The importance of odor in nest site selection by a lodger bee, Centris Bicornuta Mocsáry (Hymenoptera: Apidae) in the dry forest of Costa Rica

ECOLOGY, BEHAVIOR AND BIONOMICS

The importance of odor in nest site selection by a lodger bee, Centris Bicornuta Mocsáry (Hymenoptera: Apidae) in the dry forest of Costa Rica

SB Vinson^I; GW Frankie^II; A Rao^I

^IDept of Entomology, Texas A&M University,College Station, TX, USA

^IICollege of Natural Resources, Univ of California, Berkeley, CA, USA

^{Correspondence}

ABSTRACT

The more common lodger bee occurring in the dry forest of Costa Rica, Centris bicornuta Muscáry), has been observed nesting in new nest cavities drilled into wooden blocks placed next to cavities used by another female within 2-3 days. In contrast, new nest cavities placed in similar areas with no nesting Centris nearby were not used for weeks. These observations suggest that the presence of nesting bees may play a role in nest site selection. To confirm our observations, new nest cavities were placed in areas with or without nesting. We found nest initiation in newly placed nest cavities only in areas where bees were actively nesting. To examine the possibility that nesting locations are not unique, we placed new nest cavities in new locations either with (a) a number of completed nest cavities or (b) placed alone. Within three days we only found bees nesting in the newly placed nest cavities in situation "a". The results suggested that odor might be involved. We next compared nesting in new cavities placed alone with cavities contaminated with either (a) nest entrance plug material, (b) nest nectar, (c) nest pollen or (d) a combination of pollen and nectar. Nesting was significantly low in cavities placed next to cavities with nest entrance plug material (a), and high in cavities placed next to cavities "b, c, or d". The results suggest that pollen and /or nectar odor play a role in the location of potential nest sites.

Keywords: Nest initiation, nesting site location, provision odor

Introduction

Nidification studies on several twig nesting or lodger bees in the genus Centris have been conducted in Costa Rica (Frankie et al 1988, 1989, Vinson et al 1993, 1996, 2006, Vinson & Frankie 2000) and in Brazil (Morato et al 1999, Pereira et al 1999, Jesus & Garófalo 2000, Morato & Campos 2000, Aguiar et al 2006, Mendes & Rego 2007). In Costa Rica, we (SBV & GWF) have focused on a group of wild wood-hole nesting Centris that nest in dry forest during the dry season. These studies have provided information on the structure of nests, as well as, nesting materials needed, cavity diameters preferred and some information on provisions (Vinson et al 2010, Frankie et al 1993) also provided information regarding the nesting-habitat preferences of this group of Centris bees. In most all of these studies, trap nests consisting of bundles of 12 wooden sticks with holes of several sizes drilled into them (Bee Nest Monitoring Bundles [BNMB]) were used (Frankie et al 1988, 1993, Vinson & Frankie 2000).

However, we have also noticed (personal observations) that when new BNMB were first placed in the field or placed at a new location, it would often take a week or two to get nesting in these BNMB. If, however, new BNMB were added within a few feet of where bees were actively nesting, then these new BNMB were being nested in within a day or two. This brought up a question: are these nesting bees attracted to the activity of other bees or is it possible that an odor might play a role? We had observed that some ground nesting Centris species tend to aggregate (Vinson et al 1987) while others do not (Vinson & Frankie 1977). It is also well known that bees use both visual and olfactory cues in their location and recognition of their own active nests (Wcislo 1990, 1992). However, Inouye (2000) looked at the role of visual and odor cues in nest recognition and suggested that odor did not appear to play a role. There is even less known about the possible role of vision, sound or odors in nest site discovery, particularly if the bees tend to aggregate.

In looking at the literature and a review by Morato & Martins (2006) concerning proximate factors affecting the nesting behavior of solitary wasps and bees indicated that little has been done or reported regarding the role odor in influencing the location of nesting sites or the location of potential nesting cavities for solitary wood cavity nesting species. Torchio (1984) observed that Osmia lignaria Cresson (Megachilidae) preferred to nest near, but not in old nest cavities. In contrast, Megachile rotundata (F.) was found to prefer to nest in used nesting boards rather than new ones (Fairey & Lieverse 1986, Fairey & Lefkovitch 1993). The response by M. rotundata appeared to be due to the odors of old nest contents (Stephen & Torchio 1961). Pitts-Singer (2007) reported that female O. lignaria were attracted in a "Y-tube" only to female cocoons. In contrast, M. rotundata was attracted to intact nest cells, fecal material on the outside of cocoons, leaf pieces used to line cells and extracts of the leaf pieces while Megachile pugnata Say were attracted to paper straw nesting material with cocoons and feces (Pitts-Singer 2007).

We setup a study to see if odor or bee activity (sight or sound) might play a role in stimulating one of the more common wood-hole nesting species, C. bicornuta, in the dry forest to initiate a nest in an unused trap nest stick that was or was not placed next to a nested-in trap nest stick.

