PRELIMINARY OBSERVATIONS OF THE BENEFICIAL INSECT FAUNA ON BRACKEN FERN PTERIDIUM AQUILINUM (L.) KUHN.

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

ABSTRACT

The qualitative and quantitative aspects of the beneficial insect fauna found on bracken fern, Pteridium aquilinum, were investigated in 2003-2004 and are discussed in the article. 667 insect specimens were sampled, representing 22 species belonging to 5 insect orders. More individuals were sampled from the bracken plants populating spruce forest site, than from those within oak forest site. On the contrary, the number of the collected species was observed greater on the bracken plants within the oak forest. The predominant group of beneficial insects recorded from both the sites were Formicidae.

Key words: Pteridium aquilinum, bracken fern, beneficial insect fauna Formicidae.

INTRODUCTION

Bracken fern, Pteridium aquilinum, is considered one of the most successful invasive plant species in the world [3,12,14]. As a native plant, it is not normally considered a weed in bushland or within conservation areas, though it may dominate such areas and impair the establishment of other native species.

On the other hand, as a floristic component of natural ecosystems bracken offers a habitat for many wildlife including invertebrates. It is a food source for many species of insect herbivores [9,10]. The knowledge on the beneficial insect fauna using Pteridium aquilinum as a refuge or as a transient resource is still far from complete.

One fact well established in literature is the presence of Formicidae (Hymenoptera), visiting the nectaries of bracken. Ants are being found on the plants since the very beginning of their vegetation, even before the fronds develop. Sugar secretion by the plants and, consequently, the ant activity, decrease during frond enlargement to reach low levels at plant maturity [15]. Insect communities of Formicidae on British bracken are represented by Myrmica spp. and Formica lemani [7]. Formicidae, as generalist herbivores, tend to use the most abundant and most readily available food resources at a particular time. They are therefore an important element of balance in biocenoses [4].

The aim of the study is to assess the species composition of the beneficial insects inhabiting, infesting and visiting bracken fern plants.

OBJECTS AND METHODS

The observations were carried out in 2003 and 2004, in 7–10 days intervals, since the beginning of May untill the end of August. Two natural habitats of Pteridium  aquilinum sub. Aquilinum were undere investigation within Ślęża Massif (Lower Silesia, Poland). Site I was a spruce monoculture, developed in human-transformed habitat of acidophilic mountainous beech forest (Luzulo luzuloidis-fagetum community), whereas site II was submontane acidophilous oak forest (Luzulo-Quercetum community).

In order to determine the species composition of the insect guilds on bracken fern, 20 plants were being randomly selected for observation at each date of analysis in a particular season. These plants` above-ground parts were thoroughfully inspected and the insects were collected using small aspirator. Samples were always taken in similar conditions: at a time between 10 a.m. and 1 p.m., in sunny weather, at the air temperature >20°C. The taxonomical identification of the insects was accomplished according to characters described in the keys [1,2,4,5,8,11,13].

RESULTS AND DISCUSSION

From the above-ground parts of the bracken plants 667 individuals of the beneficial insects were collected in 2003 and 2004. They represent 22 species belonging to 9 insect families. In 2004 the number of collected individuals was less than 50% of the number collected in 2003 (Table 1). The collected beneficial insects were most abundant in the oak forest site (64.4% and 89.6% in 2003 and 2004 respectively). In the material from the other, the spruce forest site, the herbivorous and visiting species predominated.

Formicidae (Hymenoptera) were the dominant insect group irrespectively of the site and in both years of the study (Table 2). Altogether, 457 ants of 5 species were collected. The species are: Myrmica rubra L., Lasius niger L., Formica exsecta Nyl., Formica cinerea Mayr and Formica fusca L.

Coleoptera are represented in the studied material by the species of Coccinellidae and Cantharidae, first of the families prevailing and the collected ladybirds being Coccinella septempunctata L. and Propylea quatuordecimpunctata L.

Cantharidae, in turn, are represented by 4 species: Cantharis figurata Mann., Cantharis nigricans Müller, Cantharis obscura L. and Cantharis rustica Fall.

Among the predatory Heteroptera, specimens of Nabis rugosus L., Nabis ferus L. were collected most often and Himacerus apterus Fab. – less frequently; all belonging to Nabidae. Less abundant were Anthocoris nemorum L., Orius minutus L. and Lyctocoris campestris Fab. (Anthocoridae). Of the other heteropterans, Blepharidopterus angulatus Fall. and Psallus varians varians H-S marked the presence of Miridae and single specimens of Picromerus bidens L. testified of Pentatomidae.

Incidentally, Neoitamus cyanurus Loew (Diptera, Asilidae) and Chrysopa carnea Steph. (Neuroptera, Chrysopidae) were recorded. Although the insect abundance was higher on bracken plants within the spruce forest site (Table 2, 400 versus 277), the number of the identified species (14) was lower there compared to the oak forest (20).

Bracken fern forming dense stands before the establishment of the forest canopy, particularly in pine forests and in mixed coniferous forests, is perceived by forestry as a weed species. As such, it is most often subject to control with chemical herbicides [6]. Such disturbance results inevitably in reducing the abundance and general richness of the beneficial fauna relying on Pteridium aquilinum as one part of their ecological niche.

CONCLUSIONS

1. During the two-year study, insects belonging to 22 beneficial species were recorded from the bracken fern fronds and leaves.

2. The general insect abundance was greater on the bracken fern in spruce forest, whereas their species diversity  was higher in oak forest.

3. The predominant group of insects at both the studied sites were Formicidae.

4. Other insects represented eight families of different orders: Coccinellidae, Cantharidae, Nabidae, Anthocoridae, Miridae, Pentatomidae, Asilidae and Chrysopidae.

REFERENCES

Accepted for print: 17.12.2008

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