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Conservation biological control by means of ground cover offers an interesting alternative to chemical control of the twospotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae), a key pest of clementine mandarins. The aim of this study was to investigate whether this tactic could actually achieve significant reductions in crop losses caused by this pest. The three most common cover practices used in citrus in Spain were compared: wild cover, a cover of Festuca arundinacea Scherb. (Poaceae), and bare soil. The action threshold was more often exceeded in wild cover than in bare soil or F. arundinacea. When expenditures and revenues were balanced, the most favorable cover was F. arundinacea (between 44.4 and 74.5% cost reduction relative to the most expensive one). F. arundinacea as a cover crop is a conservation biological control strategy highly recommendable for clementine producers. Although its use did not reduce mite populations below the action threshold, the population decreases obtained made the adoption of this tactic a beneficial alternative both ecologically and economically. This work was partly funded by MICINN (AGL2005-07155-C03/AGR and AGL2008-05287-C04/AGR), and theConselleria d’Agricultura, Pesca i Alimentació de la Generalitat Valenciana (ATC06/058-TS/06/UJI/03)

Tetranychus urticae Koch is a cosmopolitan mite considered as the most polyphagous species among spider mites. This mite is a key pest of clementine mandarins in Eastern Spain, where Spanish clementine production concentrates. Crop management practices can affect the population dynamics of this mite and, consequently, its impact on the orchard. Microsatellite markers were used to study mite population genetics from two commercial orchards which had been managed differently following Integrated Pest Management (IPM) or Organic Pest Management (OPM) schemes during four consecutive years. A multiplex system including 20 microsatellite loci was designed specifically and allowed an efficient and inexpensive genotyping of individual mites. We found that the IPM population had a stronger fluctuation of population structure and higher genetic diversity compared to OPM population. Thus, our study concludes that crop management has an impact on the population genetics of T. urticae which may be related to the alternation of some acaricides under IPM. © 2013 Springer Science+Business Media Dordrecht., JGS-M was supported by a Spanish Ministry of Science and Innovation “Ramón y Cajal” postdoctoral contract. BS-M was supported by Spanish Ministry of Science and Innovation “INIA-CCAA” postdoctoral contract. This work was partially funded by the Ministerio de Ciencia e Innovación (MICINN) projects AGL2005-07155-C03/AGR, AGL2008-05287-C04/AGR and AGL2011-30538-C03, and by the Fundació BANCAIXA-Universitat Jaume I projects P1-1A2005-03 and P11B2008-02.

Tetranychus urticae Koch is a cosmopolitan phytophagous mite considered as the most polyphagous species among spider mites. Population genetic studies using molecular markers such as microsatellites have proven to be extremely informative to address questions about population structure, phylogeography and host preferences. The aim of this study was to increase the available molecular tools to gain insight into the genetic structure of T. urticae populations of citrus orchards, which might help in their management. Five microsatellite DNA libraries were developed using probes with the motifs CT, CTT, GT and CAC following the FIASCO protocol. Positive clones, those that included the insert with the microsatellite, were detected using the PIMA-PCR technique. Combinations of primers were designed on 22 out of 32 new microsatellites loci and their polymorphism was tested in four populations sampled along the eastern coast of Spain. Eleven successful amplifications were obtained. Cross amplification was tested in the tetranychids Aphlonobia histricina, Eutetranychus banksi, E. orientalis, Oligonychus perseae, Panonychus citri, Tetranychus evansi, T. okinawanus and T. turkestani, and the phytoseiids Amblyseius swirskii, A. cucumeris, A. andersoni, Euseius stipulatus, Neoseiulus barkeri, N. californicus, Phytoseiulus persimilis and Typhlodromus phialatus. Eight successful cross amplifications were obtained.

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.

The citrus leafminer, Phyllocnistis citrella, is a pest native to Southeast Asia which threatened the citrus industry in the Mediterranean region upon its introduction in 1993. Immediately afterward, a classical biological control program was implemented in Spain. The exotic parasitoid Citrostichus phyllocnistoides was the only introduced parasitoid to become established. In 2006, data on both the incidence of P. citrella and the impact of its natural enemies were collected following the same protocols used in 1997-1999 when C. phyllocnistoides was not yet present. C. phyllocnistoides constituted 99.4% of the parasitoids collected in 2006 corresponding to a decrease in the incidence of P. citrella from 3.2-5.1 to 1.8-2.4 mines per leaf in 1997-1999 and 2006, respectively. Mortality caused by natural enemies on P. citrella in 2006 was 93.3% (18.0% parasitism, 40.8% feeding punctures and 34.5% predation). C. phyllocnistoides, which preferentially parasitizes P. citrella second instar larvae, has displaced most of the indigenous parasitoids that moved onto P. citrella mainly parasitizing third instar larvae, upon its introduction. Because C. phyllocnistoides is an idiobiont parasitoid and preferentially parasitizes P. citrella second instars, this stage has become dominant in the orchards. The shift in the relative abundance of P. citrella larvae has prompted generalist predators to prey mostly on second instars and has contributed to the displacement of the native non-specific parasitoids, which principally utilize third instars, from the system. Both indigenous predators and the introduced parasitoid are key players in the natural regulation of P. citrella. © 2009 Elsevier Inc. All rights reserved.