Winter, 1992 (v5n2)
Bugg, Robert L.
Article written for Sustainable Agriculture Technical Reviews.
Introduction
According to Vockeroth and Thompson (1987), the Syrphidae (Diptera) comprise three subfamilies, 180 genera, and about 6,000 described species. Adults of many syrphid species resemble stinging bees and wasps. Larvae of some species are aphidophagous; these are in the subfamily Syrphinae. Common nonaphidophagous syrphids on Californian farmlands include Eristalis spp., the larvae of which live in liquified manure or in sewage ponds; Eumerus spp., the larvae of which feed on bulbs of plants; and Syritta pipiens, the larvae of which live in manure or compost. The uninitiated observer may mistake these for aphid predators.
Adult aphidophagous Syrphidae
are frequent flower visitors, and the morphology of the mouthparts suggests
that certain species are predominantly nectarivorous, where as others
are pollinivorous (Gilbert, 1981). Adults require honeydew or nectar and
pollen to ensure reproduction, whereas larvae usually require aphids to
complete development (Schneider, 1969). This seems to indicate that complementary
foods are required for completion of the life cycle. However, there are
exceptions to the rules: lacking aphids, larvae of several species can
subsist on plant materials such as pollen (e.g., Melanostoma
and Allograpta obliqua-Schneider, 1969; Toxomerus
[Mesograpta sp.]-Cole and Schlinger, 1969).
Adult syrphids can be sampled by a variety of methods, including visually scanning crops while walking, aerial netting, suction traps, Malaise traps, and water traps. For assessing eggs, larvae, and pupae of aphidophagous Syrphidae, whole-plant removal from the field and examination in the laboratory proved superior to quick inspection of plots (while walking) and to detailed visual inspection of plants in the field (Lapchin et al., 1987). Keys to Syrphidae are found in Cole and Schlinger (1969) (adults), and Heiss (1938) (1arvae and puparia).
Aphidophagous Species
World-wide, there is a fairly
large number of species of aphidophagous syrphids. For example, at least
49 species of Syrphidae attack green peach aphid, Myzus persicae (Sulzer)
(Van Emden et al., 1969). Some of the aphidophagous syrphids most common
in California were described by Smith and Hagen (1956) as follows:
- The large hover fly, Scaeva pyrastri (L.) may consume over 500 aphids during its larval stages. Adults are 1.27 cm in length. Abdomen is dark, with six white, curved stripes. Larvae are light green with a white dorsal longitudinal stripe.
- The western hover fly, Syrphus opinator Osten Sacken: adult is slightly smaller than S. pyrastri, has a yellow face, is slightly smaller, and has two black spots and two bands extending across the abdomen. The larvae are spiny and yellow or brown.
- Metasyrphus meadii (Jones) is similar to S. opinator but has black markings on the face, and the thorax is shiny.
- The bird hover fly, Eupeodes volucris Osten Sacken: the female looks similar to the large hover fly, but is only 0.85 cm long; males have a narrow cylinder at the tip of the abdomen.
- The chevroned hover fly, Allograpta obliqua (Say) is about 0.85 cm or less in length, and slenderer than E. volucris. This species has transverse yellow bands on the abdomen, and two oblique yellow marks near the tip. The larvae are smooth and green, with a broad white median strip. The breathing tubes are prominent.
- Sphaerophoria cylindrica (Say) is about the same size as A. obliqua, but has a narrow cylindrical abdomen. Larvae are greenish yellow and more or less transparent.
Syrphid Behavior
Aphid colonies are ephemeral
resources: they can appear quickly and just as suddenly disappear due
to predation, parasitism, fungal epizootics, declining host-plant quality,
changes in weather, or dispersal. Therefore, it may be important to predators
to locate aphid infestations quickly. Because of their strong flight and
ability to hover and inspect foliage for aphids, syrphids may be especially
adept at this. Based on mean number of aphids on plants with predators
divided by mean number of aphids on all collard plants, syrphids sometimes
appeared better at locating aggregations of aphids on collards than were
Coccinellidae (Coleoptera) or Chrysopidae (Neuroptera) (Horn, 1981).
