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Biological Characterization of Two Sibling Species in Sycophila mellea (Hymenoptera: Chalcidoidea: Eurytomidae) in Britain, Parasitoids of Tetramesea spp. (Hymenoptera: Chalcidoidea: Eurytomidae) in Poaceae



Hamid Ghajarieh, Hassan A. Dawah and Mike Bruford
 
ABSTRACT

The taxonomic status of Sycophila mellea (Curtis) species complex (Hymenoptera: Eurytomidae), reared from Tetramesa linearis (Walker) and T. brevicornis (Walker) feeding in the grasses: Elymus repens (Viv) and Festuca rubra (L.), respectively, were investigated using host preference study. The results of host preference experiments demonstrated that the S. mellea (ex: T. linearis and ex: T. brevicornis) were attracted predominantly to their native host plants, E. repens and F. rubra, respectively. In conclusion, the results from host preference study strongly suggest that members of S. mellea (ex: T. linearis and T. brevicornis) represent different taxa.

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Hamid Ghajarieh, Hassan A. Dawah and Mike Bruford, 2007. Biological Characterization of Two Sibling Species in Sycophila mellea (Hymenoptera: Chalcidoidea: Eurytomidae) in Britain, Parasitoids of Tetramesea spp. (Hymenoptera: Chalcidoidea: Eurytomidae) in Poaceae. Journal of Entomology, 4: 40-45.

DOI: 10.3923/je.2007.40.45

URL: https://scialert.net/abstract/?doi=je.2007.40.45

Introduction

Parasitic organisms generally and herbivorous insects and insect parasitoids in particular, are rich in sibling species (Claridge, 1988; Dawah et al., 2002) and they demonstrate that within a genus, diversification occurs with little morphological differentiation; this makes taxonomic separation of the species very difficult. Chalcidoid wasps are generally very difficult to identify because of little apparent morphological differentiation.

S. mellea is a parasitoid of T. linearis and T. brevicornis, feeding on two host plants, E. repens and F. rubra, respectively (Dawah et al., 1995). In the literature it is regarded as a single biological species (Fitton et al., 1978), yet the fact that it attacks hosts specific to plants growing in different habitats suggests that it might be a complex of species (Anga, 1991).

Parasitoid specificity to their host habitat has been used to support the taxonomic separation of closely-related species of Hymenoptera (Dawah et al., 2002). For example, Dawah (1988) in his host choice experiments, demonstrated that species of the Pediobius eubius (Walker) (Hymenoptera: Eulophidae) complex were attracted to their particular host plants through olfactory responses to odours emanating from them. He found that females prefer the odour of the host food plant from which they were reared, i.e., known host to non-host food plant. He found that the Pediobius ovipositing on the grass Elymus repens was a different species to that ovipositing on Dactylis glomerata L. He stated that this result clearly supported the taxonomic separation of species of the Pediobius complex.

Vet et al. (1984) showed that Asobara tabida (Nees) (Hymenoptera: Braconidae) appeared to be either attracted to fruit or to decaying leaves. Further investigation revealed that this species was in fact composed of two closely-related sibling species, each specialized in its own micro habitat (Kenis and Mills, 1998).

Campan et al. (2002), Mattiacci et al. (2000), Havill and Raffa (2000), Rutledge and Wiedenmann (1999) and Ding et al. (1989) reported the role of plants and other cues characteristic of habitats containing hosts and these have been shown to be important in host habitat location.

More evidence for the importance of host plant odours has been obtained from behavioural studies in the laboratory, using Y-tube olfactometers and wind tunnel experiments (Zanen and Carde, 1991; Potting et al., 1995; Nealis, 1986).

Dawah et al. (2002) examined the behavioural responses of female Pediobius obscurus Dawah and Al-Haddad and P. planiventris (Thomson) to four grass species; E. repens, B. sylvaticum, B. pinnatum (L.) Beauv and D. glomerata, infested with larvae of their Tetramesa host. They found that closely related species of P. planiventris reared from T. fulvicollis in B. sylvaticum and P.obscurrus reared from T. angustipennis in A. pratensis (L.) are associated with their host food plant in a way supporting their genetic and taxonomic separation. Since no parasitoid larvae or larval remains were found in D. glomerata, they assumed that no oviposition by either of the Pediobius species occurred. They also found that female Pediobius oviposited only on the host species from which they were normally reared as natural host food plants.

