Amblyseius swirskii Athias-Henriot is a well-known predator that is used for controlling the population of two-spotted spider mites (TSSM), Tetranychus urticae Koch, and greenhouse whitefly (GHWF), Trialeurodes vaporariorum Westwood, in strawberry greenhouses. To find the effective factors that influence the efficiency of this predator, the predation rates of A. swirskii fed on (Ι) TSSM in the presence and absence of the pollen, webbing, and GHWF, as well as on (II) GHWF in the presence and absence of the pollen, and GHWF-produced honeydew were determined. Furthermore, developmental time, fecundity, and population growth rate of this predator under the same conditions were measured. Our results showed that A. swirskii was able to reduce TSSM population, while the spider mite webbing had an adverse effect on the performance of the predator. Therefore, the presence of the predator population at the time of the infestation is crucial to the success of biological control. It can be concluded that the alternative food sources such as pollen and GHWF-produced honeydew play an important role in maintaining the predator population in the absence of pests. Moreover, the results indicate that using the pollen and another pest along with the target pest can promote the predator density. A. swirskii consumed lower numbers of TSSM when concurrently offered with GHWF and/or maize pollen, and lower numbers of GHWF in the presence of pollen. On the other hand, in the presence of alternative food or alternative prey, the fecundity of the predator was much higher.

The interspecific competition is a widespread phenomenon among the herbivores and is an important factor in shaping the herbivore communities. Plants usually mediate competition among herbivores via induced resistance, which can affect pest density, either by altering the performance or host preference of herbivores. The strawberry plants host the two-spotted spider mites (TSSM), Tetranychus urticae Koch and greenhouse whitefly (GHWF), Trialeurodes vaporariorum (Westwood) in greenhouses. We clearly observed the adverse effect of TSSM or GHWF prevalence on colonization and outbreak of the other one, therefore the possibility exists that feeding-induced changes in plant and interspecific competition are the effective factors in shaping their densities. In this study, laboratory experiments were carried out to determine whether previous feeding by TSSM on strawberry (Camarosa variety) affects the performance and oviposition preference of GHWF (as a competitor), and vice versa. Lower egg number, and the slower development as well as delayed oviposition were observed when the pests fed on infested leaves than control leaves. Furthermore, when the pests were given the choice between the non-infested and infested leaves, both pests significantly preferred the non-infested leaves. Our study concluded that feeding-induced changes in quality of host plant adversely affected the performance and oviposition preference of both pests. We suggested that these results can help to explain the different population densities of pests in presence and absence of other species and can attract the attention of farmers to the effects of interspecific competition on the densities of pests.

We quantified the life table parameters and predation capacity of a generalist predatory mite, Typhlodromus bagdasarjani Wainstein and Arutunjan on five monotypic diets, including Tetranychus urticae Koch (TSSM) eggs in the presence (SW) and absence (SN) of webs, Trialeurodes vaporariorum Westwood (GHWF) eggs (G), honeydew (H), and maize pollen (M) as well as three mixed diets, including SN + M, SN + G, and G + M. Our results showed that the individuals fed on the mixed diets had a considerably shorter developmental time and pre-oviposition period (APOP), higher oviposition days, higher fecundity and population growth rate than those raised on the monotypic diets. Furthermore, we found that the mixed diet of TSSM and GHWF eggs was the most favorable diet, resulted in the highest fecundity and population growth rate, shortest developmental time and APOP. While TSSM eggs alone in the presence of webs and honeydew were the worst diets resulted in the longest developmental time, lower oviposition day, higher fecundity and population growth rate. Our data determined that TSSM has more nutritional benefits than GHWF for T. bagdasarjani. We observed the positive effects of pollen addition to prey on the predatory mite's immature and adult life-history characters; however, it reduced the predation rate. Overall, maize pollen could enhance ecosystem services provided against spider mites and whiteflies by positively impacting the increase of T. bagdasarjani population. This predator may be more effective when two prey species are available than when only one species is present.

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