Aphidophagous syrphids are
high-performance insects and, although strong fliers, fare relatively
poorly when weather is cold, wet, or windy (Lewis, 1965a). In the Sacramento
Valley, Eupeodes spp., Scaeva pyrastri, and Syrphus
spp. are often abundant from late spring through early summer, but seem
to disappear with the advent of hot weather. By contrast, the smaller
species Toxomerus spp. and Paragus tibialis (Fallen) are
most common during the summer (Bugg and Wilson, 1989). In coastal areas,
the larger species often remain abundant during the summer (Bugg, personal
observation).
Adult females of several
syrphid species determine whether to oviposit based on the size of aphid
colonies. Several syrphid species discriminate against older, larger colonies
in favor of smaller "promising" colonies (Kan, 1988a, b, c).
However, syrphid species vary as to the size of aphid colonies or aggregations
"preferred." Chandler (1968a) showed that for Platycheius
spp. and Syrphus ribesii (L.), different aphid densities elicited
peak numbers of syrphid eggs per plant. Chandler (1968b) reported that
Platycheirus manicatas (Meigen) oviposited selectively on uninfested
plants adjoining those that are heavily infested. This response was seen
to cabbage aphid (Brevicoryne brassicae L.) on brussels sprouts,
and to bean aphid (Aphis fabae Scopoli) on faba bean. Syrphids
that oviposit on plants that have been poorly colonized by aphids may
be especially good at keeping aphids at low densities.
Managing Vegetation to Enhance Biological Control by Syrphidae
Wind inhibits activity by
adult syrphids. Hedgerows, windbreaks, or shelterbelts can protect croplands
in windy areas, and they provide some protection to windward as well as
to leeward. Shelter can reduce soil erosion, and improve photosynthetic
and water-use efficiency by crop plants, and can lead to locally elevated
temperatures in the sheltered areas (Van Eimern, 1964). These practical
considerations raise the possibility of using wind shelter to enhance
biological control by aphidophagous syrphids.
Using segregating traps and
painted pan traps containing water and detergent, Lewis (1965a) showed
that syrphids occurred in areas sheltered by artificial windbreaks (0.915
m in height, made with horizontal slats, 45 percent open area), and that
of all 13 insect taxa assessed (diurnal and nocturnal), syrphids showed
by far the greatest tendency to concentrate in the sheltered area. Aphids
also settle selectively near shelter (Lewis, 1965b), so the net effect
of windbreaks on aphid control is in question.
Because hedgerows and windbreaks
often contain flowering plants used by syrphids, effects of shelter and
of flowers may be confounded. Bowden and Dean (1977) used suction traps
to assess the distribution of adult syrphids on both sides and at two
distances from a high (7 m) hedgerow. Prevailing wind did not seem to
influence the distribution: syrphids were consistently more abundant on
the western side, which was more diverse floristically.
Pollard (1971) believed that shelter influenced syrphid oviposition, but that flowers did not. In Pollard's experiment, potted brussels sprouts plants were placed in various habitats, then retrieved and inspected for syrphid eggs. Adult syrphids were more abundant in areas with flowers, but oviposition was depressed in unsheltered areas, regardless of whether flowers occurred nearby. However, Pollard's was not a true factorial experiment, and the two factors of interest were neither manipulated nor controlled. The experiment also lacked systematic interspersion of treatments and rigorous statistical analysis.
Nectar and Pollen Sources
Floral resources are clearly
valuable to adult syrphids. Nectar serves principally as an "energy
food" to sustain the strong flight; pollen sustains ovariole development
(Schneider, 1969). Table 1 lists some of the nectar sources used by aphidophagous
syrphids, including a variety of trees, shrubs, and forbs. The table refers
to research conducted in both North America and Europe. As indicated in
table 1, flowers of some cover crops, such as buckwheat (Fagopyrum
esculentum, Polygonaceae) and tansy phacelia (Phacelia tanacetifolia,
Hydrophyllaceae) are especially attractive to adult syrphids (see
Ozols, 1964). Sweet alyssum (Lobularia maritima, Brassicaceae)
flowers are also heavily visited (Bugg, personal observation), and this
species is commonly included in proprietary "insectary cover crop"
seed mixes (e.g., Germain's Incorporated, Harmony Farm Supply, Lohse Mill
Inc., Pacific Coast Seed, Peaceful Valley Farm Supply) (Bugg and Waddington,
in press). Knuth (1908) compiled detailed records of flower visitation
by insects, including syrphids.