The present study investigate whether members of S. mellea (ex: T. linearis) and S. mellea (ex: T. brevicornis), important primary parasitoids of two phytophagous Tetramesea, T. linearis on E. repens and T. brevicornis on Festuca rubra, respond to the host food plant in a way that could support their taxonomic separation.

Materials and Methods

Collection Sites
Grasses were collected from 1999 to 2000 from the following numbered localities in South Wales and England (Ghajarieh, 2003).

Rearing Individual Larvae from Stems
Rearing techniques of Tetramesea and Sycophila species larvae employed here was described by Dawah (1987). Grasses were collected in November 2000, when the larvae within had completed feeding (Graham and Claridge, 1965). The stems were dissected by scalpel without damaging the larvae when the larvae were fully fed and in diapause and placed in individual gelatine capsules and labeled. They were placed in a shed where they were constantly checked until Sycophila larvae were reared to adulthood. All males and females of Sycophila and Tetramesa used in all experiments were fed with 10% (w/v) honey solution. Grasses were identified using Hubbard (1954). Larvae were identified to genus using Dawah and Rothfritz (1996) and host plant association.

Host Preference Tests
The behavioural responses of the host food plant preferences of S. mellea female were tested for three grass species: Elymus repens, Festuca rubra and Phalaris arundinacea (L.), infested with larvae of the following Tetramesa hosts: T. linearis, T. brevicornis and T. longicornis, respectively. The Phalaris arundinacea/T. longicornis association was used as a control. There is no report that S. mellea species complex attack P. arundinacea.

The technique used here is described in Dawah et al. (2002). Rearing larvae of Tetramesa and Sycophila. The plants used were at the same developmental stage, with a compact shoot on each. The grasses were planted in pots (15 cm in diameter) in a greenhouse. The host plants E. repens, F. rubra and P. arundinacea were infested with their natural hosts, Tetramesa linearis, T. brevicornis and T. longicornis, respectively at the mid of June 2000. This was done by releasing mated females of the three species into a cage measuring 25 x 25 x 50 cm 3. Whilst in the vials, mating was assumed to occur naturally and insects were left at least for 72 h. being introduced for oviposition. At the end of this period, the three species were released separately into cages containing their natural host plants for oviposition. The plants were then left for ten days (Dawah et al., 2002), to allow the Tetramesa larvae to emerge from the eggs and whilst one infested stem of each the three grass species was then positioned at random inside the experimental cage. The cages used for this part of experiment were wooden-framed, measured 60x40x40 cm, with the top and the sides covered with nylon mesh. Sleeves were fitted to permit access without allowing the insects to escape.

The general experimental procedure was as follows: Both sexes of S. mellea that were used in these experiments were virgin and have no experience with odour of the grasses. Rearing the larvae separately ensured the virginity of wasps. A mating period of 2-3 days was allowed (Dawah, 1988), a sufficient time to allow successful mating in a C.T. room (26±°C). One mated female of each Sycophila was placed inside the cage at a point equidistant from each of the stems and the insect allowed to select the host food plant. There were eight experimental replicates for each S. mellea population (ex: T. linearis and T. brevicornis).

Results and Discussion

The dissections of stems of the three grasses, E. repens, T. linearis and P. arundinacea confirmed that all stems used in the experiment were infested with Tetramesa. Larvae of Sycophila were recorded however only in the natural host food plants. No evidence of the presence of larvae was found in the stems of other plant species. No Sycophila larvae were found from P. arundinacea (Table 1), so it could be assumed that no oviposition by either of the two population of Sycophila took place in that grass.

