IMPACT OF HERBIVOROUS DOCK LEAF BEETLE (GASTROIDEA VIRIDULA DEG.) RELEASING ON THE DEVELOPMENT OF MOSSY SORREL (RUMEX CONFERTUS WILLD.)

Dariusz Piesik, Robert Lamparski
Department of Applied Entomology, University of Technology and Agriculture, Bydgoszcz, Poland

ABSTRACT

The field experiments were conducted in 2003, 2004, and 2005 to evaluate a direct effect of the dock leaf beetle (Gastroidea viridula Deg., Coleoptera: Chrysomelidae) releasing on mossy sorrel (Rumex confertus Willd.). Significantly greater amount of larvae was recorded on plants on which adults were previously released. Also greater leaf damage index and lesser height of mossy sorrel plants were observed on those plants in comparison to control.

Key words: weeds, herbicides, mossy sorrel, Rumex confertus Willd., biological control, the dock leaf beetle, Gastroidea viridula Deg..

INTRODUCTION

The Rumex weeds are considered to be ones of the most dangerous, uncultivated plants in the world due to their very strong expansion, which results from an abundant seed production [2]. The broad-leaved dock (Rumex obtusifolius) is a dangerous weed of pastures because it reduces the quantity and quality of fodder for cattle [3]. Also mossy sorrel (Rumex confertus), 2 m high perennial plant, occurs commonly in the world [8, 12], as well as it establishes new stands in Poland every year. It contains high amounts of oxalic acid and the lethal poisoning of animals can occur when large quantities of the plant are consumed.

The chemical control of this weed is difficult. After applying herbicides, the aboveground parts of the plant dry out, but shortly afterward, new leaves emerge [14, 15]. In addition, unjustified chemical treatments can induce the breakdown of plants’ resistance [1, 10]. Moreover, the herbicides can be directly toxic to herbivorous insects, thus increasing emigration and/or decreasing immigration [4] and can reduce the weed biomass for herbivorous insects and/or may change the nutritional status of the plant [22, 26]. Finally, pesticides may deplete the food resources or alter the food suitability for insects [6]. In that case less prey may limit predatory and parasitic species, or they may consume such amounts of contaminated food items that toxic effects occur [24].

An alternative method to the pesticide treatments includes biological control and the use of herbivorous insects [7, 27]. Gastroidea spp. is of special interest among the numerous species inhabiting Rumex spp. [23]. The dock leaf beetle is an oligophagous herbivore, which prefers consuming leaves of dock (Rumex spp.) species. Because of its feeding specialization this beetle is considered a potential biological control agent of dock plants [11]. Additionally the dock leaf beetle is a promising biological control agent, because it has a high reproductive potential [19], a short life cycle, no alternative hosts, and the number of their predator are low. Moreover, G. viridula, especially the larvae, can defoliate whole dock plants if they occur in high densities [3].

Several studies have investigated the effect of potential biocontrol agent (G. viridula) on R. obtusifolius, but little is known about biological control of R. confertus. The present study was designed to screen the possibility of biological control of mossy sorrel weeds by G. viridula. Furthermore, the experiments are addressing the possible application of those insects for the control of R. confertus.

The aim of the study was to evaluate the introduction of G. viridula adults on R. confertus, as well as the influence of larvae on the growth of mossy sorrel plants.

METHODS AND AREA OF THE STUDIES

The experiments were carried out in 2003, 2004, and 2005. The field trials were located in the natural habitat condition of R. confertus in Bydgoszcz vicinity on the marshy meadow near Vistula river, Northern Poland (53°13’N, 18°15’E).

The chrysomelid beetles, G. viridula were chosen as model species. They seem to be relevant for many reasons. First of all, these insects are characterized by low mobility. Also, oligophagous preferences for Polygonaceae spp. allow choosing these insects as possible biological control agents for their host plant.

The results of height of mossy sorrel plants and number of the dock leaf larvae were subjected to an analysis of variance. Mean amounts were separated after analysis of variance using Tukey’s test for significant differences at α = 0.05.

The field experiments.

1. Five R. confertus plants (in 5*5 replicates) were chosen for the G. viridula releasing experiment. The end of April on every caged plant in amount of 25, 38, 50, 63, and 75 pairs released adult insects, both females and males. After two weeks, when the females laid eggs, every cage was removed. Five control plants were kept without any cages under natural site conditions. The field studies were carried out over whole growing season; from spring to autumn. After emergence, the number of larvae was recorded every second week. Captured larvae were counted precisely and then released. In parallel, on mentioned objects the height of mossy sorrel plants was measured every second week.

2. G. viridula larvae feeding on the mossy sorrel plants were observed to evaluate leaf damage index. The field studies were carried out every second week over whole growing season.

A five-degree-injury scale was used, where:

Following a five-degree-injury scale leaf damage index was calculated, basing on Townsend and Heuberger equation:

RESULTS

Two periods characterized mossy sorrel development. This has been demonstrated over the three experimental years. First period shown intensive growing of Rumex plants from the end of April up to the end of July (Table 1).

