Description & Statistics
Syrphidae is a large family with more than 4,004 species known by 2000. They are spread worldwide, but are most diverse in the tropics. Important diagnostic characters include an antenna with only three segments, the 3rd segment bearing a style or arista; the wing’s spurious vein is between R and M; the anal cell is closed near the wing margin. The head is semi-circular and almost as wide as the thorax. The body is colored variously, and sometimes metallic green or blue. The eyes are large, commonly holoptic in males.
Syrphids are diverse biologically, many being mimics of Aculeate Hymenoptera. Most entomophagous species are predaceous on aphids, scale insects and cercopids, but a few attack larvae of Lepidoptera. Some syrphids are scavengers, phytophagous or myrmecophiles. Several species have been used in the biological control of aphids, but with no marked success.
The genus Microdon may be found in ant nests, possibly as a commensal. Volucella sp. lives as a scavenger in the nests of bumblebees and wasps. Several species, particularly of the genera Eumerus, Mesogramma and Merodon, are phytophagous, and species of other genera develop in the sap exuding from tree wounds. However, most species are predaceous on aphids, although a few attack Chermidae, dactylopine Coccidae, Cercopidae and lepidopterous larvae (Clausen 1940/62).
Species with predaceous larvae represent a considerable number of genera, with little variation in behavior among them. In studies of the food habits of aphid feeding species in California, Campbell & Davidson (1924) noted that the eggs are usually laid singly ont he leaf or bark surface among the aphid and mealybug colonies. Females may be capable of depositing several hundred eggs, with a maximum for one day of ca. 25. In feeding experiments it was found that up to 400 aphids are consumed by a single larva during development; but this number varies with the size of the species and stage and species of host provided (Clausen 1940/62). Several species complete their development on 100 or fewer 1st to 4th instar aphids. Scarcity of food results in the prolongation of the larval stage (Metcalf 1917).
In the British West Indies, Salpingogaster nigra Sch. is a common natural enemy of the sugarcane froghopper, Tomaspis saccharina Dist. (Guppy 1913, 1914). Eggs are laid in small clusters directly into the spittle mass of the host and hatch in 2-3 days. Larvae complete development in 9-10 days, during which time a total of 30-40 froghopper nymphs are destroyed. During attack, the maggot usually pierces it dorsally behind the first abdominal segment. The pupal stage lasts 9 days and the species is thus able to produce successive generations at less than one month intervals as long as moisture conditions are favorable. The biological control of the froghopper in Trinidad has been attempted with this predator.
The larvae of Syrphus rapalus Wlk. in New Zealand are well adapted for attack on caterpillars of Venusa verriculata on the foliage of cabbage trees (Miller & Watt 1915). The latter congregate on the inner surface of the outer leaves which enclose the heart spike and the Syrphus larvae, which are negatively phototactic, favor the same location. In Europe, Xanthandrus comptus Harr. has been observed feeding on larvae of Pieris brassicae L., Cnethocampa pinivora Tr., etc., and seems to be a general predator on various caterpillars. Observations on 12 colonies of Cnethocampa by H. D. Smith (1936) revealed that Xanthandrus larvae were present in practically all of them, with the population of 11-12 colonies being largely or entirely destroyed. An average of 2.5 larvae were found in each nest. In one colony that contained 80 caterpillars (1st instar?) all had been killed. This species shows an exceptional host range. Besides preying on various caterpillars, it is more generally an aphid predator. In other species larvae pupate among the host colonies on foliage, while in still others they descend to the ground and form puparia among debris.
Life cycles of species feeding on aphids and mealybugs are quite uniform and cover a period of 16-28 days from egg laying to adult emergence. Egg incubation requires 2-3 days in summer, and larval and pupal stages are about the same. There are 5-7 generations annually, with winter being passed as pupae. S. rapalus is a notable exception in that the larval period takes several months.
Attention was called to a pronounced tendency toward migration on the part of the mature larvae, this being induced by the need for moist surroundings for pupation (Kamal 1939). In California alfalfa infested with aphids Kamal noted a marked scarcity of mature larvae and puparia. A search revealed that large numbers of larvae had moved to the field margins and pupated in the damp soil along embankments, some having penetrated to 10-15 cm. In certain favored areas the puparia were massed in great numbers.
For detailed descriptions of immature stages, please see Clausen (1940).
Campbell, R. & W. Davidson. 1924. So. Calif. Acad. Sci. Bull. 23: 1-9, 59-71.
Cole, F. R. 1969. The Flies of Western North America. Univ. Calif. Press, Berkeley & Los Angeles. 693 p.
Fluke, C. L. 1929. Wisconsin Agr. Expt. Sta. Res. Bull. 93: 1-47.
Glumac, S. 1961. 11th Internatl. Cong. Ent. 1: 202-6.
Hull, F. M. 1949. Trans. Zool. Soc. London 26: 257-408.