Neoseiulus californicus (McGregor) is a promising agent for successful Tetranychus urticae Koch control through conservation techniques, in strawberry crops in La Plata (Buenos Aires, Argentina). In prey-predator interaction, initial relative densities have an important effect on system dynamics. The economic threshold level (ETL) used for this pest in the present study was 50 active mites per leaflet. In our laboratory experiments, initial T. urticae to N. californicus ratio had a significant effect on the population abundance of T. urticae at a 7-day period. When pest/predator ratio was 5/1 (at initial pest densities from 5 to 15 females/leaflet) the final number of active T. urticae/leaflet was significantly lower than the ETL, while at 20 females/leaflet this number did not differ from the ETL. At 7.5/1 ratio, the final number of active T. urticae/leaflet, at initial pest densities from 5 to 15 females/leaflet, reached the ETL without surpassing it. At 10/1 and 15/1 ratios, pest densities exceeded the ETL only at 15 initial T. urticae/leaflet. Most greenhouse and field observations were consistent with the predictions of a graphical model based on experimental results. This predator was very effective in limiting pest densities at a 7-day period and within the range of pest-predator ratios and absolute densities used in this study. Conservation of N. californicus promoting favorable pest/predator ratios may result in early control of T. urticae.

The spiders are one of the most important predators that produce the mortality of the plague insects. The spider community was sampled with nets, plant pocketed and pit fall traps on soybean crop and the wild vegetation on the borders (2h manual capture). The samples are done during the crop development (December to mid-May) and, in the fallow period (mid-May to November), in order to know spiders specific composition, to compare specific diversity through Shannon index, and, to analyze the colonization process. Total specific richness was 37 species grouped in 13 families, and the dominant species was Misumenops pallidus (Keyserling) (Thomisidae). The higher diversity was recorded in wild vegetation (H = 2.829, and 25 species), then the herbal stratum of crop showed the highest affinity (H = 2.140, and 28 species), lowest diversity was recorded during the fallow period (H = 1.050, and 12 species). The early species colonizing the crop (December) belong to the families Thomisidae, Anyphaenidae and Oxyopidae, after them Araneidae and Corinnidae were recorded, the late colonizing were the Salticidae species. Thomisidae species were always the most abundant, predominating from February to April. The lowest density of species of spiders was found on December in the border crop, but the distribution, from the border to the center crop, from January to March, did not show any gradient. The occurrence of several species belonging to different guild in all the plots suggests that the spiders are an important limiting effect on insect populations, as soon as the importance of the preservation of natural areas in techniques of biological control.

The system formed byN. viridula and its almost specific parasitoidT. giacomellii has been studied in the Buenos Aires province, Argentina for 26 generations of the former and 20 generations of the latter. An intensive study over 9 host generations and key factor analysis made possible the identification and quantification of different mortality factors. The main factors determining the fluctuations in total generation mortality were: egg mortality due mainly to parasitism byT. basalis; nymphal predation on 1, 2 and 3 instar nymphs and migration; and adult mortality and reduction on potential natality due to parasitism byT. giacomellii. Adult mortality and reduction of potential natality, due to parasitism byT. giacomellii, was density dependent. This factor is capable of maintaining the simulations generated by a simple model based on key factor analysis, within the range of observed densities. These results suggest that parasitism byT. giacomellii can regulate the population, even in a small area of 450 m.

The role of the aggregated distribution of attacks, a castration effect (cessation of reproduction of the parasitized hosts) and the differential selectivity for hosts by the parasitoid Trichopoda giacomellii on the population regulation of its host, Nezara viridula, was investigated using a detail‐rich simulation model. The model parameters were estimated either in the laboratory or under field conditions. Additionally, key parameter values were estimated by fitting the model to field data of nine generations of the host and the parasite, respectively. The model proved to be robust, with a high coincidence between the parameter values obtained by fitting the model and the field/laboratory estimates. In both cases, the model reproduces the regular oscillations observed in the real system. Stability analysis revealed that the model could have different dynamic behaviors, unstable or stable depending on the parameter values. It was shown that the aggregated distribution of attacks, as described by the negative binomial distribution with a low k value (aggregation parameter of the parasitoid's egg distribution among hosts), was the dominant mechanism associated with the stability of the model, although there was some degree of interaction with the other two mechanisms (the castration effect and the differential selectivity for hosts). However, neither of these two mechanisms was able to regulate the parasite–host populations in the absence of the aggregated distribution of attacks. The field/laboratory estimate of parameters falls in all cases in the stability region with normal population densities, while the best parameter estimates by fitting the model sometimes fell in the region of the parameter space where the parasite became extinct. This is one of the few cases in which the theoretical stability behavior conclusions derived from simple mathematical models could be assessed with a complex and realistic simulation model based on field/laboratory data, and the dominant stabilizing mechanisms determined. Specifically, the best fit and the field estimates of the parameter k, an indicator of the degree of aggregation of the distribution of attacks, coincided in values less than 1, as predicted by theory for stable parasite–host populations. Larger k values lead to instability.