TENTHREDINIDAE (HYMENOPTERA) ASSOCIATED WITH BRACKEN FERN PTERIDIUM AQUILINUM (L.) KUHN. IN ŚLĘŻA MASSIF

Aleksandra Halarewicz
Department of Botany and Plant Ecology, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

Three species of Tenthredinidae feeding on bracken fern were recorded within the area of Ślęża Landscape Reserve (Lower Silesia). Larvae of oligophagous species Aneugmenus padi were present during all the 3 years of study in spruce forest site. They were most abundant there at the turn of June and July.

Key words: Hymenoptera, Tenthredinidae, Aneugmenus padi, bracken fern, Ślęża Massif.

INTRODUCTION

The insects associated with ferns are mainly Homoptera, Coleoptera and Lepidoptera [4]. In the order Hymenoptera the species attacking ferns are restricted to three families, with 90% of species belonging to the family Tenthredinidae [5].

Each one of the bracken fern insect species exploits the frond in its own characteristic way. Females of Tenthredinidae use ovipositor to insert their eggs into the tissue of the host plant. Larvae of some of the species live externally and bite large pieces out of the pinnae (chewers), others, inside the tissues, induce galls (gall-formers). A few species in this group develop larvae that are leaf miners or stem borers [3].

There are 8 species of Tenthredinidae known to be associated with bracken fern, Pteridium aquilinum (Dennstaedtiaceae) in the British Isles [7]. It has been found that communities of bracken – feeding insects in North America (Mexico, Arizona) are faunally impoverished. Compared to Europe, common sawflies in these countries are represented by 2 species, other than in Europe [7,14]. Studies accomplished during the last decade in South America have broadened the knowledge of fern-infesting Tenthredinidae [11,12,13]. No Hymenoptera have been observed among the insect herbivores of bracken in Australia [10]. As the appropriate data on biology of many tenthredinid species are still unsatisfactory, in the case when only the adult specimens are sampled it is a fairly difficult task to determine their host plant species [2]. On the other hand, it has been found that even the common sawflies species that are obligatory parthenogenetic, like Aneugmenus padi, show genetic variation in larvae [9].

There is not much information available about the distribution of common sawflies in Poland [6,15,16,17]. Namely, the incidence of two species: Tenthredo livida L. and Aneugmenus padi L. [syn. Aneugmenus coronatus Klug.] was reported on ferns [15]. Regrettably, the author does not refer to the host plant more specifically.

The aim of the study was to outline the species composition of common sawflies associated with Pteridium aquilinum populating Ślęża Massif. The area features a specific geomorphological structure and climatic conditions favourable to faunistic diversity.

STUDY AREA AND MATERIAL

The observations were carried out in 2003–2005, since the beginning of May to the end of August of each year, at two natural habitats of Pteridium  aquilinum sub. aquilinum within  Ślęża Massif  (Lower Silesia, Poland). Site I is described as spruce monoculture developed in human-transformed habitat of acidophilic mountainous beech forest (Luzulo luzuloidis-fagetum community) of Radunia (a separate crest in Ślęża Massif), whereas site II is submontane acidophilous oak forest (Luzulo-Quercetum community) of Ślęża.

In order to determine the species composition of Tenthredinidae on P. aquilinum at the two sites, 20 plants growing there were thoroughly inspected at each observation date. On the approach to the plant the adults spotted on it were first collected by the small aspirator, then the insects' larvae were counted on the leaves. Samples were always taken in similar conditions: between 10 a.m. and 1 p.m., in sunny weather, in temperature > 20°C. The taxonomical identification of the insects was accomplished according to characters described in the keys of Muche [8] and in that of Blank & Ritzau [1].

RESULTS AND DISCUSSION

In the course of the 3 growing seasons, 18 adult specimens of common sawflies were caught, representing 3 species. They are: polyphagous Tenthredo livida and two oligophagous species: Aneugmenus padi and Stromboceros delicatulus  (Table 1).

Feeding of the larval instars of Tenthredinidae on the bracken leaves was not demonstrated for all the three species that appeared on the plants as adults. Larvae of Stromboceros delicatulus  and Tenthredo livida were never found on the inspected plants. Their absence from bracken may result from adult insects choosing another host plant, as P. aquilinum presents just one of their dietary options.

Bracken plants in spruce forest were always colonized only by the chewing larvae of Aneugmenus padi. Single larvae of the same species were feeding also on plants at the site II, in oak forest, but only in 2003. For their negligible density at the site II, the population dynamics is only shown for site I (Figs. 1-3). First instar larvae appeared on the leaves at the end of May each year, but they were most abundant since the end of June till the end of the first decade of July. Last instar larvae were observed by the end of July or, like in 2004, were still present on plants at the beginning of August. Percentage plant infestation in 2003 and 2004 did not exceed 40% and only in 2005 was as high as 55%. The larvae considerably depleted green mass of  the bracken plants (Fig. 4). The presence of the larvae of Aneugmenus padi during 3 years on plants of the same population of P. aquilinum verifies the fact that the species colonises primarily host plants close to its own overwintering site.

CONCLUSIONS

1. During the three-year study, adult Tenthredinidae belonging to 3 species were recorded from the bracken fern plants in selected forest associations with in Ślęża Massif.

2. Only the larval instars of Aneugmenus padi were feeding on the leaves of the observed plant.

3. The highest density of the larvae of Aneugmenus padi was observed in spruce forest in the first decade of July, in all the years of the study.

REFERENCES

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Accepted for print: 10.12.2009

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