This study reveals that various measures of bee diversity-including abundance, richness, and assemblage patterns are influenced by vane color and light reflectance patterns when passively sampling bees with vane traps. In particular, brightly colored vanes with higher light reflectance within 400–600 nm range attracted a greater diversity of bees in traps placed in a livestock pasture ecosystem. Effectiveness of blue and yellow vane traps had been compared previously in different ecosystems, for instance in apple orchards17, both woodland and open agriculture farmland13, and adjacent to Helianthus spp. (Asteraceae) field27. In all these studies, blue vane trap captured more bee species and 5–6 times more individuals compared to yellow vane trap.
In the current study, we assessed a different design and size of vanes and a wider array of vane colors and reflectance patterns attached to sample collection jars. In particular, we used bright blue and yellow vanes that were made of plastic sheets covered with a micro-prismatic retro-reflective sheeting that provides better daytime and nighttime brightness as well as high visibility and durability. These vanes showed higher light reflectance and captured the most bees and bee species in this study (Table 2). Similar material was used on red vanes as well, but the light reflectance from those vanes was relatively lower, and as a result captured fewer bees. Traps with bright blue vanes performed especially well in terms of rates of bee capture (Fig. 2; 11.1 bees per trap per sampling date) and rates of species accumulation (Fig. 3). Bright yellow traps exhibited the second highest values for capture rates (Fig. 2; 6.6 bees per trap per sampling date) and species accumulation (Fig. 3), but these rates were not deemed significantly different from some other colors in which the reflective sheeting was not used, such as dark yellow, dark blue and purple.
Bees use visual clues for detection, recognition, and memorization of floral resources in the foraging landscape7,28. The intensity of light reflected from different colors of vanes in traps affect number of bees attracted toward the trap10. Most bees can recognize colors that fall between 300 to 600 nm visual spectrums29. While the information related to the vision of many solitary and wild bees is not available, in the case of honey bees (Apis mellifera), color vision is trichromatic with highly sensitive photoreceptors at 344 nm (ultraviolet), 436 nm (blue) and 544 nm (green)30.
In this study, colored vanes at a higher light reflectance between 400 to 600 nm attracted the highest number bee species in these passive traps. Capture rate differed among traps with different colored vanes in the current study, which can be explained by sensitivity of visual spectrum of bees and variation in the light reflectance of vanes of these traps. For example, bright blue vanes had two peaks of higher light reflectance, initially in 450–455 nm range and second peak with > 800 nm. Such higher reflectance peak within the optimal range of bee vision may have played an important role in attracting abundant and diverse bee species to these passive traps. Similarly, bright yellow captured second largest number of bees, also had higher light reflectance peak within 600 nm but gradually decreased with increasing wavelength. Though bees have color spectrum from UV to orange31, they are sensitive to color spectrum between blue, green and ultraviolet32, which is a type of trichromatic vision system28. In one study33, red color vanes showed relatively lower light reflectance within 600 nm range, but had higher reflectance later in the spectrum, and this could be a reason why a low number of bees were collected in the traps. Past research showed contradictory views regarding the ability of bees to perceive red color. For instance, an early researcher in this field33, reported that bees recognize red color objects; however, other researchers had reported inability of bees to perceive34 or discriminate red from other colors35,36. It was argued that the bees see up to 650 nm in the visual spectrum and may not miss red colored flowers while foraging. However, other factors such as background (vegetation) color could also be contributing to bees’ ability to navigate different vane or flower colors in a livestock pasture landscape. Generally bees use color contrast to locate flower source, and hence neutral colors such as white are usually ignored29. Ultraviolet signal can make flowers more or less attractive to bees depending on whether it increases or decreases color contrast37. For example, UV color component in yellow38 and red39 flower increases chromatic contrast of these colored flowers with their background contributing attractiveness to the flowers. However, UV-reflecting white flowers decreases attractiveness for bees40.
Different species of bees responded to different colors of vane traps. Out of the 49 bee species collected in this study, only nine bee species were found in all vane color types, whereas 14 species were found in only one trap color. For instance, out of five bumble bee species, two were found in all six vane colors, one was found in five colors, and two species (Bombus bimaculatus and B. fervidus) were only found in the traps with bright blue vanes. Many of the species that were only found in one trap color- Calliopsis andreniformis (1, bright yellow), Ceratina dupla (1, bright yellow), Diadasia afflicta (1, bright blue), Diadasia enavata (1, dark blue), Halictus rubicundus (1, dark yellow), Hylaeus mesillae (1, red), Lasioglossum tegulare (1, bright blue), Lasioglossum trigeminum (1, purple), Megachile montivaga (1, dark yellow), Melitoma taurea (1, bright blue), Svastra atripes (1, bright blue), and Triepeolus lunatus (1, dark yellow) were singletons and it was impossible to know if this represented a true preference or pattern. Our analysis of assemblage patterns after aggregating bees at the genus level, did show a gradient-like response in bee-color associations (Fig. 4), ranging from dark blue to yellows (with no strong associations found with red vanes). These patterns may be used to guide future (passive trap-based) sampling efforts to monitor bee diversity or to target specific bee species in livestock pastures or other ecosystems. While the bright blue and yellow traps with reflective sheeting were particularly attractive to bees, dark blue and purple traps also had relatively high levels of abundance and richness and collected higher number of Melissodes. Purple, as a color, is less commonly used than blue and yellow traps in bee monitoring. While this study shows that purple may be a viable option for bee collection, it’s similar assemblage pattern (Fig. 4) and low level of complementarity with dark blue traps (Table 2) suggests that it may be redundant with blue traps that are already commonly used. Differences in species- and sex-specific associations of bees with different colors of sampling traps had also been reported in previous studies41.
Most of the bees collected in the current study were from Halictidae family (77.6%) followed by Apidae. However, few bee species in the families Andrenidae, Colletidae, and Megachilidae were collected. Consistent with our findings, others42 reported that bees of the Halictidae family were the most abundant bees in rangeland of Texas. The most common species found in this study were Au. aurata, L. disparile, L. imitatum, and Ag. texanus). In our previous studies we have found similar bee diversity in this study region18. Pollinator species richness and diversity as well as population distribution in livestock pasture vary during the season43. Mid-July to mid-August is the latter half of the summer season in the Southeastern USA, and the sampling period may have missed bee species that emerge earlier in the season and are reported in other studies42,43.
Overall, the findings of this study showed that the wild bees responded differently to passive traps with colored vanes of different light wavelength and reflectivity when deployed in a livestock pasture ecosystem. Among six different colors of vanes (dark blue, bright blue, dark yellow, bright yellow, purple and red), the bright blue traps captured the highest number of individuals and species of bees. This could be due to an appropriate match between the visual spectrum of bees and the light reflectance spectrum of vanes, which were made of a micro-prismatic retro-reflective material. Bees responded similarly to traps with other colors of vanes, except for red vane traps, which captured the lowest number of bees. The findings of this study would be useful in understanding bee vision and responses to passive traps, and, such information would help in optimizing bee sampling methods for future monitoring efforts.
Source: Ecology - nature.com
