Testing the habituation assumption underlying models of parasitoid foraging behavior

Hyperparasitism of Acroclisoides sinicus (Huang and Liao) (Hymenoptera: Pteromalidae) on Two Biological Control Agents of Halyomorpha halys

Halyomorpha halys (Stål) is an invasive Asian pest that causes severe crop losses on various crops. Nowadays, management strategies against this pest mainly rely on pesticide use, but biological control with egg parasitoids is considered the most promising long-term and sustainable solution. Trissolcus japonicus (Ashmead) and Trissolcus mitsukurii (Ashmead) are Asian egg parasitoids already present in Europe and are the most effective biological control agents of H. halys. Therefore, these two species are considered for biological control programs in Europe and other parts of the world. Acroclisoides sinicus (Huang and Liao) is a pteromalid parasitoid wasp that frequently emerged from H. halys egg masses collected in northern Italy. This species has been hypothesized to be a hyperparasitoid of Trissolcus spp. parasitoids. This study was carried out under laboratory conditions where A. sinicus was tested in no-choice and two-choice experiments to assess the host preference between T. japonicus and T. mitsukurii. Olfactory responses of A. sinicus from volatiles emitted from different potential hosts were also tested. In all trials, A. sinicus showed a clear preference for parasitizing H. halys eggs previously parasitized by T. mitsukurii compared to T. japonicus. In no-choice experiments, the impact of the hyperparasitoid on T. japonicus was low, showing an exploitation rate of 4.0%, while up to a 96.2% exploitation rate was observed on T. mitsukurii. Acroclisoides sinicus was also attracted by volatiles emitted by egg masses parasitized by T. mitsukurii, while no response was observed to egg masses parasitized by T. japonicus or not parasitized. Therefore, according to the results obtained here, A. sinicus could limit the population development of T. mitsukurii, while lesser effects are expected on T. japonicus.

Learning can be detrimental for a parasitic wasp

Animals have evolved the capacity to learn, and the conventional view is that learning allows individuals to improve foraging decisions. The parasitoid Telenomus podisi has been shown to parasitize eggs of the exotic stink bug Halyomorpha halys at the same rate as eggs of its coevolved host, Podisus maculiventris, but the parasitoid cannot complete its development in the exotic species. We hypothesized that T. podisi learns to exploit cues from this non-coevolved species, thereby increasing unsuccessful parasitism rates. We conducted bioassays to compare the responses of naïve vs. experienced parasitoids on chemical footprints left by one of the two host species. Both naïve and experienced females showed a higher response to footprints of P. maculiventris than of H. halys. Furthermore, parasitoids that gained an experience on H. halys significantly increased their residence time within the arena and the frequency of re-encounter with the area contaminated by chemical cues. Hence, our study describes detrimental learning where a parasitoid learns to associate chemical cues from an unsuitable host, potentially re-enforcing a reproductive cul-de-sac (evolutionary trap). Maladaptive learning in the T. podisi-H. halys association could have consequences for population dynamics of sympatric native and exotic host species.

A neuronal arms race: the role of learning in parasitoid-host interactions

Parasitic wasps and their larval hosts are intimately connected by an array of behavioral adaptations and counter-adaptations. This co-evolution has led to highly specific, natural variation in learning rates and memory consolidation in parasitoid wasps. Similarly, the hosts of the parasitoids show specific sensory adaptations as well as non-associative learning strategies for parasitoid avoidance. However, these neuronal and behavioral adaptations of both hosts and wasps have so far been studied largely apart from each other. Here we argue that a parallel investigation of the nervous system in wasps and their hosts might lead to novel insights into the evolution of insect behavior and the neurobiology of learning and memory.

Host Kairomones Influence Searching Behavior of Trissolcus japonicus (Hymenoptera: Scelionidae), a Parasitoid of Halyomorpha halys (Heteroptera: Pentatomidae)

The brown marmorated stink bug, Halyomorpha halys (Stål), is a highly polyphagous species native to Asia that has become a serious invasive agricultural and nuisance pest across North America and Europe. Classical biological control host range evaluations have revealed egg parasitoid Trissolcus japonicus (Ashmead) to be the primary candidate biocontrol agent for field release against H. halys. However, these evaluations only provide us with the physiological host range of T. japonicus. Other Trissolcus species have demonstrated that contact kairomones from different host species elicit varied responses in the parasitoids' host foraging behaviors. To assess T. japonicus response to host kairomones, mated naive females were exposed to leaf surfaces contaminated with adult kairomones from its preferred host, H. halys, or from a native nontarget host, Podisus maculiventris (Say) (Heteroptera: Pentatomidae). Red maple, apple, and soybean were used as plant substrate treatments. The wasp's residence time on the leaf surface, linear walking velocity, and angular walking velocity were observed and measured using Noldus EthoVision XT tracking software. Within each leaf treatment, T. japonicus displayed stronger behavioral responses on leaves contaminated with contact kairomones from H. halys. The parasitoid resided on H. halys contaminated leaves for approximately twice as a long as it did on P. maculiventris contaminated leaves. Further, both species' kairomones elicited significant decreases in parasitoid walking velocity on all tested substrate types. Overall, our study suggests that kairomone-based behavioral studies can be used to further evaluate the host specificity of T. japonicus and can be an invaluable supplement to classical biocontrol host range testing regimes.