|Cheilosia caerulescens Meigen, a destroyer of houseleek [Sempervivum tectorum]
J. d'Aguilar & R. Coutin (1988) Bull.Soc.ent.Fr. 92:307-9
Syrphid larvae are mainly known as carnivores, particularly as aphid predators. In fact, they have the most varied habitats. Some are aquatic, living as zoophages [sic] in clean (Chrysogaster) or polluted (Eristalis, Helophilus) water; some are saprophages or coprophages (Rhingia); some are necrophages or commensals in the nests of social insects (Volucella, Microdon...); finally, a good number of them are phytophages, and can sometimes be considered as pests. This is the case in Eumerus and Merodon, which destroy ornamental bulbs of Liliaceae and Amaryllidaceae, or Eumerus tricolor which damages the roots of salsify (Tragopogon porrifolius L.) or even Mesograpta polita Say which feeds on cereal pollen in North America.
The genus Cheilosia has many species generally with phytophagous larvae. They are found in the stems, even in tree trunks [footnote: certain american species are the cause of a disease of conifers called 'Black Check'], leaves, plant roots, or even in mushrooms as in Cheilosia scutellata Fallen in Tuber uncinatum Chatin (Coutin, 1983). A certain number of species are found in mountainous regions devloping in more than 100 different plant species, and particularly in alpine plants.
In September 1967 we found in a garden in Versailles leaves and whole rosettes of the houseleek, Sempervivum tectorum L., mined by stout syrphid larvae. When reared, these larvae eclosed into adults in spring 1968. Pursuing the inquiry around Versailles, one of us (R.C.) saw in the rockery of his own garden in Velizy (Yvelines) the same damage on the same plant species.
The adult was finally identified as Cheilosia caerulescens Meigen. This is a fairly widely distributed species in Europe, to be found in flat country as well as in mountainous regions (Alps, Pyrenees). The species has been observed on Sempervivum tectorum, S. arachnoideum L., Geum montanum L., Solidago virgaurea L. and Tussilago farfara L.
We have followed some aspects of the life cycle of this fly in a garden in the Paris region (Velizy). Cheilosia caerulescens overwinters as a puparium in the soil under the rosettes of the houseleek. The first adults appear at the beginning of May. Flight over the plants is quite slow and suggests searching behaviour. They forage also from a diverse array of neighbouring spring flowers as well as from their host-plant. There occurs at this time an apparent oviposition: females, after landing on a rosette, creep between the fleshy leaves and disappear for an instant, reappearing later. The real oviposition is later, and normally does not seem to start before the beginning of June. Copulation was not observed. When ovipositing, the female lays her eggs in twos or threes on the base of the leaves. A few days later the larvae penetrate the leaves at the edge, hollowing out the mesophyll in tangled sinous galleries. As rearing allowed us to confirm, each larva attacks successively 4 or 5 leaves to be sure of full growth. [When fully grown] the larvae measures about 11 mm long. The damage they do is important, and is particularly visible at the end of June; certain rosettes can be totally destroyed, the mined leaves turning brown and rotting. The first pupae are formed at the beginning of July, and the new adults can be collected and observed in activity from the middle of July. They produce a second generation, of which the larvae finish their development between mid-august and the beginning of September. Pupae are found in the soil of planting places at the end of September, overwintering in these places. Observations in the field and rearings done between 1969 and 1976 confirm these data.
Cheilosia caerulescens is therefore a typical bivoltine species, overwintering as a pupa, and which develops preferentially in Sempervivum tectorum. In a rockery where two or three species of Sempervivum are cultivated side by side, we have noticed that only S. tectorum is regularly and strongly attacked, whilst S. montanum Jacq. more rarely and S. arachnoideum not at all. Damaged plantings rapidly lose a large of their decorative character, even all of it. The two generations of larvae eat the rosettes of the preceeding year, whilst larvae of the second generation sometimes can develop on the new rosettes formed in spring.
Coutin, R. (1983) [Flies (rabassiere - don't know what this means!) and truffle flies] Bull.Fed.nat. Product. Truffes 6:17-22
1: Cheilosia caerulescens, imago plundering a capitulum of Doronicum.
2: A rosette of leaves of the houseleek, Sempervivum montanum, sheltering two eggs of C. caerulescens at the base of the leaf.
3: Detail of the preceeding photo.
4: Leaves of the roof houseleek, S. tectorum, mined by larvae of C. caerulescens, and showing the exit holes of the larvae.
5: Detail of a leaf of S. tectorum showing a larva of C. caerulescens in its gallery, just before it finishes its development.