Of a Solitary Stem-Nesting Bee, Osmia Rufa (L.)

Original article

The function of the vestibulum in nests of a solitary stem-nesting bee, Osmia rufa (L.)

Karsten Seidelmann

Institut für Zoologie, Martin-Luther-Universität Halle-Wittenberg, Domplatz 4, 06099 Halle (Saale), Germany

(Received 2 April 1998; accepted 21 November 1998)

Abstract - Nests of the stem- or hole-nesting megachilid bee, Osmia rufa, were analysed to help clarify the function of the outermost empty chamber of the nest, the vestibulum. Only nests in an exposed nesting environment had a long vestibulum, whereas nests protected from sun and temperature fluctuations (within a honey bee hive body) had short vestibuli or none at all. The rate of parasitism of the first cell from nests with a vestibulum did not differ from nests without a vestibulum in both nesting environments. The short vestibulum built by O. rufa in protected nests probably represents the remaining space of a nest hole that is too short for an additional cell. Nests in exposed environments suffered much higher mortality in all brood cells owing to parasites attacking open cells during nest construction. Additionally, mortality caused by the cleptoparasitic drosophilid, Cacoxenus indagator, one of the main parasites of the Red Mason bee, increased greatly from the rear to the entrance of nests. When nesting bees detect the cleptoparasitic fly, they do not provision the outermost space of nesting holes, thus constructing a vestibulum, to avoid misinvestment due to the high risk of parasitism. In nests exposed to normal weather factors, vestibular cells probably shelter the brood also from high fluctuating temperatures. © Inra/DIB/AGIB/Elsevier, Paris

Osmia rufa / vestibulum / nest architecture / avoidance of parasitism

1. INTRODUCTION the nest structures are a defence against parasites, predators and nest destroyers. By models et al. Nests of many Aculeata have empty cells probability Tepedino [21] were in variable numbers and positions [9, 10, able to demonstrate that empty, yet sealed 19]. To date, the function of empty cells has cells can reduce the success of parasites of remained unclear. Usually it is thought that mud-nesting Hymenoptera. These species

E-mail: [email protected] attach several cells on a surface or construct × 160 mm in length) in each block. To test for subterranean nests with cells branching off possible influences of the nesting environment on the of a these from a main tunnel in the soil. production vestibulum, nesting blocks were placed either in an intact (’protected Empty cells are also found in twig- or environment’) or dismantled (’exposed environ- hole-nesting, xylophilous Aculeata, espe- ment’) honey bee hive body (box with no combs cially megachilid bees that construct their or bees). The protected nest blocks could be reached via the entrance of brood cells in a linear order one behind the only regular flight the hive. Thus, nests in the intact hive were pro- other. In such linear nests an empty cell is tected from direct weather influences. In the usually found between the final provisioned exposed environment, the entire front side of the cell and the nest plug. These so called hive box was removed. Nesting blocks in such vestibular cells are believed to be a protec- dismantled hives were sheltered from rain but tion against parasitism of the nest [7, 11, exposed to normal temperature fluctuations. 12, 22]. Krombein [9] assumed that the par- A total of 1 712 complete nests of Osmia rufa asites that inserted their ovipositor through from within 22 nest blocks were evaluated. the nest plug no longer exist. Therefore the A nest was considered complete when the nest- tube contained several brood had a vestibulum has become a behavioural relict ing cells, regular nest and, in cases where the nest had a without current value. plug, adaptive vestibulum, if the outermost brood cell was com- According to Rust [15], a vestibulum is pletely sealed off by a cell partition. These restric- typical for all Osmia species. However, dur- tions were necessary because at the end of the season old females also seal incomplete the analysis of Osmia rufa nests around flight ing nests [18]. In this case, it is difficult to interpret vestibuli were observed to Halle, Germany, the empty space between the nest plug and the be facultative structures. This was not to be first provisioned cell. under their assumed function in expected In all nests, the thickness of the nest defense. the lack of complete parasite Furthermore, plug (except protected environment in 