Bumblebees "Agri Ticinensis''
A peculiar assemblage
(preliminary version - 20 June 2008)
In May 1832, Antonio Pensa defended his doctoral dissertation  before the medical faculty of the University of Pavia. His thesis, bearing the explicative title De Insectis Venenatis Agri Ticinensis (On the venomous insects of the Pavia countryside) is essentially an enumeration of families and species of insects, accompanied by brief descriptions and occasional comments. Seven species of bumblebees are mentioned as present in the surroundings of Pavia: Apis (Bombus) ligustica Spinola (i.e., Bombus argillaceus (Scopoli)), A. (Bombus) terrestris Linnaeus, A. ruderata Fabricius, A. (Bombus) lapidaria Linnaeus, A. (Bombus) sylvarum Fabricius, A. (Bombus) muscorum Linnaeus, and A. (Bombus) hypnorum Linnaeus. Over 170 years later, it may be of some interest to assess what bumblebee species are to be found today in the area, and to compare the present situation with Pensa's list. This is what I shall try to do in this note, revisiting Pensa's list in the light of the results of field observations and collections carried out in the vicinity of Pavia during the years 2001-08. It must be said right away that the observations have not been systematic, as the coverage of the study area has been spotty at best, and collecting anything but unbiased. In particular, they cannot form the basis of a quantitative analysis of the bumblebee populations in the area. On the other hand, they can give an idea of the species to be found and of their preferred habitats.
1. The study area, its climate, vegetation, and patterns of land use. The town of Pavia (the Ticinum of the Romans) is situated in the southern part of Lombardy (maps), in the heart of the Northern Italian plain, on a major river, the Ticino. The term "Ager Ticinensis" is somewhat vague and it is not clear what Pensa exactly meant with it, but it certainly referred to a territory much larger than the study area, and including it entirely. The area I have explored is a small part of the province of Pavia, extending mostly to the NW of the town, up to a distance of approximately 14 km from its centre (maps). It consists of a level alluvial plain, except for shallow indentations carved by streams, the major of which is the wide valley of the Ticino river. Elevation varies between 61 and 98 metres above sea level. The climate is moderately continental, with cold winters, hot summers, and moderate rainfall (about 800 mm annually), occurring throughout the year but particularly in late spring and in autumn. Urban areas are few and, except for the town of Pavia, limited in extent; for its greatest part, the rest is agricultural land. The area has been under intensive cultivation for centuries, and the original vegetation has been almost entirely removed, except for remnants of broadleaf woodland along the Ticino. The main crop is rice, followed by maize; there are also extensive poplar plantations. The permanent irrigated hay-fields which were formerly rather frequent, and presumably constituted good foraging places for bumblebees, have almost entirely disappeared following a sharp decline in cattle raising. The area therefore appears to be hostile to bumblebees; these are generally found only in marginal places, such as fallow fields, margins of roads and canals, irrigation ditches, and edges of rice fields. In fact, the latter habitats are important foraging areas, as the presence of abundant moisture allows them to support throughout the summer a continuous succession of flowers, in particular long-corolla ones such as clovers and vetches, at least where weed control is practised with moderation.
2. The present bumblebee fauna. During the years 2001-08 I have observed eleven species of bumblebees, namely Bombus muscorum (Fabricius, 1793), B. sylvarum (Linnaeus, 1761), B. pascuorum (Scopoli, 1763), B. hortorum (Linnaeus, 1761), B. ruderatus (Fabricius, 1775), B. argillaceus (Scopoli, 1763), B. vestalis (Geoffroy in Fourcroy, 1785), B. hypnorum (Linnaeus, 1758), B. pratorum (Linnaeus, 1761), B. terrestris (Linnaeus, 1758), and B. lapidarius (Linnaeus, 1758).
- Bombus muscorum has been found in few localities (map), always associated with humid habitats (canals, ditches, rice field margins), where it seems to favour Tufted Vetch Vicia cracca and Clovers Trifolium sp.. I have also seen it foraging on White Dead-Nettle Lamium album, Alkanet Anchusa officinalis, Comfrey Symphytum officinale and Marsh Woundwort Stachys palustris. Queens have been observed as early as mid-April, and workers as late as the beginning of September.
- Bombus sylvarum frequents much the same habitats as muscorum, but does not seem to show as strong an affinity for humid environments. It is definitely more numerous than muscorum, though not really common (map). It favours much the same flowers as muscorum, but I have also found it on Red Dead-Nettle Lamium purpureum (together with pascuorum, argillaceus and terrestris), White Dead-Nettle Lamium album, Self-Heal Prunella vulgaris and Alkanet Anchusa officinalis in spring and early summer, and on Knapweed Centaurea sp. in late summer. I have observed queens as early as 17 March, and males as late as the first decade of September.
- Bombus pascuorum is the most common species, together with terrestris, and is found virtually everywhere, from urban areas to the woodlands along the Ticino. It shows the usual long colony cycle; queens have been observed from the third decade of March, while workers are abroad throughout September, and I have seen young queens and males as late as mid-October.