By the end of that month plants started to die which seems to be normal in their natural habitat. Moreover, drying was observed immediately after seeds production. The second period of intensive development was noticed from the beginning of August to the end of September. Biocontrol agents decreased host plants height as compared to control. In all years of experiment, treatments with released pairs of adults significantly affected plants development, especially including biomass and reproduction of R. confertus Willd. Height of mossy sorrel weeds was correlated with number of adults released on the host plant. Increased number of G. viridula adults released by the end of April resulted in increasing number of larvae and in parallel in decreasing height of plants. Due to increased larvae abundance and decreased development of plant less seed production was observed. The most significant damage was observed on those plants on which 75 pairs of adults was released. However only the greatest numbers of released insects (63 pairs and 75 pairs) and the consecutive generations were able to eliminate mossy sorrel plants. Probably only systematic stress of G. viridula adults and larvae can impact those weed plants.

The first larvae generation was growing from mid-May to mid-June, while the second and third one in July and at the end of August and beginning of September (Table 2).

Emphatically across growing season the greatest number of larvae was observed on those plants on which 75 pairs of adults were released. The weed biomass and its population declined. Moderate herbivorous effectiveness characterized the other treatments. Moreover the combination with 25 pairs of released beetles was similar in case of larvae abundance as compared to control. Those plants were damaged by herbivorous insects, but were still green. From the beginning of August, when all control and treated plants were growing up again, similar tendency was observed.

Very interesting results were achieved from the leaf damage index (LDI) experiment (Fig. 1).

Average LDI showed real impact of preying on R. confertus plants. For all treatments significantly higher amounts of consumed leaves as compared to control were observed. G. viridula feeding on mossy sorrel has affected plants development. Increased number of G. viridula adults released on plants had result in increasing LDI. For the treatments of 63 and 75 pair the infestation reached by the end of July almost 100%. Similar tendency was observed for the other treatments, although lesser and later in time. All plants started drying by the end of July, although those with the highest amount of released insects a bit earlier. In the beginning of August new leaf rosettes were observed again. However no plants development has been recorded for the treatments of 63 and 75 pair of released insects. It may suggest, that regular stress by high amount of insects can affect the mossy sorrel development. Further experiments are needed to investigate the mossy sorrel weed plants overwintering after injury.

DISCUSSION

Biocontrol has its basis in the fact that when organisms invade new environments they often leave their adapted natural enemies behind and they may erupt in great abundance [5]. The most suitable term of beneficial insect releasing is when herbivore levels are low. There are many instances of the introduction of natural enemies to control pest populations. The most frequent concern against biological control is that it might be dangerous to the beneficial organisms [9]. However, Van Klinken and Edwards [25] argued that the fundamental host range of insect herbivores commonly used as biocontrol agents is unlikely to change.

Early work showed [14] that under natural site conditions G. viridula has damaged the vegetative weight of mossy sorrel to a large extent. Whittaker et al. [29] described this species as a very important biological factor, as well as part of an integrated programme for regulating the development of undesirable plants. The results of our experiments provide sufficient evidence that at the beginning larvae skeletonized the leaves and then made holes. G. viridula is an effective biocontrol agent with highly host specific orientation. These results are in agreement with the others who reported similar prey activity [16, 18].

In this experiment, we noted that adult’s introduction caused an increase of the larval population. In consequence, the greater number of larvae was observed. Looking at the trends especially plants with 63 and 75 pairs of released adults were stressed by significantly higher amount of larvae. In particular, this is of usefulness for the biological control of mossy sorrel. Greater number of larvae of all instars on mossy sorrel plants resulted in the higher leaf damage index. Following that suggestion, high-injured leaves have lesser surface for photosynthesis. Also, significantly damaged plants are subjected to the pathogens.

Biological control seems to be one possible tool for the further consideration. We anticipate that releasing of G. viridula will increase its population and thus affect R. confertus growth and reproduction. Previous study [17] demonstrated that herbicide treatment affected plant growth, but new leaf rosettes were produced by the end of the summer. However, herbicide-treated plants were significantly less attractive food in comparison to the control plants [17]. Larvae, subjected to direct toxic effects from herbicides, were at least repelled from the odour of herbicides. Sotherton and Moreby [21] reported similar tendency, although for the other herbicides. It suggests that the herbicide treatment can change the physiological status of the plant [13, 28] and thus reduce survival of larvae [20].

CONCLUSIONS

1. Chrysomelid adults released resulted in greater G. viridula larvae population on R. confertus as compared to control mossy sorrel plants.

2. Greater leaf damage index (LDI) occurred on plants, on which adults were released; especially on those with 63 and 75 pairs of the dock leaf beetles.

3. Consequent larval preying of every consecutive generation, during the growing season, resulted in reducing the height of mossy sorrel plants.

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Accepted for print: 16.11.2006

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