Abstract. 1. Seasonal patterns in insect population fluctuations have rarely been examined in rainforests of the central part of Southeast Asia where the climate is aseasonal.
2. We examined the seasonality of population fluctuations in a Bornean rainforest phytophagous scarabaeid assemblage (six species of Melolonthinae and seven of Rutelinae). We also investigated effects on abundances of these insects of irregularly fluctuating environmental changes, such as short-term rainfall, and community-level leaf flushing and flowering of canopy trees. Scarabaeids were collected over 6 years by monthly light-trapping.
3. Eight scarabaeid species had clear seasonality in occurrence and abundance. Seasonal trends were synchronised across the eight species; their monthly catches peaked from March to May and were nearly zero in other months. An analysis of correlations between environmental changes and insect abundances suggested that short-term changes in rainfall affected the synchrony.
4. In contrast, fluctuations in abundances of five other scarabaeid species were unrelated to calendar seasons. In one of the five scarabaeids, which is a flower-visiting species, the patterns in population fluctuation strongly correlated with flowering changes. In the remaining four species, no correlations were observed between environmental changes and population fluctuation patterns.
5. While previous studies have demonstrated that aseasonal patterns in population fluctuations are common in various insect assemblages in Southeast Asian rainforests, we show that clear seasonality is prevalent for the phytophagous scarabaeid assemblage.