ABSTRACT
The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is one of the most injurious pests affecting clementine orchards in Spain. In clementine trees, T. urticae inhabits the lower side of leaves where it produces silky webbing and dense colonies. It sucks the cell contents, causing chlorotic spots on the upper side of the leaves. At the end of summer, T. urticae infestations can result in characteristic fruit scarring and consequently fruit downgrading. To date, the control of T. urticae has been mainly based on chemical control. However, the use of this method is not always effective. Beneficial arthropods can be decimated, creating conditions favorable for uncontrolled proliferations of T. urticae and other pests. Currently, the integrated pest management programs in citrus aims to maximize the use of biological control. Thus, the overall objective of this thesis has been the improvement of the biological control of T. urticae in clementine orchards.
Biological control is not common in citrus nurseries where chemical control is prevalent. The systemic neonicotinoid imidacloprid applied as a drench is effective against three out of four key pests of young clementine plants in Spain - aphids, leafminers and scales. However, mites, the fourth key pest, are not controlled by imidacloprid and could be regulated by introduction of the predatory phytoseiid mites Phytoseiulus persimilis Athias-Henriot and Neoseiulus californicus (McGregor). The aim of this study was to evaluate the effects of imidacloprid applied as a drench on the demographic parameters of these two predatory mites and the compatibility of P. persimilis releases with imidacloprid to control key pest populations in young clementine plants under field conditions. The results showed that some demographic parameters of P. persimilis were affected by imidacloprid. However, their combined effect on its intrinsic rate of increase was neutral. In contrast, imidacloprid negatively affected the demographic parameters of N. californicus. Field results proved that young clementine plants could be satisfactorily protected against key pests with releases of P. persimilis combined with drench applications of imidacloprid. The combination of imidacloprid with P. persimilis releases was highly effective for management of the key pests of young clementine plants in the nursery. 
Conservation and augmentative biological control strategies have been developed to take full advantage of the natural enemies that occur in Spanish citrus orchards. Among them, the predatory mites E. stipulatus (Athias-Henriot), N. californicus and P. persimilis play an important role in the biological control of tetranychid mites. However, these predatory mites are often affected by pesticides and information about the side-effects of these products against these beneficial arthropods is essential to guarantee the efficacy of these beneficial arthropods. The side-effects of some pesticides remain unknown and the primary aim of this study is to fill this gap. We have further used this information and that collected from other sources to compare the response of these three mite species to pesticides. Based on this information, E. stipulatus has resulted as the most tolerant species, followed by N. californicus and P. persimilis. Therefore, using E. stipulatus as an indicator species in citrus may have led to the paradox of selecting presumed selective pesticides resulting in excessive impact on N. californicus and, especially on P. persimilis. Because these two latter species are considered key for the biological control of T. urticae in citrus in Spain, we propose to use P. persimilis as the right indicator of such effects in citrus instead of E. stipulatus. This change could have a dramatic impact on the satisfactory control of Tetranychid mites in citrus in the near future.
Cover crops can serve as a reservoir of natural enemies by supplying alternative food sources as pollen. In turn, pollen quality and availability can modulate phytoseiid communities. In clementine trees associated with a cover crop of Festuca arundinacea Schreber, these communities were more diverse than those associated with a multifloral wild cover crop. As a consequence, the former had a better regulation of T. urticae populations than the latter. Longer provision of higher quality pollen in the multifloral cover relative to F. arundinacea is suspected to interfere with the biological control of T. urticae by specific phytoseiid predators (P. persimilis and N. californicus) by enhancing the less efficient generalist pollen feeder Euseius stipulatus which is a superior intraguild predator. To determine whether pollen quality is behind these results, the effect of the provision of two different pollens (Carpobrotus edulis (L.) L. Bolus and F. arundinacea) on the efficacy of two phytoseiid species (E. stipulatus and N. californicus) to regulate T. urticae populations has been studied under semi-field conditions. Results suggest that pollen provision does not enhance the ability of these phytoseiids to reduce T. urticae populations. However, C. edulis pollen resulted in explosive increases of E. stipulatus numbers that did not occur with F. arundinacea pollen. Therefore, poor quality pollen may prevent pollen feeders from reaching high numbers in the field. This effect could benefit phytoseiid species suffering intraguild predation by E. stipulatus explain field results.
Biological control through augmentative releases is a common practice against some citrus pests such as Aonidiella aurantii Maskell (Hemiptera: Diaspididae), Planoccocus citri Risso (Hemiptera: Pseudococcidae), and is developing in other species such as Tetranychus urticae. In the case of the T. urticae, releases of the phytoseiid mites P. persimilis and N. californicus resulted successful in nurseries. However, under field conditions, phytoseiids are not always detected after releases and control of T. urticae has not always been satisfactory, especially in the case of N. californicus. There is no information on the behavior of these phytoseiid mites in citrus orchards. Understanding their dispersal and preying activity could be useful to improve augmentative releases. Results showed that the phytoseiids exhibited a similar spatial distribution pattern under different prey densities. Both phytoseiids were predominantly found on leaves, where T. urticae colonies were present. However, at low pest densities, N. californicus showed a higher tendency to move to the trunk. Furthermore, prey detection by molecular methods proved P. persimilis to be a superior predator, even at lower prey densities. Based on these results, hot spot augmentative releases of P. persimilis are recommended.