Bottom-up cascade in a tri-trophic system: different impacts of host-plant regeneration on performance of a willow leaf beetle and its natural enemy

1. Plant quality can directly and indirectly affect the third trophic level. However, little attention has been paid to how changes in plant quality affect the performance of predators through trophic levels, and which herbivores or predators are affected more strongly by host-plant quality. The present study examined the effects of artificial cutting of willows on the performance of a willow leaf beetle (Plagiodera versicolora Laicharting) and its predatory ladybird beetle (Aiolocaria hexaspilota Hope). 2. Laboratory experiments showed that performance (survival rate, developmental time, and adult mass) of the willow leaf beetle was higher when fed with leaves of cut willows than when fed with leaves of uncut willows. Performance (developmental time and adult mass) of the predatory ladybird was also improved when it was fed on the leaf beetle larvae that had been fed on leaves of cut willows, compared with those that had been fed on leaves of uncut willows. This indicates that a bottom-up cascade occurs in the tri-trophic system. 3. In a comparison of improved performance parameters between the leaf beetle and the ladybird, regenerated willows shortened the developmental time of the willow leaf beetle more than that of the ladybird. This indicates that the impacts of willow cutting on insect performance differ between the second and third trophic levels.