1992) and a vestibulum did not result in a marked vestibulum length were measured. The contents increase in parasitism of outermost brood of the outermost cell (= first brood cell behind the cells, although all major parasites of O. rufa vestibulum) was recorded for 1 413 nests, and and contents of all were present. The vestibulum could there- for 1 073 of these, the length brood cells were also recorded. From the fore represent remaining space which is too length of the provisioned cells of each nest, the mean small to construct an additional brood cell. length for male and female cells were calculated. To test these and thus to hypotheses, clarify In O. rufa nests, sons occupy smaller cells than the function of a vestibulum, its frequency daughters, and they are located towards the and length were recorded in nests of O. rufa entrance [14, 16, 17]. Therefore, the space which from different nesting environments, and would have been needed for an additional male cell in that nest was to the the outermost cells were exam- particular compared provisioned of the if it existed. To ined for parasites. To identify possible influ- length vestibulum, compute mean vestibulum length for nest blocks, vestibu- ences of the construction of a vestibulum lum length was assumed as zero for nests lacking on parasitism of inner brood cells, the total this empty space. nest contents of random were exam- samples To ascertain the distribution of ined. parasites within nests, every nest was equally divided into 300 sections by length and the contents of each section registered. A distribution curve of para- 2. MATERIALS AND METHODS sites could be constructed by totalling the number of parasitized cells in every section over all nests [17]. The study was carried out on nests of Osmia rufa from the Botanical Garden of the Univer- Differences in vestibulum length and entrance sity of Halle (Germany). In 1992 and 1993, plug thickness between exposed and protected females of this species were offered wooden nest nesting environments were statistically analysed blocks with 100 nesting holes (8 mm in diameter by a two-way model I ANOVA with year and nest environment as fixed effects followed by a factor NE (F = 413.62, P < 0.001). In 1993, Scheffé test for a posteriori comparison of means. the vestibuli were slightly shorter than in The existence of a vestibulum and the parasitism 1992. of the outermost cell were added as fixed effects in a model I ANOVA to analyse influences of Only 15 % (1992) and 5 % (1993) of nest parasitism rate on outermost cell parasitism. vestibuli in protected nests were large Parasitism rate (PR) was computed as number enough to accommodate an average-sized of cells parasitized per provisioned cells. A log- male cell. In exposed nests, 63 % (1992) linear analysis according to Warnstorff and Dör- and 50 % (1993) of vestibuli would have fel [23] of a four-dimensional contingency table been to accommodate another was used to detect influences on the occurrence large enough male cell of a vestibulum and a dependence of the outer- (figure 1). most cell’s contents on the existence of a vestibulum. In the contingency table, year (Y), nesting environment ( NE), and existence of a vestibulum 3.2. Thickness of the nest plug (V) were considered design variables, and the content of outermost brood cell (BC) was the Osmia nests had nest from response variable. All calculations were com- rufa plugs puted with the Statistica® package (StatSoft Inc.). 1.3 to 14.5 mm in thickness (table II). The thickness of the plug depended mainly on the occurrence of a vestibulum (F = 149.71, 3. RESULTS P < 0.001), but was not correlated with the vestibulum length (all R2 < 0.01). In nests 3.1. Frequency and length without a vestibulum, the plug was signifi- of the vestibulum cantly thicker than in nests with one (table II, all P < 0.001). An influence of the The occurrence of a vestibulum was nest environment on plug thickness was not related to the type of nesting environment: found (F = 0.64, n.s.). (NE x V 2I = 274.04, P < 0.001). A vestibulum was found in only half of the protected nests compared to ca. 90 % of exposed nests 3.3. Contents (table I). Also the mean vestibulum length of the outermost brood cell differed between the types of nesting environments. In both years, vestibuli from Parasites found in the outermost brood exposed nests were twice as long as vestibuli cells were Cacoxenus indagator (Diptera: from protected nests (table I; Scheffé-Test, Drosophilidae) in 10.4 % of nests, Anthrax all P < 0.001). Most of the variance in