VARIATION IN SPECIES SPECTRUM OF HOVERFLIES (DIPTERA, SYRPHIDAE) IN ARABLE CROPS DEPENDING ON THE COLLECTION METHOD

Grażyna Sobota, Jacek Twardowski

ABSTRACT

The studies were carried out at the Experimental Station of the Agricultural University in Wrocław in 1998-2002. The aim of the study was to compare the species spectrum of hoverflies (Diptera, Syrphidae) collected by different methods. Four methods were applied: collection in yellow water traps, sweeping, direct collection, and rearing of the pupae collected directly from cultivated plants in the field. In the yellow traps, various and numerous species of Syrphidae were captured, but weather conditions had strong effect on their abundance. In some years, in yellow traps there were found species (Metasyrphus corollae, Melanostoma mellinum) that only foraged for food or were on migration towards south of Europe and that were absent or scarce among hoverflies reared from the pupae. Hoverflies collected using a sweeping net or directly were usually not numerous and represented mainly two genera: Sphaerophoria and Melanostoma. These two collection metho

Key words: Syrphidae, yellow water traps, sweeping, direct collection, collection of the pupae, arable crops.

INTRODUCTION

The larvae of hoverflies of the subfamily Syrphinae are considered the specialized aphidophagous predators [1, 3]. The adults feed mainly on pollen and nectar of flowering plants, or honeydew produced by aphids. The pollen contains proteins that are indispensable for gametogenesis. Hoverflies, along with other aphidophagous insects (Coccinellidae and Chrysopidae), play a very important role in the reduction of aphid population numbers. Therefore, they are objects of many scientific investigations. Various methods of study are applied to identify the species composition of Syrphidae associated with arable crops. Most frequently, yellow water traps or sweeping are used. Sometimes, hoverflies are collected directly or adults are reared from larvae or pupae collected from plants. Therefore, it is often difficult to compare and discuss the results obtained by different authors. Up to now, there is no information on the effectiveness of a given method in the available literature.

Our assumption was that the species spectrum of Syrphidae in the samples would depend on the applied method of collection, so the main purpose of the present work was to evaluate the methods and find the most accurate one for studying predatory Syrphidae in the field. We carried out our study using at least two collection methods in a given crop each year.

MATERIAL AND METHODS

The studies were carried out in the fields of the Agricultural University Experimental Station in Pawłowice (about 10 km north - east of the center of Wrocław) in 1998-2002. In 2000, the numbers of Syrphidae were very low; therefore we do not present the results obtained in that year. The collection and observation of hoverflies were conducted in sugar beet, winter and spring wheat, and spring barley crops. The crops and collection methods taken into consideration in individual years are given in Table 1.

The painted fluorescent yellow traps were 5 cm high and 25 cm in diameter. The traps were filled with water containing an additive Citowett to reduce the surface tension. Three traps were positioned along the diagonal of each field. The traps were placed on stands and their position above ground was adjusted according to the plant growth, so the rim was at the same height as the apices of plants. The traps were emptied twice a week during the vegetation period, i.e., from mid April to the end of July. The Syrphidae were selected from the collected material and stored in 75% ethanol.

The sweeping net (45 cm in diameter) was used as follows: the studied area was swept 100 times (4 times 25 sweepings) along its diagonal, always near the yellow traps. During each sweeping movement, a shape of the figure 8 was made at the tips of the plants. This method was applied once a week during the vegetation period, between 10 and 11 a.m. At the same time, during one hour continuously, all observed hoverflies within the given crop were captured with an entomological net (45 cm in diameter). The collected hoverflies were killed in ethyl acetate.

The adult Syrphidae were reared from pupae collected from randomly chosen crop plants. In the laboratory, each pupa was placed in a glass tube separately. The tube was closed with cotton wool. These cotton wool plugs were moistened with pure water twice a week. After emergence of imagines, the percent of emergence was calculated. All collected hoverflies were determined at species, generic or subfamily level.