Effects of flowers on oviposition
have proven difficult to demonstrate, perhaps because of the difficulties
of spatial scale encountered with the highly vagile adult syrphids. Distribution
of syrphid flies and their oviposition on brussels sprouts was related
to flowers, in an unreplicated study involving a hedgerow (Van Emden,
1965). As mentioned earlier, Pollard (1971) contended that shelter provided
by hedges was important, but that flowers were not. For example, potted
plants were located amid standing gram, which was counted as "shelter."
But grain fields, depending on phenological stage, can also be sources
of alternate prey or pollen. Chandler (1968b) found that Senecio jacobaea
(Asteraceae), in buckets, did not influence oviposition by syrphids. These
flowers were presumably cut, which can reduce the flow of nectar. This
was not considered, although Chandler did suggest that syrphids continued
to visit the flowers.
By contrast, Sengonca and Frings (1988) showed apparent enhancement of biocontrol in a two-year, replicated study, involving tansy phacelia. This annual forb is native to California and was introduced as a bee plant to Europe during the early 1900's. Tansy phacelia was grown in interior strips, and in "islands," in conjunction with 200-m2 plots of sugarbeet. Control plots featured monocultures of sugarbeet. Densities of bean aphid (Aphis fabae) and eggs and larvae of aphidophagous hover flies (Diptera: Syrphidae) were highest in control plots. In plots with phacelia, sugarbeet yields were significantly higher, and adult syrphids (which feed on floral nectar and pollen of phacelia, and were presumably attracted thereby) were significantly more abundant. Syrphids were credited with reducing the aphids in plots with phacelia. Aphidophagous syrphids observed included Episyrphus balteatus, and Metasyrphus corollae Sphaerophoria scripta, Scaeva selenitica, and Melanostoma scalare. As reported by Blake (1990), Steven Wratten at the University of Southampton, England is also using tansy phacelia to enhance activity of syrphids, especially Episyrphus balteatus and Metasyrphus corollae. Hoverflies with the distinctive star-shaped phacelia pollen in their guts were collected as far as 200 m from strips of flowering phacelia.
Alternate Prey
Only survey studies have
been conducted on the possible role of alternate prey in enhancing biocontrol
by syrphids. Bugg and Ditcher (1989) evaluated several warm-season cover
crops as sources of alternate prey for aphidophaga: American jointvetch
(Aeschynomene americana), cowpea (Vigna unguiculata ssp.
unguiculata), sesbania (Sesbania exaltata), and hairy indigo
(Indigofera hirsuta) all supported cowpea aphid (Aphis craccivora
Koch), whereas a sorghum X sudangrass hybrid (Sorghum bicolor)
hosted corn leaf aphid (Rhopalosiphum maidis [Fitch]) and greenbug
(Schizaphis graminum [Rondani]). Aphidophaga observed included
syrphid flies (e.g., Allograpta obliqua [Say], Ocyptamus fuscipennis
[Say], Ocyptamus costatus [Say], Pseudodoros clavatas [Fabricius],
Sphaerophoria spp., Toxomerus boscii Macquart, and Toxomerus
marginatus [Say]). Sesbania appeared to be the best source of cowpea
aphid, and reservoir for pooled aphidophaga, including coccinellid beetles
and syrphid flies. Densities of aphidophagous Syrphidae were significantly
different among cover crops on three of the eleven dates assessed, September
10, 14, and 29; sesbania featured the highest densities on all three.
Bugg et al. (1990) assessed
aphidophagous Syrphidae in various cool-season cover crops in southern
Georgia. They observed Allograpta obliqua (Say), Syrphus sp.,
Eupeodes (Metasyrphus) sp., and Toxomerus marginata Say).
Whole-plot inspection for pooled aphidophagous syrphids indicated significant
differences among cover crops on 5 of the 19 sampling dates: 1) Crimson
clover and 'Cahaba White' vetch on February 22; 2) Crimson clover and
lentil on March 13; 3) Arrowleaf and crimson clovers on March 30; 4) Arrowleaf
clover, hairy vetch, 'Cahaba White' vetch, and narrow-leafed lupin, on
April 19. Thus, significant differences for adult aphidophagous Syrphidae
were only seen on a relatively few occasions. Adult syrphids seldom fly
and may seek concealed locations when the weather is windy, cold, or rainy,
and therefore may not have been observable on all sampling dates.
