Introduction and Distributional Expansion of Trechus obtusus (Coleoptera: Carabidae) in Maui, Hawai'i

Trechus obtusus Erichson (tribe Trechini), native to Europe and North Africa and introduced to the Pacific coast of North America, is recorded for the first time from East Maui Island, Hawai'i, based on collections made at Haleakala National Park in September 1998. The species subsequently expanded its distribution to include Polipoli Springs State Recreation Area, East Maui. Range expansion has averaged 3 km per year, based on documented absence of T. obtusus from the Polipoli Springs area in 1998. All Hawaiian individuals are macropterous, even though European and North American populations of T. obtusus are dimorphic for wing configuration, with the brachypterous form most common in long-established populations. The source area for the Hawaiian invasion is hypothesized to be Oregon or the San Francisco Bay area, based on the closest match in the frequency of macroptery between specimens from Hawai'i and those from those mainland areas. Monomorphic macroptery of the Hawaiian populations suggests that the founder population was small, with estimates ranging from as few as 6 individuals to as many as 25, assuming the founding propagule was drawn at random from populations in the western United States. Baseline abundance data are presented for Polipoli Springs State Recreation Area, where T. obtusus co-occurs with seven native Mecyclothorax species (tribe Psydrini), establishing the opportunity for long-term assessment of the impact of the introduced species on the sympatric native fauna. Means to identify T. obtusus in the context of the Hawaiian carabid beetle fauna are presented.