- Bombus hortorum has been found in a few localities (map), almost all in wooded areas or at the edge of woodland, mostrly along the Ticino. The species has been observed between early April and late June, foraging on Red Dead-Nettle Lamium purpureum, White Dead-Nettle Lamium album, Yellow Iris Iris pseudacorus, Bramble Rubus sp., Alkanet Anchusa officinalis, Viper's Bugloss Echium vulgare, and Red Clover Trifolium pratense. Usually, males first appear at the end of May.
- Bombus ruderatus has been observed in a single locality (map), where I have seen just one queen in mid-March, foraging on Red Dead-Nettle Lamium purpureum in a poplar plantation. However, due to the similarities between workers and males of this species and those of argillaceus, and to some extent also those of hortorum, one cannot discount the possibility that some of the workers or males which I ascribed to the latter two species might in fact have been ruderatus.
- Bombus argillaceus has been found in scattered localities throughout the study area (map), and can be locally common; it is the most abundant species after pascuorum and terrestris. The earliest queens have been observed at the end of the second decade of March, and workers generally appear one month later. Males are abroad in numbers in June, at which time the species often outnumbers all others where it occurs. By the first decade of July only scattered individuals are left, and the species virtually disappears well before the end of the month. The relative paucity of spring sightings of queens and, by contrast, the abundance of workers and males in early summer, seem to suggest that argillaceus colonies are very large, and that they undergo an explosive development between April and their final demise in late June-early July. B. argillaceus favours open areas away from trees but, unlike muscorum or sylvarum, does not seem to be particularly attached to humid situations, possibly because its shorter colony cycle allows it to largely avoid the relative aridity of the summer months. For instance, I have found many workers and males foraging on a large patch of Echium vulgare in a very dry area near the Ticino, and workers foraging in a shopping centre parking lot where Red Clover Trifolium pratense was blooming profusely. Interestingly, while T. pratense is definitely a favourite, the other clovers, such as for instance T. repens, seem to be shunned entirely. Other favourite flowers are Red Dead-Nettle Lamium purpureum, Bugle Ajuga sp., Ground Ivy Glechoma hederacea, Self-Heal Prunella vulgaris, Yellow Iris Iris pseudacorus, Vetches Vicia sp., Alkanet Anchusa officinalis and Comfrey Symphytum officinale.
- Bombus vestalis has been observed only in one locality (map), a dry open area close to woodland near the Ticino, where I have seen a single female in late May, feeding on Viper's Bugloss Echium vulgare. Other bumblebee species present in this locality were pascuorum, hortorum, argillaceus, and terrestris.
- Bombus hypnorum appears to be very rare (map). I have observed just a couple of males in early June, feeding on Bramble Rubus sp. alongside individuals of pascuorum, hortorum, argillaceus, pratorum and terrestris.
- Bombus pratorum is also very rare (map), and I have found it only in two localities. In the first of these I have observed one queen in early March, while in the second I have observed very few males between the end of May and early June, feeding on Alkanet Anchusa officinalis and on Bramble Rubus sp. together with other bumblebees belonging to the species sylvarum, pascuorum, hortorum, argillaceus, hypnorum and terrestris.
- Bombus terrestris is as ubiquitary as pascuorum, though it does not usually occur in true woodland along the Ticino, but only in clearings and woodland edges. The earliest queens usually appear during the first decade of March, and a few individuals, mostly young queens, can be observed well into October.
- Bombus lapidarius is among the rarest of the species observed (map), together with ruderatus, vestalis, hypnorum and pratorum. I have seen only very few isolated workers in one locality, foraging on Trifolium pratense in the company of abundant argillaceus and some pascuorum.
It is interesting to notice that the greatest variety of bumblebee species, and the greatest number of individuals, have been observed in cultivated land, where I have found up to six species foraging together, and not in the more "natural" areas along the Ticino. While this may in part reflect an observational bias, it must be noticed that the areas adjoining the river are often wooded, and because of their generally poor gravelly or sandy soils experience considerably more arid conditions during the summer months than the surrounding cultivated land. Perhaps because of these factors, typical bumblebee flowers, particularly key ones such as vetches and clovers, tend to be scarce, especially during the summer.