The preference of insect parasitoids towards the food plant of the host, rather than to the host itself, has been reported by many authors. Parasitoids, in the absence of their host, may be attracted to the micro habitat of their preferred host by chemical cues. van Alphen and Jervis (1996) suggested that two phases in host finding behaviour can be recognised for parasitoids: 1) habitat location and 2), host location. They stated that two sorts of stimuli operate at each level - attractant stimuli and arrestant stimuli. Attractant stimuli elicit orientation to the general area of hosts and for reduction in the distance, are involved with arrestant stimuli by parasitoids moving around host areas (Waage, 1978).

Table 1: Results of the host specificity experiment. Individual mated females of S. mellea (ex: T. linearis and T. brevicornis) were given the choice between E. repens, F. rubra and P. arundinace infested with T. linearis, T. brevicornis and T. longicornis, respectively

Much relevant research has been about orientation to microhabitats, in other words potential sources of host food such as plants (van Alphen and Vet, 1936). Godfray (1993) suggested that stimuli from the host micro habitat or food plant are one of the three broad categories of information that are used in host location. Chemical cues from the host’s micro habitat can attract parasitoids in the absence of the host itself (McAuslane et al., 1990; Read et al., 1970; Camors and Payne, 1972).

Volatiles released by other organisms may be used in host location, Greany et al. (1977), Spradbery (1970), Lewis and Jones (1971) Ramachandran and Norris (1991). Greany et al. (1977) reported that the braconid Biosteres longicaudatus (Ashmead), which attacks tephritid fruit flies, is attracted to some chemical cues released by a fungus that grows on peaches (Prunis persica). Some ichneumonids in the genera Rhyssa and Megarhyssa are attracted to volatile chemicals produced by a fungus (Spradbery, 1970). Some other important sources of short-range attractant chemicals include frass and honeydew. Thus for example, Lewis and Norris (1971) demonstrated that the braconid Microplitis croceipes (Cresson), a parasitoid of the corn earworm moth, Helicoverpa zea (Boddie), responds by antennation to 13-methyl haptaoctane, a chemical in the host frass. Ramachandran et al. (1991) identified several active volatiles from frass of the soybean looper moth, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), larvae that attract the braconid, Microplitis demolitor Wilkinson.

Most Homopterans produce large quantities of honey dew which both reveal their presence and provide food for parasitoids. The encyrtid, Microterys nietneri (Mutschulsky), responds to fructose and sucrose and some other unidentified compounds in the honeydew secreted by its host, Coccus hesperidum (L.) (Homoptera: Coccidae), the brown soft scale (Vinson et al., 1978). Tactile and visual micro habitat cues also are important in host location. The braconid Opius (alloeum) melleus (Gahan) is attracted to hawthorn berries, Crataegus monogyna Jacq, where its host feeds (Glas and Vet, 1983). Wackers and Lewis (1993) have reported that after attraction to the host plant by long range stimuli such as volatile chemicals, visual cues help parasitoids in the final stages of approach and landing.

The present work demonstrated a very strong preference between parasitoids and their host food plant (here on which its insect-host feeds) as opposed to a non-host food plant. The main conclusions were drawn from this study that females of S. mellea (ex: T. linearis) and S. mellea (ex: T. brevicornis) showed a strong response to the host plants that their Tetramesa hosts which parasitize E. repens and F. rubra, respectively. Dissections confirmed that all stems of E. repens, F. rubra and P. arundinacea were infested by their specific parasites, T. linearis, T. brevicornis and T. longicornis in the experiments. No evidence was found for the presence of S. mellea larvae in the stems of P. arundinacea. Larvae and larval remains of Sycophila were recorded in their natural host plants.

These results confirm that S. mellea (ex: T. linearis) oviposit only in E. repens and S. mellea (ex: T. brevicornis) oviposited in F. rubra. No parasitoid larvae or larval remains were observed from P. arundinaceae. So that it is assumed that no oviposition by Sycophila took place in that grass. The results also clearly show an attraction to the host plant, but not to non-host food plants. As a further step towards understanding the taxonomic status of these potential sibling species, the responses of S. mellea (ex: T. linearis) and S. mellea (ex: T. brevicornis) to their hosts supports their taxonomic separation.

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