Bugg and Ellis (1990) evaluated five prospective cover crops in Falmouth, Massachusetts. Four distinguishable taxa of aphidophagous hover flies were observed. A total of 725 syrphid adults were observed, with the breakdown as follows (numbers of specimens observed in parentheses): Allograpta obliqua (3), Sphaerophoria spp. (55), Syrphus spp. (9), Toxomerus spp. (658). Thus, Toxomerus spp. represented over 90 percent of the observations. Buckwheat (a nectar source) showed the highest densities on 3 dates; hairy vetch, Vicia villosa (infested with pea aphid), did so on 2 dates (1 tie).
Conclusion
Local oviposition by syrphids
may be more strongly influenced by shelter than by flowers, though there
are some conflicting data. It is difficult to demonstrate effects of flowers,
probably because adult syrphids are highly mobile, and benefits acquired
by pollen feeding (e.g., ovariole development) do not occur immediately.
Moreover, nectar is an energy food and enables dispersal. Therefore, landscape-scale
experiments may be needed. I found no studies on effects of alternate
prey on syrphid efficiency in agroecosystems.
Given the importance of syrphids
in field, orchard, and vegetable crops, further experiments on enhancement
should be under-taken.
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For more information write to: Robert Bugg, Information Group, Sustainable Agriculture Research and Education Program, University of California, Davis, CA 95616-8533.
| Table 1. Flowering plants and associated aphidophagous hover flies (Diptera: Syrphidae). | ||
| Nectar Source | Syrphidae Attracted | Reference |
| Buckwheat
(Fagopyrum esculentum) |
Allograpta obliqua, Ocyptamus costatus, O. fuscipennis, Pseudodoros clavatus, Toxomerus boscii, T. marginatus | Bugg and Ditcher, 1989 |
| Buckwheat and canola
(Brassica napus) |
A. obliqua, Spaerophoria spp., Syrphus spp., and Toxomerus spp. | Bugg and Ellis, 1990 |
| California lilacs
(Ceanothus spp.) |
A. obliqua, Spaerophoria spp., Scaeva pyrastri, Eupeodes volucris, Metasyrphus spp., Melanostoma spp., Toxomerus spp. | Bugg,
(personal observation) |
| Common knotweed
(polygonum aviculare) |
Allograpta spp., Sphaerophoria spp., Paragus tibialis [Fallen] | Bugg et al., 1987 |
| Corn spurry
(Spergula arvensis) |
A. obliqua, Eupeodes volucris, Melanostoma sp., Scaeva pyrastri, Sphaerophoria spp., Syrphus meadii, Toxomerus spp. | Bugg,
(personal observation) L. Linn, (personal communication) |
| Coyote brush
(Baccharis pilularis) |
Allograpta obliqua, Sphaerophoria spp., Scaeva pyrastri, Eupeodes volucris, Metasyrphus spp., Melanostoma sp., Toxomerus spp. | Bugg,
(personal observation) |
| Holly-leaved cherry
(Prunus ilicifolia) |
Allograpta obliqua, Sphaerophoria spp., Scaeva pyrastri, Eupeodes volucris, Metasyrphus spp., Melanostoma sp., Toxomerus spp. | Bugg,
(personal observation) |
| Soapbark tree
(Quillaja saponaria) |
Scaeva pyrastri, Eupeodes volucris, Metasyrphus spp., Melanostoma | Bugg, 1987 |
| Tansy phacelia (Phacelia tanacetifolia) and White mustard (Sinapis alba) | Episyrphus balteatus, Melanostoma mellinum, Metasyrphus corollae, Sphaerophoria mentastri group, Sphaerophoria scripta, Syrphus ribesii | Klinger, 1987 |
| Toothpick ammi
(Ammi visnaga) |
Allograpta obliqua, Sphaerophoria spp., Paragus tibialis. When the plant was induced by summer planting to flower out of season (during the spring), it attracted Scaeva pyrastri, Eupeodes volucris Metasyrphus, Melanostoma. | Bugg and Wilson, 1989 |
| Wild buckwheats
(Eriogonum spp.) |
Allograpta obliqua, Sphaerophoria spp., Scaeva pyrastri, Eupeodes volucris, Metasyrphus spp., Melanostoma sp., Paragus tibialis, Toxomerus spp. | Bugg,
(personal observation) Bugg and Heidler, 1979 Swisher, 1979 |
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