anthrax (Diptera: Bombyliidae) in 4.6 % of vestibulum length was associated with the nests, and Chaetodactylus osmiae (Acari: in 0.3 % of nests. These Chaetodactylidae) protected nests) was the mortality in nests species attack open cells during provision- without a vestibulum higher than in nests ing. In none of the 1 712 investigated nests with one ([V x BC]/Y1 NE2: 2I = 6.387, nest had a parasite broken through an intact P = 0.041). This was caused by increased plug and parasitized the first cell after the larval mortality apparently due to reasons nest was sealed. Parasitism by Monodon- other than parasitism (contrast: bee + para- tomerus obscurus (Hymenoptera: Pteroma- site against dead, 2I = 6.206, P = 0.012) and lidae) was rare. It occurred in less than 0.6 % not by parasitism itself (contrast: bee + dead of all nests and could always be attributed to against parasite, 2I = 0.068, n.s.). a defective nest plug or a damaged nest block. The length of the vestibulum clearly had no influence on the mortality of the outer- The lack of a vestibulum had no signifi- most brood cell (table IV). A multiple com- cant influence on the mortality of the out- parison of means did not show significant ermost brood cell (table III, [V × BC]/Y NE: differences between vestibulum lengths in 21 = 15.014, n.s.). Only in one trial (1992, nests with different outermost brood cell from 10.4 to 5.5 % from outer to inner cells, respectively, whereas mortality caused by Anthrax anthrax increased from 4.6 to 9.1 %. Mean parasitism of exposed nests (PR = 0.287) was much higher than in protected nests (PR = 0.081, P < 0.001). No differences were found in the mean parasitism of nests without a vestibulum (PR = 0.184) compared to nests with a vestibulum (PR = 0.193, P = 0.495). The amount of parasitism of the outermost cell corresponded to the parasitism rate of the remaining cells of the nest (F = 14.641, P < 0.001). Nests with a parasitized outermost cell were twice as parasitized = in the other brood cells com- contents in one nesting environment over (PR 0.304) the other (table IV, all P > 0.206). pared to nests with an unparasitized outermost cell (PR = 0.155, P < 0.001 ). There was also variation in parasitism 3.4. Parasitism of nests between nest blocks pooled for both nest and nest blocks environments and years. The parasitism rate of a nest block (total cells parasitized per The same parasites found in the outer- total provisioned cells) was correlated with most cells were also present in the inner the frequency of vestibulum occurrence cells of Osmia rufa nests. However, para- (R2 = 0.465, P < 0.001) and mean vestibu- sitism by Cacoxenus indagator decreased lum length (R2 = 0.809, P < 0.001) of all nests in this particular block. However, a by themselves. They depend on finding a better correlation of vestibulum frequency suitable hollow twig or a bore hole of a was achieved by regarding only parasitism beetle in dead wood. The nesting place is a by Cacoxenus indagator (R2 = 0.687, rare resource for these species [4, 5, 21]. P < 0.001). Under such conditions it would be expected that once a hole is found, its space would be used. If a certain of the 3.5. Distribution completely part hole remains vacant, an of parasites within nests nesting usually adaptive function would be expected to explain this space. The vestibulum repre- The main parasites Cacoxenus indaga- sents such a regular structure. tor and Anthrax anthrax were not equally distributed throughout the nests (figure 2). The frequency of occurrence and mean Parasitism by Anthrax anthrax increased length of vestibuli may give an important with distance from the nest entrance, indication of their purpose. If it is assumed whereas Cacoxenus indagator caused high- that the vestibulum plays a protective role est mortality in the outermost brood cells. against parasites, then it would be expected The sections near the entrance and the rear to have a regular occurrence as well as a of nests lacked provisioned cells (owing to minimal length. If on the other hand a the presence of the nest plug, vestibulum vestibulum represents only the space remain- and empty space); therefore, a curve for par- ing after completion of as many cells as pos- asitism rate could not be drawn for these sible in the cavity, it should be shorter than sections. a mean male cell of a given nest. Only 51 % of O. rufa nests in protected 4. DISCUSSION environments had a vestibulum, and in 90 % of these nests it was smaller than a male Many stem-, twig- or hole-nesting xylo- cell. For these protected