Abstract
The direct effects of rising global temperatures on insect herbivores could increase damage to cereal crops. However, the indirect effects of interactions between herbivores and their biotic environment at the same temperatures will potentially counter such direct effects. This study examines the potential for intraspecific competition to dampen the effects of optimal temperatures on fitness (survival × reproduction) of the brown planthopper, Nilaparvata lugens [BPH] and whitebacked planthopper, Sogatella furcifera [WBPH], two phloem-feeders that attack rice in Asia. We conducted a series of experiments with increasing densities of ovipositing females and developing nymphs on tropical and temperate rice varieties at 25, 30 and 35°C. Damage from planthoppers to the tropical variety was greater at 30°C compared to 25°C, despite faster plant growth rates at 30°C. Damage to the temperate variety from WBPH nymphs was greatest at 25°C. BPH nymphs gained greater biomass at 25°C than at 30°C despite faster development at the higher temperature (temperature-size rule); however, the effect was apparent only at high nymph densities. WBPH survival, development rates and nymph weights all declined at ≥ 30°C. At about the optimal temperature for WBPH (25°C), intraspecific crowding reduced nymph weights. Temperature has little effect on oviposition responses to density, and intraspecific competition between females only weakly counters the effects of optimal temperatures on oviposition in both BPH and WBPH. Meanwhile, the deleterious effects of nymph crowding will counter the direct effects of optimal temperatures on voltinism in BPH and on body size in both BPH and WBPH. The negative effects of crowding on BPH nymphs may be decoupled from resource use at higher temperatures.
Highlights
Global temperatures have increased by between 0.5 and 0.9 ̊C since the 1960s and are predicted to continue rising under current global CO2 emissions [1, 2]
We examine the effects of temperature on intraspecific competition in brown planthopper (BPH) and whitebacked planthopper (WBPH) and examine the relative impacts of temperature and herbivore density on damage to their rice hosts
WBPH nymphs had higher survival, more rapid development and attained a greater biomass at 25 ̊C compared to 30 ̊C. Nymphs of both species failed to develop to adults at 35 ̊C possibly due to adverse effects of the high temperature on yeast-like endosymbionts, which are essential for nymph nutrition [18, 40,41,42]; and due Intraspecific competition counters climate effects on insect herbivores to the comparatively slow growth of rice plants at 35 ̊C
Summary
Global temperatures have increased by between 0.5 and 0.9 ̊C since the 1960s and are predicted to continue rising under current global CO2 emissions [1, 2]. Warmer temperatures will directly affect insect herbivores by increasing the numbers of generations they achieve in a single season or crop cycle [5, 6]. These changes are predicted to increase herbivore damage to plants, including crops [7,8,9,10]. Evidence of increased damage to crops from insect herbivores under warmer climates is rare and often anecdotal, or suffers from the problems of ‘cause-and-effect’ associated with correlative studies [1, 7, 11]. The natural enemies of herbivore pests may have greater attack efficiency at higher temperatures, or may increase the numbers of generations they achieve in a season to a greater extent than observed among their prey [3, 15]
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