Pollination is an essential ecological function, and bees (Hymenoptera: Apoidea) are among the most important pollinators. However, there is growing evidence of decline in some bee populations, with habitat alteration playing an important role, particularly in heavily cultivated regions such as the midwestern USA (Byrne & Fitzpatrick 2009; Grixti et al. 2009; Cameron et al. 2011). Monitoring of bee abundance and diversity is essential for effective bee conservation. Bees are commonly monitored by using sampling devices such as bowl traps, or "bee bowls," which are colored bowls that are placed on the ground, or occasionally elevated, and filled with a liquid such as soapy water (Leong & Thorp 1999; Droege et al. 2010; Grundel et al. 2011; Shapiro et al. 2014). These traps are often referred to as "pan traps" in the literature, but "bowl trap" seems more appropriate because actual aluminum pans are sometimes used as pan traps (Martin 1977). Other trap types used in bee monitoring include vane traps, which consist of 2 plastic cross vanes with a collection container underneath, into which the insects fall upon contact with the vanes (Stephen & Rao 2005; Kimoto et al. 2012), and Malaise traps (Malaise 1937; Townes 1972), which are large, mesh fabric flight interception traps that collect flying insects when they contact a vertical central portion and move up a sloping roof to a collection container. The usefulness of Malaise traps for collecting bees and other insects has been demonstrated in several studies (Matthews & Matthews 1971; Noyes 1989; Bartholomew & Prowell 2005; Ngo et al. 2013), although Malaise traps were less effective than bowl traps in collecting pollinating insects in southeastern U.S. forests (Campbell & Hanula 2007). Recently, Geroff et al. (2014) assessed the effectiveness of the above methods in a west-central Illinois tallgrass prairie and found that Malaise trap captures identified the greatest bee abundance and species richness. This finding suggests that Malaise traps may be useful in assessing bee diversity in this system. In this paper, we further investigate the effectiveness of Malaise traps in assessing bee species richness, using the Chao1 statistical richness estimator (Chao et al. 2009). Data for this study were collected as part of a larger evaluation of bee sampling methods done from early Jun to early Oct 2010 by Geroff et al. (2014). That study was done in an approximately 12 ha restored prairie at Western Illinois University's Alice L. Kibbe Life Science Station (40.3658°N, 91.4067°W). More detailed information on the study location is given in Geroff et al. (2014). For the present study, data from 5 Townes-style Malaise traps (Sante Traps, Lexington, Kentucky), spaced 50 to 100 m apart, were used. Only Jun and Jul samples were used to ensure sufficient sample sizes. In each month, trapping was done on 6 d within a 10 d period, based on weather conditions (clear, calm, sunny days). In Jun, data were collected on