捕食者による非消費型効果がヒメトビウンカの表現型可塑性に及ぼす影響と害虫防除への応用

概要

Small brown planthopper (SBPH) is a serious pest of rice in East Asia. The control of the planthopper has been made mainly with pesticides, but development of alternative measures is on demand due to the environmental impacts caused by the pesticides. In the present study, I first focus on non-consumptive effects from a generalist predator, Paederus fuscipes, which is commonly found in rice paddies, on SBPHs, and investigate the presence of phenotypic plasticity of SBPHs when exposed to predator (P. fuscipes) cues. I then examine this phenomenon can be applied on SBPH control in the laboratory condition; I identify the chemical nature of predator cues and apply them on rice plants in the field to know whether SBPH population, rice plant biomass and arthropod community can be affected. The present study provides a series of new insights for better understanding of predator-prey interactions and non-consumptive effects in insects and for exploring a new approach to control agricultural pests such as SBPHs.

Chapter 1 focuses on the non-consumptive effects of the rove beetle on SBPHs. It is shown that SBPHs can respond to volatile cues from caged rove beetles or a predator body extract by reducing activity levels, and this behavioral plasticity can be reserved and recovered after removal of the predator cues. Further, the predator extract (odor risk cue) alone can elicit a response similar to that elicited by caged predators (“visual + odor” risk cues).

Chapter 2 demonstrates that the adult SBPH longevity is shorter and fecundity and weight gain are lower when they have been exposed to the predator cues during the nymphal stage though the nymph development and egg hatching rate of SBPH are not influenced by predator cues. Thus, Chapters 1 and 2 provide, for the first time, evidence of non-consumptive effects on SBPH.

In chapter 3, an additional, new finding on non-consumptive effects is given. Caged predators or a predator extract is shown to cause a higher proportion of long‐winged female SBPHs in the parent and F1 generations though the proportion of males and their wing length were unaffected. This finding gives evidence that SBPHs respond to predator cues by changing the wing morphs, and that SBPHs can have a higher ability to escape from predator attack or from the area where the predator is abundant.

In chapter 4, field spray of predator body extracts in solvents with different polarities is performed to test their effects on the SBPH population and rice plant biomass. The results show that field sprays of predator cues can decrease SBPH density and increase rice plant biomass. In addition, predator cues extracted using chloroform show stronger non-consumptive effects or trait-mediated indirect effects than those using ethanol or water.

In chapter 5, a Y-tube olfactometer study is made to examine whether SBPH could detect the presence of rove beetles (P. fuscipes) via odor from rove beetle abdominal gland secretion (AGS). Then, the chemicals of AGS are identified by a gas chromatography-mass spectrometry. Chemicals identified are exposed to a SBPH to test their effects on SBPH behavior. Female or male SBPHs are shown to distinguish the rove beetle AGS from odors without predation risks. The gas chromatography-mass spectrometry study detects sixteen chemicals that are abundant in female and male AGS. Five of them (n-undecane, n-pentadecane, n-hexadecane, n-eicosane and n-heneicosane) individually or collectively can reduce the activity level of planthoppers.

In chapter 6, a preliminary field study is made to show whether filed spray of rove beetle AGS can affect the arthropod community in the rice fields. After AGS spray, the abundance of arthropods pests such as hemipteran pests (SBPH, brown planthopper, with-backed planthopper and leafhoppers) and lepidopteran pests (rice leaf folder) decrease while arthropod natural enemies such as spiders, ladybugs and parasitoids temporary increase at the beginning of the experiment. However, the species diversity of arthropods itself is not affected.

The present study demonstrates that SBPH can sense the volatile odor from the rove beetle to avoid encountering predators by reducing its activity level but such behavioral plasticity incurs a cost of poor development and fewer offspring, causing a low population growth rate in the field and leading to protection of rice plants. Thus, this study provides evidence of predator non-consumptive effects and its potential application to pest management.