Implementing Mass Rearing of Trissolcus japonicus (Hymenoptera: Scelionidae) on Cold-Stored Host Eggs

Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), a pest of Asian origin, has been causing severe damage to Italian agriculture. The application of classical biological control by the release of Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae), an exotic egg parasitoid, appears to be one promising solution. In Italy, releases of T. japonicus in the field were authorized in 2020. In this study, some parameters that could influence the rearing of T. japonicus in insectaries were investigated. A significantly higher production of progeny was observed on host eggs stored at 6 °C (86.5%) compared to −24 °C (48.8%) for up to two months prior to exposure to parasitism. There were no significant differences in progeny production from single females in a vial provided with only one egg mass (83.2%) or 10 females inside a cage with 6 egg masses (83.9%). The exposure of parasitoids to refrigerated (6 °C) egg masses of H. halys for 72 h led to a significantly higher production of progeny (62.1%) compared to shorter exposures for 48 (44.0%) or 24 h (37.1%). A decline in production of progeny by the same female was detected between the first (62.1%) and the second parasitization (41.3%). Adult parasitoids stored at 16 °C for up to 90 days had an 87.1% survival rate, but a significant decrease in progeny production was detected. These parameters could be adjusted when rearing T. japonicus for specific aims such as the production of individuals for field release or colony maintenance.

Behavioral Response of Halyomorpha halys (Hemiptera: Pentatomidae) and Its Egg Parasitoid Trissolcus japonicus (Hymenoptera: Scelionidae) to Host Plant Odors

Insects use a range of cues to help them interact with each other and their host plants. Among these cues, olfaction plays a major role in host selection. The present study investigated the behavioral response of the brown marmorated stink bug, Halyomorpha halys (Stål), and its egg parasitoid, Trissolcus japonicus (Ashmead), to host plant-related odors. We used H. halys nymphs since their response to host odors is relatively unknown. In a Y-tube, we first evaluated the behavioral response of H. halys nymphs to whole-fruit odors of apple [Malus domestica (Borkh.)] and peach [Prunus persica (L.) Batsch)]. Subsequently, we tested the behavioral response of H. halys and T. japonicus to 18 selected synthetic volatiles previously identified from H. halys and its common host plants. In the greenhouse, we further tested H. halys attraction to the most promising of these volatiles individually and as blends. In single-choice tests, H. halys nymphs preferred odors from apple and peach over the control (no odor). In dual-choice tests, H. halys did not show any preference between apple and peach odors. Among the 18 volatiles tested, H. halys nymphs were attracted to ethyl salicylate (ES), undecane (UN), and ethyl acetate (EA) compared to the control. In the greenhouse, H. halys nymphs were similarly attracted to blends of 1:1 ratio of ES and EA but not to single compounds. Also in the Y-tube, female T. japonicus preferred the arm that had ES, β-caryophyllene, and decanal and a blend of these three compounds at a 1:1:1 ratio. Trissolcus japonicus was more attracted to the control arm than to the arm containing tridecane or α-pinene. These results indicate the potential of developing H. halys and T. japonicus attractants or/and repellents based on host plant volatiles and suggest possible adaptive responses of this pest and its egg parasitoid to similar host plant odors.

Lethal and Sublethal Effects of Conventional and Organic Insecticides on the Parasitoid Trissolcus japonicus, a Biological Control Agent for Halyomorpha halys

The egg parasitoid Trissolcus japonicus is a natural enemy of Halyomorpha halys, a polyphagous invasive pest in Europe and North and South America. Integration of chemical and biological control tactics could facilitate effective and sustainable integrated pest management programs. This study was conducted to assess (i) the lethal effects of field rates, (ii) the sublethal effects of maximum and half field rates, and (iii) the lethal effects of different routes of exposure of three organic and two conventional insecticides against T. japonicus. Maximum field rates of spinosad and sulfoxaflor resulted in acute lethal toxicity to adult T. japonicus 1 week after residual contact exposure. Maximum and half field rates of pyrethrins, the mixture of azadirachtin and pyrethrins, and clothianidin caused sublethal effects to female wasps through residual contact exposure. Furthermore, all insecticides caused acute lethal effects 1 week after ingestion by unmated female wasps. Taken together, these results suggest that careful planning is necessary to ensure compatibility between biological and chemical control for H. halys. The insecticides evaluated in this study varied in toxicity to T. japonicus and should be used with caution to conserve this natural enemy for biological control of H. halys.