RESULTS

1) Yellow water traps
Hoverflies caught in yellow traps were usually abundant (with an exception of 2001) (Table 2). The representatives of the subfamily Syrphinae made from 53.8% up to 100% of all collected hoverflies. The other represented subfamilies were Eristalinae (0% to 31.5%), Eumerinae (0% to 25.9%), Cheilosiinae (0 to 2.0%), Sphegininae (0% to 0.4%), and Milesiine (0% to 0.4%). In 2001, only few Syrphinae were caught in the traps in spring barley and few Eristalinae in winter wheat. Syrphidae collected in yellow traps belonged to 37 species. Two hoverflies were determined as Mallota spp. and some other individuals could be determined only at subfamily level. These were Syrphinae, Eristalinae, Cheilosiinae, Milesiinae (Table 2). In 1998, in the sugar beet crop, Metasyrphus corollae was the dominant species (29.9% of all collected hoverflies); the subdominant species was Sphaerophoria scripta (16.2%). The species spectra of Syrphinae in winter and spri ng wheat in 1999 were similar. In both crops, the dominant species was Episyrphus balteatus (47.0% and 42.8%, respectively). Low number of Syrphidae caught in cereals in 2001 was probably caused by unfavorable weather: frequent rainfalls and high maximum daily temperatures. Among those scarce Syrphidae, the most common was E. balteatus. In 2002, the results were different. In winter and spring wheat, the Syrphidae were abundant. In winter wheat, the most numerous species of Syrphinae was M. corollae (20.9%). Slightly less abundant were S. scripta (15.2%), Melanostoma mellinum (14.2%), and E. balteatus (11.4%). In spring wheat, the most abundant was S. scripta (17.8%). Relatively numerous were also M. corollae (14.1%) and M. mellinum (8.3%), whereas E. balteatus made only 6.7% of all collected hoverflies in that crop.

2) Sweeping
Only few syrphids, belonged to three species were collected with a sweeping net. In 1999, in winter wheat two species of the genus Melanostoma (M. mellinum and M. scalare) and S. scripta were recorded (Table 3). M. mellinum made majority of all collected species. In spring wheat, only two individuals of M. mellinum and two individuals of S. scripta were caught.

3) Direct collection
In the material collected directly with an entomological net, the hoverflies of the subfamily Syrphinae predominated (Table 4). Members of other subfamilies were caught only in sugar beet in 1998 (Syrphinae – 93.9%, Syrittinae – 4.1% and Eristalinae – 2.1%). In the years of study, the most diverse fauna of hoverflies was found in sugar beet (13 species), and the least diverse was in winter wheat (two species). In 1998, the most abundant in the samples collected directly was M. mellinum. Nearly a half of collected hoverflies (67 individuals; 46.2%) belonged to this species. S. scripta was also quite frequent – 35 flies were caught (24.1%). Other species were less numerous or only occasionally caught. In 2001, in winter wheat, only a few individuals of S. scripta and M. mellinum were recorded. In 2001, in spring barley also S. scripta (50.0%) and M. mellinum (16.6%) predominated.

4) Collection of pupae
The pupae collected from crop plants gave only the representatives of the subfamily Syrphinae (Table 5). The percent of emergence ranged from 30.7% (from pupae collected in spring barley) to 68.2% (spring wheat). The emerged hoverflies belonged to seven species. The number of determined species in each crop was similar (three to five). Almost in all crops E. balteatus predominated. In 2001 in spring barley, it made 72.3 % of all determined specimens, and in 2002 – 88.5%, and in spring wheat in 2002 – 76.7%. In 2001, only a few syrphids were reared from pupae collected from winter wheat. The most abundant species was S. scripta. This species was also numerous in samples from other crops (in 2001 in spring barley – 20.5% of all determined Syrphinae; in 2002 in spring wheat – 15.0%). S. scripta did not occur in the material from winter wheat in 2002.

