Can Tropical Insects Stand the Heat? A Case Study with the Brown Planthopper Nilaparvata lugens (Stål)

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

Effects of thermal stress on the brown planthopper Nilaparvata lugens (Stal)

This study investigated the effects of heat stress on the survival, mobility, acclimation ability, development, reproduction and feeding behaviour of the brown planthopper Nilaparvata lugens. The critical information derived from the heat tolerance studies indicate that some first instar nymphs become immobilized by heat stress at around 30°C and among the more heat tolerant adult stage, no insects were capable of coordinated movement at 38°C. There was no recovery after entry into heat coma, at temperatures around 38°C for nymphs and 42-43°C for adults. At 41.8° and 42.5oC respectively, approximately 50% of nymphs and adults are killed. In a comparison of the acclimation responses between nymphs and adults reared at 23°C and acclimated at either 15 or 30°C, the data indicate that increases in cold tolerance were greater than heat tolerance, and that acclimation over a generation compared with a single life stage increases tolerance across the thermal spectrum. The temperatures that kill around 50% of nymphs and adults also exert negative effects on development and longevity. The same exposures also lower fecundity and extend egg development time through a combination of mating groups, in which the greatest effects occur when both males and females have experienced sub-lethal heat stress. Likewise, exposure to their ULT50 reduced feeding activity in both life stages of N. lugens. The amount of honeydew excreted by females and males in the treated nymph and adult groups were 3-4x and 2-3x lower than in the equivalent control groups. Overall, sub-lethal heat stress extended egg development time, inhibited nymphal development, lowered fecundity and reduced feeding activity

Heat Stress Impedes Development and Lowers Fecundity of the Brown Planthopper Nilaparvata lugens (Stål)

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 ULT(50) (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 ULT(50) 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 ULT(50) (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 ULT(50) 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

Mean number of eggs per female after exposure of first instar nymphs and adults of <i>Nilaparvata lugens</i> at their ULT₅₀.

<p>N = 20 pairs for each mating combination. Mean values with the same letter are not significantly different at p<0.05 level.</p

Thermal activity thresholds of different life cycle stages and sexes of <i>N. lugens.</i>

<p>Mean (± SE) CT_max (A) and HCT (B). Mean values with the same letter are not significantly different (<i>p</i>≤0.05); n = 20 for first instar nymphs, adult females and males.</p

Mean (± SE) ULT₅₀ of first instar nymphs and adults of <i>N. lugens</i>.

<p>Mean values with the same letter are not significantly different (<i>p</i>≤0.05); n = 50 at each exposure temperature.</p

Temperature range of thermal activity thresholds of different life cycle stages and sexes of <i>N. lugens.</i>

<p>Changes in the CT_max (A) and HCT (B) for first instar nymphs (white bars), adult females (cross-hatch bars), and adult males (black bars); n = 20 for each life cycle stage.</p

Range of development times for adults of <i>Nilaparvata lugens</i> after exposure as first instar nymphs at the ULT₅₀.

<p>N = 50 for control and treated groups (gender ratio as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047413#pone-0047413-g001" target="_blank">Figure 1</a>).</p