Insect Pests of Rhododendron kiusianum Makino (Ericaceae), with Special Reference to Outbreaks of Inurois sp. and Arichanna melanaria Linnaeus (Lepidoptera, Geometridae) on Mts. Kuju and Kirishima

Rhododendron kiusianum Makino (Pl. 1, Figs. 1～3) which is endemic to Kyushu and grows only on high mountains such as Kuju, Kirishima and others is famous for its beautiful flowers from late spring to early summer. They decorate the whole mountains when in full bloom. Vegetations of this plant growing on the Dambaru area of Mt. Daisenzan, Kuju, and on the Ikenohara area of Mt. Unzendake belong to the natural monuments. Insect pests of this Rhododendron have never been investigated before, although occurrence of a single pest called "kishita-edashaku" and its serious damages have been known from time to time for last 40 years. Because of recent increasing news on damages of this natural monuments by insect pests, we conducted a series of investigations chiefiy en Mts. Kuju and Kirishima from winter of 1971. This paper reports the result of it, which is briefly summarized as follows: 1. Thirty-four species of insects belonging to 5 orders and 17 families were found to be pests of Rhododendron kiusianum (Table 1). 2. Among them, two species of lepidopterous insects were found to be serious and most destructive. They are Inurois sp. (Oenochrominae, Geometridae), which is the most important pest, and Arichanna melanaria fraterna (Bulter, 1878) (Ennominae, Geemetridae), the second important species. (The former is a new species and the description of it by Dr. H. inoue will be appeared in Mushi, vol. 48). 3. Inurois sp. is one of the so-called "winter moths" whose adult appears in winter and the female of them is often wingless or brachypterous. On Mt. Daisenzan, Kujti, the adult of Inurois sp., the female of which is wingless (Pl. 1, Fig. 7), appears early November, then copulates and the female eviposits the eggs in a batch (12 eggs in average) on twigs (Pl. 1, Fig. 9) or rarely on leaves also. They do not feed at all. Eggs overwinter and the first instar larva hatches toward the end of April or early May when the young leaves and shoots of the Rhododendron began to grow. They feed on leaves and flower buds for about 30 days, in which they pass through 4 instars. Toward the middle of June they pupate in the soil (not more than 10 cm in depth) in a rather tight silken cocoon. The pupal period lasts about 5 months through summer and autumn. 4. For the years 1972 and 1973, the pepulation deasities of Inurois sp. en Mts. DaiseRzan and Kirishima wefe incredibly high. For example, in a space of 0.25 m^2 of a tree of Rhededendren, an average of 90 last instar larvae of Inurois sp. was found crowded on Mt. Daisenzan in the season of 1972. As the result of an outbreak of Inurois sp. like this, no green leaves and flowers were seen at that time (Pl. 1, Fig. 4), and all the trees were almost thought to be killed. 5. Arichanna melanaria was much less in the population than Inurois sp. but the damage to Rhododendron by the larvae was estimated still high, especially on Mt. Kirishima. This species has one generation a year like Inurois sp. and the larva appears about the same time with the latter, but the pupal stage lasts only about a month so that the adult fiies in .summer. It overwinters on twigs in the egg stage which lasts about 9 months. 6. Developmental stages of the two species of geometrid moths were described. Colour polymorphism of the larva of Arichanna melanaria was revealed for the first time (Pl. 1, Figs. 10～13; Fig. 17). 7. Altitudinal distribution of the larvae of two species, Inurois sp. and Arichanna melanaria, was observed on the slope of Mt. Daisenzan from Bogazuru-plain (1,300 m) up to Dambaru (1,700 m). Inurois sp. was found numerous on the summit, then decreasing their population down to the Bogazuru-plain, while the larvae of Arichanna melanaria were dominant on the lower place (for example, 14 larvae on a tree of Rhododendron of about 2 m2 in extension but only 6 of Inurois sp.）, but becoming scarce toward the summit where only several of them per 10 m^2 were found. The reason of this sort of habitat segregation and the adaptation of Inurois sp. to the environment of Dambaru area were discussed. 8. Only a few species of insect natural enemies were obtained from both Inurois sp. and Arichanna melanaria. Among them, an ichneumon fly Callidiotes sp. (Porizontinae) was rather abundant and most active through the larval period of Inurois sp. on Dambaru, Mt. Daisenzan. At least an ichneumon fly and two species of tachinid flies were found to be parasitic on the larva of Arichanna melanaria, and the larva of a syrphid fly was found predaceous to the larva of the latter. 9. Absence of insect pests which are active during the post season of Inurois sp. and Arichanna melanaria (i.e., summer and autumn) is thought to be responsible for the recovery of leaves and therefore the survival of Rhododendron today. Therefore, it is recommended that special attention should be paid in future for watching against the emergence of any insect pests in this season.