Abstract
Immobilized preys are routinely used in agro-ecological exposure studies to quantify predation of pests under field conditions, but this method has not been validated. Our purpose was to determine the validity of using immobilized adults of the major rice pest Nilaparvata lugens, brown plant hopper (BPH), as sentinels. We used direct observation by video recording to determine the causal agents of removal of field exposed BPH sentinels with two experiments: 1) we recorded removal events of dead, immobilized BPH; and 2) we compared removal of (i) dead, immobilized BPH, (ii) live, immobilized BPH, and (iii) live, mobile BPH. Long-horned grasshoppers were responsible for most removals of dead, immobilized BPH, in both experiments. Predatory ground beetles removed most of the live, immobilized BPH, whereas frogs were the major predators of live, mobile BPH. Overall, we showed that removal of immobilized sentinel prey is not representative for predation of live, mobile prey, stressing the need for a critical assessment of commonly used sentinel methods. In addition, we found that frogs played the major role in predation of BPH in rice. As current strategies to enhance biocontrol of planthoppers in rice focus on arthropod natural enemies, this finding could have major implications.
Highlights
Immobilized preys are routinely used in agro-ecological exposure studies to quantify predation of pests under field conditions, but this method has not been validated
A common method to measure predation is the placement of immobile prey in the field and quantifying the prey removal rate by comparing the number of prey before and after a known exposure period[1,2,3]
Our data demonstrate that (1) the agents responsible for removal of immobilized brown planthopper (BPH) sentinels differed greatly in identity and relative importance from the taxa found preying on live, mobile BPH, and (2) frogs played a major role in removal of live, mobile BPH
Summary
Immobilized preys are routinely used in agro-ecological exposure studies to quantify predation of pests under field conditions, but this method has not been validated. A common method to measure predation is the placement of immobile prey in the field (referred to as “sentinel” or “prey enrichment”) and quantifying the prey removal rate by comparing the number of prey before and after a known exposure period[1,2,3]. Brooks, et al.[20] found a higher predation rate of live, mobile than of dead, immobilized prey in a freshwater macroinvertebrate system, while Steward, et al.[21] reported that predatory wasps (Vespidae) preferred pinned to unpinned larvae. An important aspect of determining the validity of a sentinel method is to assess whether the removal agent of immobilized prey is a predator of the prey in unmanipulated settings.
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