3. Concluding remarks. As we have seen, all the bumblebee species mentioned by Pensa are still present today in the vicinity of Pavia. As is well known, it is often difficult to distinguish individuals of Bombus ruderatus from individuals of the two sister species argillaceus and hortorum, the greatest potential for confusion arising between workers of the three species and between males of ruderatus and argillaceus; only the queens of argillaceus stand apart, on account of their distinctive colouring. I have seen just one individual that could convincingly be classified as ruderatus, a queen. On the other hand, I have observed several argillaceus queens and a few typical hortorum queens, plus many workers and males of argillaceus and several workers and males of hortorum. Most of the argillaceus/ruderatus workers and males that I have seen looked like classic argillaceus, the workers showing moderately darkened wings, a very broad yellow collar and a narrow interalar band, the males an even narrower interalar band and abundant yellow hair on the vertex. However, as I said, confusion with ruderatus and hortorum cannot be ruled out. A slightly puzzling finding is that of a specimen, intermediate in size between a typical argillaceus queen and a typical worker, showing strikingly darkened wings and worker-like body colours, except for all-black first and second tergites. My guess is that it is simply a large argillaceus worker or a small argillaceus queen. I have not observed in argillaceus the almost continuous size variation, from the largest gynes to the smallest workers, which occurs for instance in hortorum. However, there could well be occasional individuals that are intermediate between typical queens and typical workers in colouring as well as in body size. Another possibility is that of a hybrid between argillaceus and ruderatus; occasional interbreeding between the two species is thought to occur in their area of sympatry in south-eastern France .
Pensa's description of argillaceus, which reads "hairy bee, black, thorax yellow above with a black band in the middle", raises some questions. It is the description of a queen, and Pensa makes no mention of the chromatic dimorphism between argillaceus queens and workers or males. Furthermore, he gives measurements, expressed in lines, for each species. It is not clear whether he is using English lines or, as is more likely, French "lignes"; in any case, a line is slightly more than 2 mm. Now, the body lengths that he gives for argillaceus and ruderatus (and which, presumably, represent averages) are 11 lines and 7-8 lines, respectively. The first measurement is a realistic value for the length of argillaceus queens, which in the area explored averages about 24 mm. On the other hand, the ratio between the two measurements is close to 1.45, which corresponds to a ratio of roughly 3:1 in body weights. These ratios are suspiciously similar to those that hold in many bumblebee species, including argillaceus, between the corresponding measurements of queens and workers. So my impression is that many, if not most, of Pensa's ruderatus might just be workers and males of argillaceus. As is clear from  and , while the presence of argillaceus in the central part of the Northern Italian plain is well documented, Pensa's is virtually the only record of ruderatus in the literature for this region, all other records for Northern Italy coming from the surrounding hills or from the area of contact between hills and plain.
The presence of vestalis is remarkable. To my knowledge, this is the first record of the species for the whole Northern Italian plain . In fact, it is the first record of any Psithyrus for this region, except for a couple of very old records of campestris from the provinces of Padova and Venezia .
One puzzling aspect of Pensa's list is the absence of pascuorum, which is surprising, in view of the ubiquity and abundance of the species. Possibly Pensa regarded what we nowadays would call pascuorum as belonging to muscorum, although his description of the latter ("hairy bee, orange-red with yellow abdomen") does not fit very well with the colours of the local pascuorum, which are orange-red all over.
On the contrary, the absence of pratorum from Pensa's list is not surprising. To my knowledge, there are no records of it in the literature for the entire Northern Italian plain , and the species appears to be very uncommon at best in the territory explored. Most of the area, as we see it nowadays, does not seem to meet the ecological requirements of pratorum. In this respect, it is perhaps significant that most of the observations of the species took place in a niche environment having many of the characters of a natural woodland edge. It is also significant that hypnorum, whose ecological requirements are close to those of pratorum, was also found in the same locality, and that this was the case also for hortorum, which I almost always found in, or close to, wooded land.
Much of what has been said of pratorum applies also to hypnorum. The species occurs throughout the Alps, but there are few records in the literature for other parts of Italy, the only recent ones coming from the subalpine region and from a couple of localities in the Northern Apennines ; I have also observed the species at high elevation in the Apennines just south of Pavia. Pensa's is the only literature record for the central and western portions of the Northern Italian plain. Possibly, in his days, before the advent of mechanized agriculture, habitats suitable to hypnorum were more common, and the species was more frequently encountered.
I have found no specimens belonging to the subgenus Bombus s.s. which could convincingly be classified as Bombus lucorum (Linnaeus, 1761); virtually all the individuals I have seen appeared to be more or less typical terrestris.
1. Pensa A. 1832. De Insectis Venenatis Agri Ticinensis. pp. 31. Ticini Regii.
2. Scholl, A., Obrecht, E. & Zimmermann, M. 1992. Evidence of hybridization of Bombus argillaceus and B. ruderatus (Hymenoptera: Apidae) in a zone of contact in France: enzyme electrophoretic data. Proceedings of the XIX International Congress of Entomology, Beijing p. 53.
3. Intoppa F., Piazza M.G., Ricciardelli D'Albore G. 1995. Catalogo bibliografico delle specie di Bombidae (Hymenoptera Apoidea) segnalate per l'Italia. Apicoltura 10, supplemento. pp. 135.
4. Quaranta M. et al. 2004. Wild bees in agroecosystems and semi-natural landscapes. 1997-2000 collection period in Italy. Bulletin of Insectology 57 (1): 11-61
5. Contarini M. 1843. Cataloghi degli Uccelli e degli Insetti delle province di Padova e Venezia. Bassano: 34-35.