Molecular Identification of Trissolcus japonicus, Parasitoid of the Brown Marmorated Stink Bug, by Species-Specific PCR

The samurai wasp, Trissolcus japonicus (Ashmead), has been proposed as a biocontrol agent against brown marmorated stink bugs (BMSB), due to its ability to parasitize and kill BMSB eggs. However, the wasps’ small size makes it challenging for those untrained in morphological identification to determine the wasps’ species. To circumvent this problem, a molecular method was created to identify T. japonicus. The method uses species-specific primers, designed in this study, which target the variable region of the mitochondrial Cytochrome Oxidase 1 (CO1) locus. After confirming successful DNA extraction from samples, the PCR amplification using our primers produced 227-bp PCR products for all T. japonicus specimens and no amplification in other microhymenoptera candidates. Additionally, DNA from BMSB-parasitized eggs gave positive PCR amplification, while the control BMSB samples showed no amplification. This indicates that PCR with our primers specifically and sensitively differentiates T. japonicus specimens from other similar wasp species and discriminates between T. japonicus-parasitized and non-parasitized BMSB eggs. Finally, an in silico analysis of CO1 sequences demonstrated that our primers match the sequences of four different haplotypes of T. japonicus, indicating that our diagnostic method could potentially be applied to analyze T. japonicus populations throughout North America, Europe, and parts of Asia.

Only Females Oviposit: Chemical Discrimination of Adult Stink Bug Sex by the Egg Parasitoid Trissolcus japonicus

Egg parasitoids foraging for suitable hosts scattered in the environment rely mainly on chemical cues. Elucidating the chemical ecology of natural enemies is important in the development of effective and successful strategies for conservation biological control. In this context, the host cuticular hydrocarbons, which are exploited by several species of egg parasitoids as contact kairomones, could be used to retain them by providing information about the presence and the sex of adults of the target species: sex is important because only females of the host species lay the eggs that can be subsequently utilized for parasitoid reproduction. However, the chemical basis of host sex discrimination in egg parasitoids is not well understood. We carried out behavioral and chemical bioassays to investigate the role played by contact chemical cues left by adults of the brown marmorated stink bug, Halyomorpha halys Stål, in host egg searching behavior and adult host sex discrimination by the egg parasitoid Trissolcus japonicus (Ashmead). A first set of bioassays showed that parasitoids spent more time exploring patches contaminated with chemicals associated with adult H. halys females compared with adult males. Similar responses were displayed by T. japonicus when hexane extracts of H. halys were tested suggesting that non-polar chemical compounds are involved in host sex discrimination. GC-MS analysis of hexane extracts revealed quantitative differences in the cuticular compounds of the two sexes, with 1-hexadecene (more abundant in males) being the most important component in determining these differences. Hexane extracts of H. halys females blended with synthetic 1-hexadecene significantly reduced the wasps’ arrestment responses compared to crude extracts.

Survey for Adventive Populations of the Samurai Wasp, Trissolcus japonicus (Hymenoptera: Scelionidae) in Pennsylvania at Commercial Fruit Orchards and the Surrounding Forest

The samurai wasp, Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae), is an egg parasitoid associated with the brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Trissolcus japonicus is a candidate for classical biological control of H. halys populations. Since 2014, adventive populations of T. japonicus have been detected in 14 US states, in the Canadian provinces of British Columbia and Ontario, and in two European countries, Switzerland and Italy. Establishing baseline information about populations of T. japonicus is important, as this species is not host specific to H. halys and the potential ecological effects of the accidental introductions are not fully known. In this study, yellow sticky cards were deployed at commercial fruit orchards in nine counties in Pennsylvania separated by more than 400 km. Trissolcus japonicus was detected on cards in eight counties, and in two habitats, in the orchard and at the forest border. Other native species of Scelionidae known to attack the eggs of H. halys were also identified, including Trissolcus euschisti (Ashmead), Trissolcus brochymenae (Ashmead), and Telenomus podisi Ashmead (Hymenoptera: Scelionidae). These results are important baseline ecological knowledge for both T. japonicus, which appears to be established in orchards throughout Pennsylvania, and other native Scelionidae.