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Tetranychus urticae Koch (Acari: Tetranychidae) is a key pest of citrus that can also feed on most plant species occurring in the ground cover associated with this crop. To determine the effect of the management of the ground cover on the biological control of this mite, we have studied the dynamics of both tetranychid and phytoseiid mites in four commercial clementine mandarin orchards under three different ground cover management strategies: (1) bare soil, (2) resident wild cover and (3) a sown cover of Festuca arundinacea Schreb. (Poaceae). Our results provide evidence that the management of ground cover can affect both Tetranychid and Phytoseiid mite populations. We hypothesize that both natural enemies (top-down mechanisms) and plant resources (bottom-up mechanisms, discussed in a previous paper) play important regulatory roles. T. urticae specialist phytoseiid mites were consistently found in the F. arundinacea cover and this may explain the better regulation of Tetranychid mite populations on trees grown in association with this cover crop. In contrast, the more regular provision of alternative food (pollen) in the wild cover relative to the F. arundinacea cover, could explain the higher abundance of generalist pollen feeder phytoseiids in the former. As a result, more efficient T. urticae-specialized phytoseiids could suffer increased competition from generalist pollen feeders in the wild cover. This fact, in combination with periods of prey scarcity, could result in their disappearance from the agroecosystem, resulting in an inadequate control of Tetranychid mites in trees grown in association with a wild cover.

Fruit scarring caused by Tetranychus urticae is one of the main pest problems faced by Spanish clementine growers as it can downgrade the quality of the fruit completely. T. urticae infestations concentrate on fruit at the end of summer. This study assessed the influence of vegetative growth of clementine trees during summer and fall on the damage caused by T. urticae. Damage significantly increased in trees in which summer and fall flushes were mechanically removed. This indicates that summer and fall leaf growth is a key factor limiting mite damage on fruit. Cultural and crop protection practices which promote abundant normal summer and fall vegetative growth seem to be critical for minimizing the impact of T. urticae populations on fruit quality. © 2007 Elsevier Ltd. All rights reserved.