Material and Methods

The study reported here was conducted during the month of February 2009 in riparian areas of the dry forest, along small rivers in Guanacaste Province of Costa Rica about 8.5 km northwest of the Town of Bagaces. This area is known as Hacienda Montverde. Within this area, we had four locations (sites) that we have used for several years and in which several species of lodger bees were nesting. Within the above dry forest, we picked 12 new potential nesting sites for the study that were located approximately 0.1 to 0.3 km apart.

We initially used BNMB that consist of 11.5 x 2.03 x 2 cm wooden sticks drilled lengthwise to a depth between 7.0 cm to 11 cm depending on the following five cavity (hole) diameters: 4.5, 6.5, 8, 9.5 and 11 mm. As cavity diameters increased so did the cavity depth. To form the BNMB, two sticks of each cavity diameter (except we used four of the 8 mm cavity diameter sticks) were bundled together using masking tape and twine. The sticks were bundled into two stacks with the largest cavity diameters at the bottom and smallest at the top. As the stack was formed, the pair of sticks of the same cavity size was stacked next to each other so that the cavity of one was next to the back or non-cavity end of the companion stick (see Frankie et al 1993 for details).

To determine if female C. bicornuta prefer to nest in wooden cavities near other nesting Centris lodger bees, we initially took unused BNMB and placed them in two situations. For the first situation, we placed four unused BNMB at each of three sites (total of 12 BNMB's were placed) that we had used for several years and where we already had some BNMB's in place and in which bees were actively nesting. We also placed four unused BNMB's in three other sites (total of 12 BNMB's) not previously used that were at least a 100 m from one of our active sites, but were in the same forest.

The BNMB's from both situations were collected four days later to confirm what we had noted before; that the cavities in the newly placed BNMB were nested in when placed with actively nesting BNMBs, but these cavities were not nested in when placed away from such nesting activity (see results for details). The completed BNMB's were returned to the laboratory where the sticks with completed nests of C. bicornuta, as evidenced by a nest plug coated with a distinctive thick yellowish oil-like material, were removed. Since C. bicornuta prefers to nest in the 8 mm and 9.5 mm cavities (personal observations) we focused on these cavity sizes and recorded the number with bee nests and without for each site and situation.For our second study to confirm the importance of the presence of a completed nest next to an available nest cavity, we placed available cavities next to or separate from a completed nest in a cavity. For this we used cavity diameters of 8 mm and 9.5 mm that were completed from the above study. These were re-bundled for test two into a number of stacks each consisting of five sticks, one on top of the other and alternating the completed nest entrances. We also formed a number of stacks of five sticks as described, but of unused sticks only. These bundled stacks of unused five nest sticks, as well as, bundled stacks containing five sticks with nests were placed in ten sites at two different unused locations (a total of 20 sites).

At each site the stick bundles were placed in two situations. One set of the unused stick bundles was hung along with and next to one bundle of the completed bee nest sticks on a nail that had been placed in the side of a tree about 1.5 m above the ground (situation A). In the second situation (situation B) one unused stick bundle was hung on another tree, as described earlier, but by its self. Each situation had 50 nest cavities. The two situations were at least 15 m to 25 m away within each of the locations. Three days later the unused stick bundles were monitored for bee nesting activity in both situations and compared using an independent two-sample t-test (Table 1).

Based on the results of the above test, we setup a study to determine if the nest plug's oily material (Fig 1) or some other components (provisions) of a completed nest might attract potential nesting females. We first collected the nest plug material with the aid of a small spatula and placed the contents into a small wide mouth vial that was capped between collections. The nest sticks were split length-wise and the nectar (See Vinson et al 2006) was collected with the aid of a 50 μl capillary tube in which the contents of the tube could be sucked in and out with the aid of a small rubber bulb attached. This material was also placed into a wide mouth vial that was capped between collections. The interface between the nectar and the nectar moist pollen was collected with another small spatula and placed into another small wide mouth vial capped between collections. The cream colored pollen at the back of the cell was packed hard and appeared dry. This pollen was extracted from the cell wall with tweezers, cleaned of any nectar or cell wall material. This pollen was removed as chunks that were placed in a watch glass and were broken apart with the aid of a nail head. The crushed pollen mass was then scraped into a wide-mouth vial and capped between collections.

