Summary1. Most plant species rely upon multiple lines of defence that develop or change functions with leaf age. To date, the effects of leaf age on multiple defence traits have been insufficiently elucidated. For Mallotus japonicus (Euphorbiaceae), we report a shift from direct to indirect defence against herbivory. That shift occurs gradually during leaf ageing. 2. First, the densities of trichomes and pellucid dots, the numbers of extrafloral nectaries (EFNs) and pearl bodies, and the number of ants on leaves of M. japonicus were examined in the field at weekly intervals. Secondly, herbivore species and the degree of herbivory were examined. Thirdly, the effects of ants on herbivore exclusion were investigated experimentally in the field. Finally, to confirm the effectiveness of trichomes and pellucid dots for defence against two herbivore species [the adult flea beetle, Aphthona strigosa (Coleoptera: Chrysomelidae), and the inchworm, Ascotis selenaria (Lepidoptera: Geometridae)], both young and middle-aged leaves were tested using laboratory bioassay. 3. On newly emerged leaves, densities of trichomes and pellucid dots were extremely high, but they decreased rapidly with leaf age. The respective quantities of EFNs that were actually secreting nectar, the pearl bodies, and the ants on middle-aged leaves were greater than those on leaves of other ages. On M. japonicus, we found 11 herbivorous insects, among which Ap. strigosa and As. selenaria were the principal herbivores. 4. The damaged leaf areas of ant-excluded plants were much larger than those of control plants. Most herbivory was attributable to As. selenaria. Laboratory bioassay revealed that against Ap. strigosa and small As. selenaria larvae, trichomes and pellucid dots in young leaves functioned well, although those in middle-aged leaves did not. 5. Synthesis. Young M. japonicus leaves are defended against herbivores through the use of direct defence traits such as trichomes and pellucid dots. However, the plant uses indirect defence traits for middle-aged leaves. This shift from direct defence to indirect defence with leaf ageing supports the 'optimal defence theory': plants have evolved multiple defence traits to maximize their fitness.
The ability of the gregarious larval endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) to adjust progeny sex ratio and clutch size was investigated. The sex ratios (proportion of males) of field clusters were diverse, but many (70%) were female‐biased. Nearly 10% yielded males only, suggesting a low percentage of unmated females in the field. In over half of the clusters containing females, the sex ratio was below 0.3. Superparasitism was common in the field, and females were believed to increase progeny sex ratio when attacking previously‐parasitized hosts. However, in a single oviposition bout, sex allocation was not precisely controlled both in the field and laboratory. In the laboratory, the number of eggs laid in a day tended to decrease with increasing female age. For females which were offered two hosts per day and for those offered three hosts per day, this value became nearly the same several days after the start of oviposition. The total number of hosts which a female could parasitize during her lifetime was often less than 40. Some of the old females which attacked more than 40 hosts produced male‐biased clutches; this was due to sperm depletion, because sperm remained viable throughout a female's lifetime. The amount of sperm used in a single oviposition bout seemed fixed and was not dependent on the number of eggs laid. Females with much oviposition experience did not produce new eggs to compensate for deposited eggs, and the efficiency of egg use (deposited eggs/total eggs) was more than 80%.
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