Muhammad Adnan Bodlah

Host choice, settling and folding leaf behaviors of the larval rice leaf folder under heat stress.

Extreme high-temperature events are the key factor to determine population dynamics of the rice leaf folder, Cnaphalocrocis medinalis (Guenée), in summer. Although we know that adult of this insect can migrate to avoid heat stress, the behavioral response of larva to high temperature is still unclear. Therefore, impacts of high temperature on behavioral traits of C. medinalis including host choice, settling and folding leaf were observed. The results revealed that these behavioral traits were clearly influenced by high temperature. The larvae preferred maize leaves rather than rice and wheat at normal temperature of 27°C, but larvae experienced a higher temperature of 37 or 40°C for 4 h preferred rice leaves rather than maize and wheat. Capacity of young larvae to find host leaves or settle on the upper surface of leaves significantly reduced when they were treated by high temperature. High temperature of 40°C reduced the leaf-folding capacity of the third instar larvae, but no effects were observed on the fourth and fifth instar larvae. Short-term heat acclimation could not improve the capacity of the third instar larvae to make leaf fold under 40°C.

Behavioural adaptation of the rice leaf folder Cnaphalocrocis medinalis to short-term heat stress

Under ongoing climate warming, both the degree and number of high-temperature events in summer may increase, and behavioural adaptation is an important ecological strategy employed by insects to cope with such events. The rice leaf folder, Cnaphalocrocis medinalis Güenée, is a serious insect pest of rice fields in summer. Population outbreaks have become more frequent in the last ten years. In addition to adult migration, rice leaf folders are thought to have other thermal adaptations. Therefore, the behaviours of larval and adult rice leaf folders, such as leaf folding (making shelter) and habitat selection for pupae and eggs, were observed on rice plants under heat stress. The results showed that larval shelter-making velocities significantly decreased during or after four hours of heat exposure, and shelter size decreased as the temperature increased. Larvae preferred to pupate on young rice leaves at 27°C and middle-aged leaves at 30°C, but they strongly preferred older leaves when reared at 34°C. Female moths generally preferred to oviposit on the top of young leaves, but they preferred the middle and lower leaves for egg deposition when exposed to 36 and 40°C, respectively. Furthermore, more eggs were distributed on the lower surfaces of rice leaves with an increase in heat stress. These behavioural responses of rice leaf folders to heat stress indicate that this pest has great potential to adapt to high temperatures; therefore, the possibility of a population outbreak will remain high despite global warming.

Rice Leaf Folder Larvae Alter Their Shelter-Building Behavior and Shelter Structure in Response to Heat Stress

Abstract Behavioral thermoregulation is a key strategy for insects to cope with heat stress.The rice leaf folder Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae) larvae usually fold one leaf to construct a leaf shelter. The larvae are vulnerable to heat stress, and the temperature in summer is often beyond the optimal range of them. Shelters confer protection against environmental stress but unclear whether larvae will alter shelter-building behavior when encountering heat stress. We observed the shelter-building behavior of larvae during and after heat shock, and then examined the shape and structure of shelters. Larvae spent more time in selecting a site and building a shelter during and after heat shock than at the optimal temperature. More than 70% of larvae folded two or three leaves to build a shelter during and after heat shock, but more than 60% of larvae only folded one leaf at the optimal temperature. Larvae built more single-leaf longitudinal shelters at the optimal temperature, but they built more multileaf overlapping shelters during and after heat stress. Larvae constructed a short leaf shelter using a small amount of silk binds when they were exposed to 40°C for 4 h. The rice leaf folder larvae can alter their shelter-building behavior and shelter structure in response to heat stress.