nests, it may be philous Aculeata such as O. rufa are not concluded that the vestibulum represents able to excavate cavities to form their nests remaining space which was too short to construct an additional brood cell. In contrast, protected nests. The construction of a almost every nest in exposed environments vestibulum is not correlated with the para- had a vestibulum twice as long as those of sitism rate of the particular nest. It depends protected nests. Vestibuli with a mean length on parasitism of the whole nest block by of 20-30 mm occurred also in more than species attacking cells under construction. In 80 % of O. rufa nests in trap nest studies Halle, main parasites of this group are [1-3, 8]. Therefore, vestibuli in exposed Anthrax anthrax and Cacoxenus indagator. environments seem to have a functional The bomylid, A. anthrax, is a rare enemy advantage. of Osmia rufa, but in the botanical garden of Halle it causes regular mortality [16]. The in occurrence large fluctuations the A. anthrax females hover in front of the nest and of a vestibulum length argues against blocks and throw their eggs in the nest the defense function of the vestibu- parasite entrances, obviously unnoticed by the bees. lum. Parasites are able to locate and attack Cacoxenus indagator is a widespread para- O. rufa nests in both nest environments. site of Osmia rufa causing high mortality Thus, the bees should not make a difference regularly [3, 6, 14, 16, 17]. This drosophilid in their nest architecture between environ- sits on the surface of the nest block waiting ments. a vestibulum Additionally, may for its chance to parasitize. Occasionally it defend enemies that attack a only against walks into nesting holes to lay eggs on the sealed nest the nest wall by drilling through provision mass. The parasite can be easily or the of plug. Among major parasites detected by the nesting females and is some- O. rufa, only M. obscurus shows this times attacked. If the bees detect C. inda- behaviour Due to its this [16]. sting length, gator on their nest (or nest block) during is able to structures species hardly penetrate construction they should let the outermost thicker than 4 mm An nest [17]. ordinary space of the nesting hole remain empty to offers since plug enough defence, especially avoid the risk of high parasitism. Due to the bee cocoons lie at the rear partition of the increasing probability of parasitism by and therefore at the distance cell, greatest C. indagator closer to the nest entrance, the from the nest However, the observed plug. outermost space suffers highest parasitism. of the nest if a vestibu- strengthening plug Provisioning of a cell next to the nest lum is indicates a defence lacking possible entrance would lead to very high parasitism role of the vestibulum against parasitic because the parasite would not have to enter If both alternative the wasps. strategies - the nest and risk being caught by the bee. construction of a and vestibulum, strength- Therefore, leaving the outermost cell empty of the nest have the same effect, ening plug - by constructing a vestibulum might be a the little extra cost to build a thicker required strategy to avoid misinvestment in a brood nest could be used the bees to plug easily by cell which has an unacceptably high risk of better exploit rare nesting holes. being parasitized. Parasitism in the first brood cells is Under such circumstances, a higher par- slightly but not significantly increased in asitism of the outermost cells would be nests without a vestibulum compared to expected in exposed nests without a vestibu- nests with a vestibulum. In addition, nests lum. However, mortality of the outermost with parasites in the first cell do not have cell caused by parasitism is not independent shorter vestibuli than others. Nevertheless, of the rate of parasitism in the remaining the occurrence and mean length of a vestibu- cells of the nest. This suggests that particu- lum is related to parasitism. Exposed nests lar nests face different risks of parasitism suffered higher mortality due to parasites depending on their location within the nest attacking cells during provisioning. These block [ 17]. Part of the nests without vestibuli nests had more and longer vestibuli than were probably exposed to less parasitism resulting in the observed reduction of nids de nombreux apoïdes solitaires et xylo- expected mortality. In addition, this effect is philes comprennent entre le bouchon qui probably responsible for the stopped increase ferme le nid et la première cellule approvi- of parasitism