Each of these nest materials were collected repeatedly until we had about ten ml's of each. To determine if these nest materials had any activity in attracting C. bicornuta, we first took two sticks with 8 mm cavities and bundled them together with the cavities facing the same direction. A small cotton plug was placed in the cavity of one of the pair of sticks about 4 mm deep. These paired sticks (one with a cotton plug and one unplugged) were taken to the field to be placed in another ten different sites at two locations (total of 20 tests of each situation), as described above. In each site, four pairs of sticks were hung on a nail as described earlier with each pair placed on a different tree that were six to twelve meters apart. Just before hanging, the sticks with the cotton plug were charged by placing approximately 25 μl of one of the four nest materials on the plug. These were applied to the cotton plug of the pair of sticks. The nest plug oily material was applied with a spatula while both the nectar and nectar-pollen mixtures were applied with a capillary tube as described above. The pollen was mixed with a little water to form a paste that was applied with a spatula. Each pair of sticks was marked as to the treatment they received and hung on four different trees at each site, as described above. This was repeated until we had four pairs of sticks each with a different material applied to the plug and placed at each site.

They were examined three days later for evidence on nesting by C. bicornuta in the hole next to each of the four different treatments. The results were compared (nest plug oily material, pollen, nectar, and nectar + pollen) using a Kruskal Wallis mean rank sum test for independent samples.

Results and Discussion

For test one each BNMB had six of the 8 mm and 9 mm cavities for a total of 72 placed in each situation. Of the 72 cavity nest sticks removed from the BNMB at the site with nesting bees, 43 either had completed nests or the nests were nearing completion. These completed nest sticks were set aside so they could be used for the next set of experiments.

In contrast, of the 72 cavity nest sticks removed from the BNMB's that we had placed in sites that to our knowledge were not being used by nesting bees only one had a cavity with a nest started, as evidenced by a cell in the back of the cavity with pollen. These results confirmed what we had observed in the past, that nest sticks placed near nesting bees are more likely used than nest sticks placed in a new location. But this could be due to odor or to the activity of the nesting bees that are flying around the nests (movement) and creating buzzing sounds.

In our second test where we placed unused five stick nest bundles (unused cavities) either alone or with a completed five stick nest bundle (cavities with bee nests), significantly more bee nests were initiated in the unused nest cavities that were placed next to previously completed nest cavities than nest cavities that were placed alone as shown in Table 1 (t = -5.25, df = 98, P < 0.0001). In both cases the sites had not been used in the past for any of our on-going studies. These results eliminated sound or bee activity as a factor and suggested that odor might be involved.

In the third study where we placed an unused nest cavity next to an unused nest cavity that we had added a cotton plug a 2-3 mm from the entrance and to which we either added the nest entrance oil-like material used to seal a completed nest or some of the provisions (nectar, pollen and a combination of nectar and pollen), the results are shown in Table 2. In this test the nest plug oil-like material was significantly less attractive resulting in less nesting in the adjourning cavity as compared to the other materials (Kruskal Wallis rank sum test, H = 15.35, df = 3, P < 0.0002). There were no significant differences between the pollen or nectar or a combination of the two. In fact all three appeared to be active as 93% of the available cavities associated with these three situations were being nested in. It is not clear if the same odors are involved in the pollen and nectar samples or if different odors are involved. If the latter is true, one might expect a heightened or synergistic response, but the results do not indicate any such effect.

Unlike the Xylocopidae that can make their own cavities in wood (Gerling 1989), some bees, including the species studied here, nest in wooden cavities but are unable to make their own cavities. In fact, they depend on many wood-boring insects that play an important role in providing the wooden cavities that the lodger bees need. Further a number of the larger wood boring insects, such some Cerambycidae, tend to aggregate (Allison et al 2004, Akbulut et al 2008, see picture 2 at http:www.birderslounge.com/2009/05/the-mystery-of-the-wood -boring-insect/). Thus, a female wood cavity nesting bee that can detect and respond to another female bee that is nesting or has just completed a nest would have a high probability of locating cavities that have not yet been used. Odor of other nesting females could thus play an important role in finding such aggregations of wooden holes generally produced by various species of wood boring insects.

The discovery that some Centris are attracted to the odor associated with nests in progress may also have some important implications in the management of certain wood hole nesting solitary bees. Particularly in attracting certain solitary wood-cavity nesting solitary bees initially to artificial wood hole or cavity nesting units such as those described by O'Dell (1997).

The results also suggest that the nest entrance oil-like material is less active in attracting nesting females and supports the suggestion (Vinson & Frankie 2000) that the nest plug material is important in preventing nest cavity usurpation of newly completed nests.

Acknowledgments

We wish to express our deep gratitude to D A S Stewart for allowing us to conduct our research on his ranch. We also express our appreciation to both Dr Jerome G Rozen, Jr and Rollin Coville for their help in splitting the nest sticks to make notes on the various cells and pass them on to us to collect the contents of the newly completed nest cells for our study. We thank Texas Agrilife, Texas A&M University and the California Agricultural Experimentation Station for partial support.

Received 24 November 2009 and accepted 15 June 2010

Edited by Kleber Del Claro - UFU

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Publication Dates

• Publication in this collection
  09 May 2011
• Date of issue
  Apr 2011

History

• Received
  24 Nov 2009
• Accepted
  15 June 2010