The two spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a major cause of pest damage worldwide. Its host range includes among many the strawberry crop, a high value crop in Denmark as well as in many other temperate and subtropical regions. Chemical control of T. urticae have been challenged by the polyphagous nature of T. urticae and its ability to reproduce exceedingly fast under optimal conditions. T. urticae have shown to be able to develop resistance towards chemical pesticides. Pest control by chemical means also have the unfortunate side-effect of reducing populations of naturally occurring enemies.

The potential and ability of phytoseiid predatory mites to control T. urticae have long been recognized. Less known is the potential of insect predators to control this pest and especially the naturally occurring generalist predators. Knowledge of the natural occurrence and diversity of predatory insects and predatory mites in Danish strawberry fields and surrounding vegetation is lacking, as is the knowledge of the potential of generalist insect predators to control T. urticae.

The overall objective of this PhD thesis was to investigate the trophic interactions between natural enemies, in particular generalist predators and the two spotted spider mite, T. urticae, in strawberry. This was done by investigating interactions of T. urticae and its natural enemies as influenced by cropping practice and the surrounding vegetation (Manuscript I) as well as more specifically defined predator-prey interactions (Manuscript II and III) and interactions occurring between groups of natural enemies (Manuscript IV).

Cropping practice influenced the density of T. urticae profoundly, with a ten times higher abundance found in conventional compared to organic strawberry fields (Manuscript I). Cropping practice also influenced the abundance, distribution and species richness of the arthropod natural enemies. The ratio of predatory mites per T. urticae was consequently found to be considerably higher in the organic compared to the conventional strawberry fields, approaching the recommended ratio for biological pest control of T. urticae. The abundance of insect predators was not different between the two cropping systems, while the abundance of hymenopteran parasitoids was. A higher species richness of predatory mites was found in the conventional field margins compared to the organic field margins, while insect predator species richness was found only to be higher in the organic field edge compared to the conventional field edge.
The contribution of the generalist insect predator, Anthocoris nemorum L. (Hemiptera: Anthocoridae) to control of T. urticae was investigated by molecular gut content analysis of field collected predators (Manuscript II). Based on the present results, it is not only evident that they contribute to the natural regulation of T. urticae, but also that they seem to do this from late June to early August. The proportion of predators with T. urticae DNA in their gut content decreased during this time, while the density of T. urticae only increased non-significantly.

The antipredator response of T. urticae (Manuscript III) was influenced by predator specialization. Chemical cues of a specialist predator, Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) induced a reduced oviposition rate of T. urticae, while chemical cues of both the specialist and the generalist predator, Orius majusculus Reut. (Hemiptera: Anthocoridae) induced an increase in dispersal rate of T. urticae. Increased dispersal of prey increases the likelihood of the prey encountering other natural enemies.

The study of the tritrophic interaction between entomopathogenic fungi, arthropod predators and T. urticae as influenced by specialization of the natural enemies (Manuscript IV), revealed an effect of fungal specialization on the resulting predator behaviour. Specialist fungal spores, of Neozygites floridana Weiser and Muma (Zygomycetes: Neozygitaceae), significantly increased prey encounters by predators and increased the likelihood of predators handling prey in the presence of these spores, while generalist fungal spores, of Metarhizium brunneum (Metchnikoff) Sorokin (Ascomycota: Clavicipitaceae), significantly reduced the predation rate of T. urticae. However, behaviours such as time spent searching, feeding, resting and grooming were not significantly affected by the presence of either species of fungal spores.

In conclusion, many T. urticae natural enemy interactions influence the resulting trophic dynamics in strawberry agroecosystems. Findings confirm the important role of cropping system on pest and natural enemies, and the resulting infestation levels point to the potential role of generalist insect predators to control of T. urticae in strawberry. It furthermore confirms the important role of antipredator responses and predator responses to entomopathogenic fungi as affected by specialization. The studies also raise new questions and potential avenues of further studies. These include the contribution by insect predators to control of T. urticae and larger scale studies of interactions between T. urticae predators and entomopathogenic fungi, which would expand our knowledge and ultimately benefit biological control of T. urticae in strawberry.