by C. indagator near the nest sionnée un espace vide dénommé vestibule. entrance (figure 2). The curve represents Afin d’expliquer la fonction de cette cellule there only nests without a vestibulum. vide à l’entrée du nid, 1 712 nids d’Osmia ont été ouverts et Les nids The construction of a large vestibulum rufa analysés. de 22 blocs nichoirs démon- instead of an additional brood cell in most of provenaient tables avaient été soit dans une the nests in exposed environments may also qui placés ruche intacte nids soit dans suggest a possible thermoregulatory func- (« protégés »), tion of the vestibulum. Immature stages of une ruche dont on avait ôté le panneau fron- tal nids O. rufa are relatively temperature sensitive. (« exposés »). Already at 32 °C the mortality of larvae and La fréquence et la taille des vestibules est du lieu. Les nids pupae increases precipitously [13]. In a hole indépendante exposés pos- sédaient un vestibule à 90 % drilled by a beetle in dead wood, a vestibu- (tableau I). Dans 55 % des cas les vestibules étaient suf- lum may be able to protect the outermost cells from excessive warming in sunshine fisamment grands pour qu’une cellule à cou- and to buffer temperature variations. There- vain supplémentaire soit construite. En 51 % seulement des nids fore, larvae in exposed nests without a revanche, proté- vestibulum should suffer higher mortality gés avaient un vestibule et 10 % seulement in the outermost cell due to reasons other de ces vestibules auraient pu contenir une than parasitism compared to nests with a cellule à couvain supplémentaire (figure 1). vestibulum. This effect could not be demon- Là où manquait le vestibule, la fermeture strated. The only difference was found in du nid était renforcée (tableau II). L’exis- the protected environment where it was not tence du vestibule n’a pas eu d’influence expected. But because of the low number sur le contenu de la première cellule à cou- of nests without a vestibulum in exposed vain. Les nids sans vestibule n’étaient pas environments, the data base of this study is plus parasités que les nids avec vestibule too small to accept or reject a thermoregu- (tableau III). De même, on n’a pas trouvé de latory function of the vestibulum. Precise différence dans la longueur du vestibule temperature measurements within nests to entre les nids parasités et les nids non para- test this hypothesis have yet to be performed. sités (tableau IV). On n’a pas trouvé un seul As O. rufa nests also have vestibuli in twigs nid où un parasite ait réussi à pénétrer dans or stems where temperature regulation un nid obturé. effects should be negligible, this effect can La fonction du vestibule ne peut donc pas not be regarded as its main function. résider dans la défense contre les parasites, qui attaquent le nid après sa finition. Une fermeture moyenne suffit à empêcher les ACKNOWLEDGEMENTS ichneumons comme Monodontomerus obscurus de pénétrer dans le nid. En outre, les I want to thank an referee for pro- anonymous peuvent localiser et envahir les viding valuable criticisms on an earlier draft and parasites nids de sorte ne faut R. Ziegler for his suggestions. This work was cachés, qu’il pas supported by a scholarship from the Studien- s’attendre à des différences dans la position stiftung des deutschen Volkes. du vestibule en fonction du lieu de nidifi- cation. Les vestibules courts et rares des nids pro- Résumé - La fonction du vestibule dans tégés représentent donc certainement les nids de l’abeille solitaire Osmia l’espace restant, insuffisamment grand pour rufa (L.), qui nidifie dans des tiges. Les construire une autre cellule. En revanche il faut reconnaître une fonction adaptative aux Niströhre zu klären, wurden 1 712 Nester vestibules des nids exposés, puisque presque von Osmia rufa geöffnet und analysiert. Die tous ces nids possèdent un vestibule. Nester stammen aus 22 zerlegbaren Holz- La construction d’un vestibule dépend de Nistblöcken, welche entweder in einer nor- l’attaque des blocs nichoirs par les parasites, malen Bienenbeute (’geschützte Nester’) qui déposent leurs œufs dès la phase oder in einer Beute, deren Vorderfront ent- d’approvisionnement dans les cellules à cou- fernt worden war (’ungeschützte Nester’), vain encore ouvertes. untergebracht waren. La fréquence de la construction d’un vesti- Die Häufigkeit und Größe des Vestibulums bule et de sa longueur moyenne a été corré- war vom Standort abhängig. Ungeschützte lée avec le parasitisme de l’ensemble du Nester enthielten zu 90 % ein Vestibulum bloc nichoir par Cacoxenus indagator et (Tabelle I). Bei 55 % dieser Vestibuli hätte Anthrax anthrax. Alors que A. anthrax der Raum für die Anlage einer weiteren compte parmi les parasites peu fréquents Brutzelle ausgereicht. Demgegenüber be- d’O. C. est un rufa, indagator parasite saßen nur 51 % der geschützten Nester ein cause une important qui régulièrement forte Vestibulum und nur 10 % dieser Vestibuli mortalité. Ce ne drosophilidé parasite pas waren groß genug für eine weitere Brutzelle toutes les cellules du nid hôte avec la même (Abb. 1). Fehlte das Vestibulum, so wurde probabilité (figure 2): le taux de parasitisme der Nestverschluß verstärkt (Tabelle II). Die des cellules augmente fortement quand on se Existenz eines Vestibulums beeinflußte den de l’entrée du nid. les rapproche Puisque Inhalt der ersten Brutzelle nicht. Nester ohne femelles d’O. détecter rufa peuvent la pré- Vestibulum waren in diesem Bereich nicht sence de C. elles évitent le indagator, risque stärker als Nester mit Vestibu- de très élevé des cellules exté- parasitiert parasitisme lum (Tabelle III). Auch die mittlere Länge rieures en laissant celles-ci vides. Le vesti- des Vestibulums unterschied sich nicht zwi- bule en résulte semble le qui représenter schen parasitierten und unparasitierten résultat éviter un investissement mal pour Nestern (Tabelle IV). Es konnte kein einzi- placé, ou une stratégie de réduction du ges Nest werden, in dem ein Para- Pour un insecte tel O. gefunden risque. que rufa, qui sit den intakten überwunden niche dans du bois mort, le vestibule Nestverschluß pour- hatte. rait avoir aussi une autre fonction : celle Die des Vestibulums kann daher d’améliorer le microclimat du nid, puisque Bedeutung nicht in einer Abwehr von Parasiten l’espace rempli d’air peut atténuer les fortes liegen, welche das Nest nach seiner variations extérieures de Fertigstellung températures, wie Monodonto- lorsqu’il y a par exemple ensoleillement. angreifen. Schlupfwespen © Inra/DIB/AGIB/Elsevier, Paris merus obscurus werden bereits von einem durchschnittlichen Nestverschluß abge- Osmia rufa / vestibule / nid / parasitisme wehrt. Außerdem können Parasiten auch verdeckte Nester lokalisieren und befallen, so daß keine Unterschiede in der Anlage eines Vestibulums in Abhängigkeit vom Zusammenfassung - Die Bedeutung der Nistort zu erwarten wären. Vorzelle in Nestern der solitären, röhren- Die kurzen und seltenen Vestibuli der bewohnenden Biene Osmia rufa (L.). Die geschützten Nester stellen daher sicherlich Nester vieler solitärer, xylophiler Apoidea den Restraum der Niströhre dar, welcher enthalten zwischen dem Nestverschluß und zur Anlage einer weiteren Zelle nicht mehr der ersten verproviantierten Zelle einen lee- ausreichte. Demgegenüber muß den Vesti- ren Raum, das Vestibulum. Um die Bedeu- buli der ungeschützten Nester eine adaptive tung dieser leeren Zelle am Eingang der Funktion zukommen, da fast alle Nester ein Vestibulum enthalten, für dessen Anlage [2] Bayer-Helms F., Untersuchungen zum auf die Verproviantierung einer weiteren Fortpflanzungsverhalten solitär lebender Wild- bienen an den Beispielen Osmia rufa und Brutzelle verzichtet wurde. Megachile versicolor (Hymenoptera: Megachil- Die Anlage eines Vestibulums erwies sich idae), Diplomarbeit, Göttingen, 1993. Brechtel Die Stechimmenfauna des Bienen- vom Befall der Nestblöcke durch Parasiten [3] F., waldes und seiner Randbereiche unter welche ihre Eier bereits in der (Südpfalz) abhängig, besonderer Berücksichtigung der Ökologie kunst- Phase der Verproviantierung in die noch nestbewohnender Arten, Pollichia, Bad Dürkheim, 1986. offenen Brutzellen legen. Bei einer zusam- menfassenden aller Nester ohne [4] Chandler L., Interspecific competition in Osmia Analyse lignaria SAY - Osmia cordata Robertson, Proc. Diskriminierung des Neststandortes war die North Central Branch, Entomol. Soc. Am. 16 Häufigkeit der Anlage eines Vestibulums (1961) 18-19. und dessen mittlere Länge mit der Parasi- [5] Danks H.V., Populations and nesting sites of des Nistblockes durch some aculeate Hymenoptera nesting in Rubus, tierung gesamten J. Anim. Ecol. 40 (1971) 63-77. Cacoxenus und Anthrax anthrax indagator [6] Gathmann A., Bienen, Wespen und ihre Gegen- korrelliert. Während A. anthrax zu den sel- spieler in der Agrarlandschaft: Artenreichtum tenen Parasiten von O. rufa zählt, ist C. inda- und Interaktionen in Nisthilfen, Aktionsradien und Univ. Göt- ein welcher Habitatbewertung, Dissertation, gator Kardinalparasit, regel- tingen. Cuvillier, Göttingen, 1998. eine hohe Mortalität verursacht. 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