Spiders (Araneae) occur in high abundance in all terrestrial ecosystems including agro-ecosystems. They are a very heterogeneous group of animals with different hunting tactics and therefore they play very different ecological roles. At family level these tactics are rather similar thus properties and behaviour found in different species of one family can be seen as characteristic for the whole family. Especially in orchards little is known about their role and probably it is undervalued. Therefore a comprehensive review (based on about 500 articles) of spiders as natural enemies of pest species of different crops was made resulting in information about the expected prey spectrum at family level. A qualitative evaluation of pest-spider relationship was carried out for a whole range of agro-ecosystems and the results are transposed to spider groups inhabiting the orchard ecosystems.
In a fundamental research project on integrated plant protection in orchards in Hungary (Apple Ecosystem Research) more than 2000 animal species were described for apple orchards. Until now the spiders were not studied in this project. The aim of this study is to describe the species richness and dominance order of spider communities inhabiting the canopy and the herbaceous-layer of apple and pear orchards in Hungary. Altogether 20283 individuals were collected belonging to 165 identifiable species. Considerable overlap has been observed between the spider fauna of apple and pear orchards.
Special attention is paid to the differences in spider fauna of orchards situated in different growing regions, because this knowledge can contribute to improve regional IPM programs. The great differences indicated that the composition of spider communities is basically determined by geographical locations. Although both the pesticide treatments and the different prey densities can significantly influence the densities of spiders, their effects on the composition of spider communities is limited.
The effect of conventional (based on broad-spectrum insecticides, e.g. OP's and pyrethroids) and integrated (based on selective chemicals, mainly IGR's) pest management systems on the canopy, herbaceous-layer and ground level inhabiting spider communities was investigated. The results lead to the conclusion that in case of applying integrated pest management there are possibilities to develop more complex spider communities. The negative effect of broad-spectrum compounds on spiders can be observed only on the canopy and to a lesser extent on the herbaceous-layer but not at the ground level. Regardless the pesticide treatments the composition of spider communities was similar.
The age of the orchards can significantly influence the spider density in the canopy through the prey density. In young (more vigorous) orchards, where the size of the canopy was smaller and the density of the pear lace bug ( Stephanitis pyri ) higher, significantly more complex hunting spider communities were present than in the same treated old orchards. This relationship was not observed in case of the guild web-building spiders. At the same time the diversity of the canopy inhabiting spider communities was higher in the old orchards, regardless of the chemical treatments.
The effect of the border of orchards on spider communities was investigated and it was found, that if selective insecticides were used the immigration into the orchards was significantly higher. While in case of applying broad-spectrum insecticides the canopy spider densities did not differ significantly between the outer rows and the interior rows of the orchards.
A considerable overlap exists between the spider communities of the canopy, the herbaceous-layer and the adjacent vegetation. Despite chemical treatments, exchange of individuals occurs and provides possibilities for re-colonization of spiders in the orchards from the herbaceous-layer and from the surroundings after pesticide treatments.
The most promising group of spiders in orchards is the clubionid spiders (Clubionidae) with as dominant species: Clubiona pallidula , Clubiona phragmitis , Cheiracanthium mildei . These spiders actively hunt on vegetation and never make a web for catching prey. Some species are winter-active, move and even hunt in winter. The low feeding rate in winter months at low temperature indicates that the winter-feeding will be of minor importance for natural pest control. In early spring when most of the other predators and parasitoids are not yet active, these spiders prey on pests that overwintered in the orchard like larvae of leafrollers (Tortricidae) and have a significant effect on suppression of pest populations.
Considerable predation by spiders was observed of the key pear pest, the pear suckers ( Cacopsylla spp .) and of the pear lace bug ( Stephanitis pyri ) common in IPM orchards in the vegetative period. In the latter case it was observed that the clubionid spider Ch. mildei showed a positive numerical response to prey density in the field, indicating density dependent mortality resulting in a better natural control.
The predatory capacity of clubionid spiders was estimated to be 3.3 mg at 10 °C to 5.7 mg at 20 °C per day with a model based on digestion and egestion characteristics. This indicates a daily potential killing rate of 3-6 small (L 1 -L 3 ) caterpillars of leafrollers depending on temperature. The size of the population in an untreated apple orchard was estimated to be 60.000 clubionids / ha (22 per tree) by mark-recapture method using double-release protocol in spring. These two findings indicate that spiders can be important in reduction of orchard pests, indeed.
The data provided in this thesis indicate that the role of spiders as natural control agents in orchards can be augmented. In orchards where Integrated Pest Management is applied, and where the use of broad-spectrum pesticides is minimized, an excellent possibility is available to develop more complex and abundant spider communities, which can contribute to a better suppression of pests.
|Qualification||Doctor of Philosophy|
|Award date||27 Apr 1999|
|Place of Publication||S.l.|
|Publication status||Published - 1999|
- natural enemies
- insect pests
- agricultural entomology