DISCUSSION

The use of yellow water traps is the most common method for collecting Syrphidae in purpose of determining their species spectrum and abundance. The attractiveness of such traps to the imagines of these flies is unquestionable [5, 8, 11, 13]. This is mainly due to the strong attraction to yellow color shown by Diptera, including Syrphidae, and among them individuals not associated with a given crop. Therefore, the use of only this method may lead to false conclusions. It is advisable then, to use not only yellow traps for comparison of Syrphidae populations in different habitats. Hickman et al. [9] have come to the similar conclusion. They collected Syrphidae in winter wheat and adjacent habitats. The weather conditions during the collection period had the most profound effect on the number and species composition of Syrphidae in the traps. For example, in 2001, the number of trapped hoverflies was very low in comparison to results obtained when other methods were applied, due to frequent rains and high daily temperatures. Some of the collected Syrphidae, e.g., Metasyrphus corollae, used to be found in the yellow traps quite often, whereas on the plants infested by aphids they did not occur at all. It may be presumed that that species just foraged for food and for the oviposition site within a given crop. Ciepielewska [2] states that weather has an influence on the intensity of flight of Syrphidae but does not affect the population dynamics of hoverflies. Another factor affecting the number of hoverflies caught in yellow water traps may be the abundance of flowering plants. Schneider [12] reported that the attractiveness of yellow color might increase when the availability of flowers decreases. On the other hand, more Syrphidae were caught into traps located close to the flowering plants than into traps away from them, although the distance that hoverflies had to cover was similar [12]. According to the hypothesis by Hickman et al. [9], the Syrphidae may assume yellow color to be a pote ntial source of food (i. e. flowers) and this may be the reason why hoverflies, especially those hungry ones, fly around such traps. It must be considered that probably not all species react to yellow color in a similar way. Haslett [7] found that Episyrphus balteatus did not show any preference to yellow color in the field. These results were not quite accepted by other authors [14]. They argue that no preference for yellow color occurs only in young imagines. Another issue is the migration of hoverflies in the Palearctic, especially the migration flights of E. balteatus and M. corollae [3]. Tenkumberg and Poehling [15] reported that the aphidophagous species of hoverflies migrate to overwintering sites in southern Europe or northern Africa and come back to northern regions in spring when their food plants flower and aphid populations develop. In Germany, there are differences in the timing of migrations of Syrphidae to the cereal fields between the north and the south of the country. In northern Germany, Syrphidae appear in cereals 2-3 weeks later than in the south of the country, which is probably due to the distance of 600 km between these two regions the flies have to cross. However, the delayed appearance of syrphid larvae in the north may be due to the differences in environmental conditions. Consequently, a number of factors and not only searching for oviposition sites may influence the species spectrum of aphidophagous hoverflies caught in the yellow water traps. This may explain our finding of quite a high number of Metasyrphus corollae in yellow traps in some years, while the species did not occur among hoverflies reared from pupae. The number of Syrphidae caught in yellow traps depends also on their position in relation to the plants. The majority of researchers place the traps at the level of plant apices. Hickman et al. [9] proved that significantly fewer hoverflies were caught in winter wheat in traps located on the ground. Individual species differ in their searching beh avior. It seems however, that the hoverflies caught into traps put on the ground are more connected with a given habitat. Those that are caught (although they are sometimes quite numerous) in traps positioned at plant tops level are in many cases accidental species in a given crop.

Quantitative sweeping and direct collection narrowed the species spectrum of Syrphidae. The application of these two methods allowed the collection of hoverflies of mostly two genera: Sphaerophoria and Melanostoma. The basic shortcoming of these methods is the collection during a limited period of the day, so the daily activity of different species is not considered [6, 17]. Other factors should also be taken into account: atmospheric conditions and the fact that Syrphidae are very good fliers and extremely restless animals [10]. It clearly appears that sweeping and direct collection are very selective methods.

The results of our study show that the most credible method of assessment of species composition of aphidophagous Syrphidae is the rearing of adult hoverflies from pupae collected from plants. However, the degree of parasitization of hoverfly population must be considered. Individual species of Syrphidae may be parasitized in a different degree. It must also be taken into account that females of certain species do not oviposit on plants infested by aphids or oviposit in some distance from the aphid colonies. Such behavior is characteristic of the genus Platychirus [4]. According to Wnuk [16], the reason is the cannibalistic behavior of larvae.

The choice of a proper method depends on the purpose of the study and on the plant species. Our results lead to a general conclusion that more than one method must be applied if a full species spectrum of Syrphidae in arable crops is to be identified.

CONCLUSIONS

1. The species spectrum of the family Syrphidae caught in yellow water traps was broad and represented by individuals from six subfamilies. All together, 37 species were identified. The abundance of hoverflies in these traps was usually high, but strongly dependent on weather conditions.

2. Metasyrphus corollae and Melanostoma mellinum were abundant in water traps in certain years and were absent among hoverflies reared from pupae collected from plants. These species probably just foraged for food or were on migration towards the south of Europe.

3. The hoverflies collected with a sweeping net or collected directly were usually not numerous and represented mainly two genera: Sphaerophoria and Melanostoma. These methods of collection are applied within a short time period and they do not allow the collection of syrphids penetrating lower parts of the plants.

4. The most credible method for identification of the species spectrum of aphidophagous Syrphidae in arable crops is the rearing of hoverflies from pupae collected from plants infested by aphids. However, the degree of parasitization of individual species must be taken into account.

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