The brown planthopper Nilaparvata lugens (Stål) is the most serious pest of rice across the world, especially in tropical climates. N. lugens nymphs and adults were exposed to high temperatures to determine their critical thermal maximum (CTmax), heat coma temperature (HCT) and upper lethal temperature (ULT). Thermal tolerance values differed between developmental stages: nymphs were consistently less heat tolerant than adults. The mean (± SE) CTmax of nymphs and adult females and males were 34.9±0.3, 37.0±0.2 and 37.4±0.2°C respectively, and for the HCT were 37.7±0.3, 43.5±0.4 and 42.0±0.4°C. The ULT50 values (± SE) for nymphs and adults were 41.8±0.1 and 42.5±0.1°C respectively. The results indicate that nymphs of N. lugens are currently living at temperatures close to their upper thermal limits. Climate warming in tropical regions and occasional extreme high temperature events are likely to become important limiting factors affecting the survival and distribution of N. lugens.

This study investigated the effects of sub-lethal high temperatures on the development and reproduction of the brown plant hopper Nilaparvata lugens (Stål). When first instar nymphs were exposed at their ULT50 (41.8°C) mean development time to adult was increased in both males and females, from 15.2±0.3 and 18.2±0.3 days respectively in the control to 18.7±0.2 and 19±0.2 days in the treated insects. These differences in development arising from heat stress experienced in the first instar nymph did not persist into the adult stage (adult longevity of 23.5±1.1 and 24.4±1.1 days for treated males and females compared with 25.7±1.0 and 20.6±1.1 days in the control groups), although untreated males lived longer than untreated females. Total mean longevity was increased from 38.8±0.1 to 43.4±1.0 days in treated females, but male longevity was not affected (40.9±0.9 and 42.2±1.1 days respectively). When male and female first instar nymphs were exposed at their ULT50 of 41.8°C and allowed to mate on reaching adult, mean fecundity was reduced from 403.8±13.7 to 128.0±16.6 eggs per female in the treated insects. Following exposure of adult insects at their equivalent ULT50 (42.5°C), the three mating combinations of treated male x treated female, treated male x untreated female, and untreated male x treated female produced 169.3±14.7, 249.6±21.3 and 233.4±17.2 eggs per female respectively, all significantly lower than the control. Exposure of nymphs and adults at their respective ULT50 temperatures also significantly extended the time required for their progeny to complete egg development for all mating combinations compared with control. Overall, sub-lethal heat stress inhibited nymphal development, lowered fecundity and extended egg development time.

1 The influence of acclimation on the cold and heat tolerance of Nilaparvata lugens was determined by measurements of the critical thermal minimum and maximum (CT min and CT max ), chill and heat coma temperature (CCT and HCT) and lower and upper lethal temperature (LLT 50 and ULT 50 ). 2 First-instar nymphs were acclimated for 5 days at 15 • C and for 2 days at 30 • C and compared with a population maintained at 23 • C; for the adult comparisons, first-instar nymphs were reared at 15, 23 and 30 • C until adult emergence, requiring development periods of 50-55, 30-35 and 18-20 days, respectively. 3 The thermal tolerance limits of both age groups changed significantly with acclimation and were correlated with rearing temperature. 4 Across the 48 separate measurements of thermal tolerance (CT min , CCT, CT max , HCT, LLT 50 and ULT 50 of nymphs and adult males and females), the temperature differential across the three indices of cold tolerance after acclimation at 15 • C compared with a population maintained at 23 • C were between 0.5-2.8 and 2.9-5.0 • C for nymphs and adults, respectively. By comparison, acclimation at 30 • C increased heat tolerance in terms of changes in the CT max , HCT and ULT 50 , and the temperature differentials compared with the 23 • C population were between 1.1-3.3 • C for nymphs and 0.3-1.6 • C for adults. 5 These data indicate that, under the acclimation regimes applied to N. lugens, increases in cold tolerance were greater than heat tolerance.

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