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Phytophagous hemipterans of the family Pentatomidae, commonly known as stink bugs, are important pests of several crops, being soybean (Glicine max (L.) (Fabales: Fabaceae)) one of the most important and attacked crops. In Argentina the most abundant species linked to this crop are: Nezara viridula (L), Piezodorus guildinii (West.), Diceraeus furcatus (Fab.) and Edessa meditabunda (Fab.) (Hemiptera: Pentatomidae), being the first two the most relevant (Cingolani et al. 2014a). From the whole complex of soybean?s pentatomids, P. guildinii causes the deepest damage to the seeds (Depieri & Panizzi 2011) and the greatest foliar retention that negatively affects the harvest process (Corrêa-Ferreira & Azevedo 2002), being also this species more resistant to commonly used pesticides (Temple 2011).Stink bugs are attacked by a diverse complex of natural enemies in the Neotropics, being parasitoids of the eggs and those of the adults the main ones. Among them, egg parasitoids (Hymenoptera: Platygastroidea) are very common and usually can lead to efficient local regulation of stink bug populations (Zerbino & Panizzi 2019). Regarding the other guild of parasitoids, several species of flies (Diptera: Tachinidae) and microhimenopterans (Hymenoptera: Encyrtidae) parasitize adults of all stink bug species in the complex of soybean pests, except for P. guildinii (Liljesthröm & Ávalos 2015), what represents an empty niche. From the early seventies, the cosmopolitan N. viridula was the predominant soybean pest in the Neotropical region, but from 2000 on, this species has sharply decreased in abundance (Panizzi & Lucini 2016). On the contrary, in the last two decades the relative abundance of P. guildinii has been increasing significantly and has currently become the most important species in several provinces of Argentina and other countries (Akin et al. 2011, Cingolani et al. 2014, Corrêa-Ferreira 2008, Massoni et al. 2008, Temple et al. 2013) The lack of the guild of parasitoids of the adult attacking P. guildinii may contribute to the explanation of its current expansion and pest status. Several physiological and behavioral characteristics influence the parasitoid acceptance behaviour and its reproductive success in a given host (Henry et al. 2008), whereby under particular circumstances a parasitoid may include a non-preferred species among its potential hosts. The range of hosts of a parasitoid species is determined by the ability of the females to search for hosts and select them (Godfray 1994, Vinson 1998). In turn, in the process of parasitism, the quality of the host also influences the successful development and emergence of parasitoids? progeny. The parasitoids show phenotypic plasticity in their host choices (Vos & Vet 2004), and their performance will be strongly influenced by the factors that act on this plasticity. Phenotypic plasticity is the ability of a genotype to express different phenotypes by adapting their physiology, morphology or development in response to changes in the environment (Colinet et al. 2007, Pigliucci 2001). The size of the parasitoids is one of the morphological characteristics that better evidenciate this plasticity, given that it varies depending on the host species on which the parasitoid develops the level of superparasitism and the age of the host, among others. The duration of the immature stages, as well as the fecundity of the adults, can also be influenced by the species of host used. Roitberg et al. (2001) defined ten important life history characteristics to be taken into account in the study of the performance of parasitoids, among which are mentioned: size, longevity, fecundity, mating ability and dispersal capacity.In relation to the particular states of the life history of the parasitoid, the age of the female parasitoid can negatively affect the oviposition rate, and the emergence and the sex ratio of the offspring (Powell & Shepard 1982). Indeed, the absence of adequate (preferred) hosts as the parasitoid ages (females with a high egg load and time-limited) could influence on her decision to parasitize a non-preferred host. It is known that selectivity decreases with a decrease on the life expectancy of a parasitoid, lowering the threshold of quality below which the hosts are rejected (Rosenheim 1999). The Tachinidae family is currently considered to be in adaptive radiation: the host ranges of different species of tachinids are highly variable over time and space, and probably in a process of expansion (Sitreman 2001). In this context we made tests with Trichopoda giacomellii (Blachard) (Diptera: Tachinidae), a spontaneous parasitoid of N. viridula to evaluate its performance when using P. guildinii as host. We offered six adults of P. guildinii to a young (1-3 days old; n=28) or old (4- 7 days old; n=26) female of T. giacomelli. In both cases the females were naïve and previously mated. In each case, the number of eggs attached by the fly to the host was counted throughout observational revisions (five of them within the first five hours of the experiment and an additional revision 72 hours after the beginning of the experiment). The visualization of at least one egg attached to the body of the host was considered as a sign of parasitism. We performed a Chi square test to compare the number of replicates parasitized in each treatment and a Wilcoxon test to evaluate the total number of eggs layed per female in each treatment.Six of the 28 (0.214) young parasitoid females and 11 of the 26 (0.423) old parasitoid females accepted and parasitized P. guildinii adults (Figure 1). This parasitism proportions were not significantly different between them (Chi square p-value=0.17)Young females parasitized on average 3 of the offered hosts laying 1.14 eggs (± 3.38 SD) on each one. On the other hand, old females parasitized on average 2.27 of the offered hosts laying 2.04 eggs (± 3.74 SD) on each one. We found no significant differences in the number of eggs laid in each host between treatments (p-value=0.12).From the 18 P. guildinii that were attacked by young parasitoids we obtained 9 pupae (emergence proportion=0.5) which resulted in 5 adult flies. In the case of the old parasitoids we obtained 8 pupae (emergence proportion=0.32) from the 25 P. guildinii that were attacked by old parasitoids. However, only one individual was capable to moult into an adult fly. A few individual of F2 were achieved too, but in this case they were from the mate of F1 parasitoid adults that parasite N. viridula, because when we try to offer P. guildinii to F1 flies, they rejected to parasite. We need to do more replicates with flies of F1 generation. Figure 1. Proportion of hosts parasitized by young and old female parasitoids.These data, although preliminary, indicate that T. giacomellii attacks P. guildinii and is able to successfully develop its complete cycle on this host, what results of interest for the design of integrated management strategies for this pest. This is of particular interest in a scenario where N. viridula populations is decreasing in density and P. guildinii is becoming more abundant.Although the proportion of parasitism between treatments didn?t show statistically significant differences, it seems to be a trend in old females to parasitize more than young ones. This may be linked to an effect of parasitoid age on their host?s selection and acceptance behaviors. On the contrary, although young parasitoid females seem to be more selective than old ones refusing more to parasitize P. guildinii, a non-preferred host, the progeny of these females may had had a better chances to develop viable progeny, as we obtained a slightly greater amount of flies developed from hosts parasitized by young females than by old females. Parasitoids of the adult stage of insects are capable to reduce the reproductive capacity and longevity of their host, as well as the amount of damage that the pest can cause to the crop. The combined action of this guild of parasitoids together with the parasitoids that attack the egg stage of hosts (oophagous parasitoids) can be very beneficial for the management of this pest species.The interest in sustainable plant protection techniques is growing globally. Biological control of pests through natural enemies is one of the most important options to reduce pest populations without damaging the environment. Among natural enemies, dipteran tachinids are parasitoids, antagonists of phytophagous insects, although the potential of tachinids in biological control is still underestimated, as host-parasitoid interactions of these biocontrol agents have been poorly studied. Increasing this knowledge will contribute to a better use of these beneficial insects.

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