)of a rice plant to pupate when reared at 27℃,and a preference for the middle-aged leaves(L3-L5)at 30℃(4h per day),but a strong preference for the lower older leaves(L4-L6)at 34℃.One day old adults were exposed to heat stress at 34,36 and 40℃ for four hours per day for 1,2 and 3 successive days.After heat stress,adults were released on rice leaves and egg deposition frequency on the rice leaves(L1-L6)was observed.The results showed that adults of rice leaf folder move towards the lower older leaves of rice to oviposit when encountered heat stress,and more eggs were oviposited on the down surface of rice leaf.