Contrasting reproductive traits of competing parasitoids facilitate coexistence on a shared host pest in a biological control perspective

BACKGROUND: Interspecific competition in insect parasitoids is an important ecological phenomenon that has relevant implications for biological pest control. To date, interspecific intrinsic (=larval) competition has been intensively studied, while investigations on extrinsic (=adult) competition have often lagged behind. In this study we examined the role played by parasitoid reproductive traits and host clutch size on the outcome of extrinsic competition between Trissolcus basalis (Wollaston) and Ooencyrtus telenomicida (Vassiliev), two egg parasitoids of the pest Nezara viridula (L). Laboratory experiments were conducted by allowing both parasitoid species to exploit an egg mass made of 10, 20, 30, or 40 hosts through single or simultaneous releases. Furthermore, under field conditions, egg masses consisting of 10 or 40 hosts were exposed in a tomato crop in order to validate laboratory investigation. RESULTS: The results show that the egg mass size is an important predictor of extrinsic competition in our study system as a higher proportion of T. basalis emerged from large egg masses, while O. telenomicida dominated in small egg masses. Analysis of reproductive traits of parasitoid species indicates that T. basalis has superior abilities in host exploitation compared with O. telenomicida. CONCLUSIONS: We found that contrasting reproductive traits of two competing egg parasitoid species facilitate coexistence on a shared stink bug host. This work also highlights the importance to consider extrinsic competitive interactions between parasitoid species in a biological control perspective. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry

Trichoderma harzianum Strain T22 Modulates Direct Defense of Tomato Plants in Response to Nezara viridula Feeding Activity

Plant growth-promoting fungi belonging to genus Trichoderma are known to help plants when dealing with biotic stressors by enhancing plant defenses. While beneficial effects of Trichoderma spp. against plant pathogens have long been documented, fewer studies have investigated their effect on insect pests. Here, we studied the impact of Trichoderma root colonization on the plant defense responses against stink bug feeding attack. For this purpose, a model system consisting of tomato plant, Solanum lycopersicum cv Dwarf San Marzano, Trichoderma harzianum strain T22 and the southern green stink bug, Nezara viridula, was used. We firstly determined stink bug performance in terms of relative growth rate and survival on tomato plants inoculated by T. harzianum T22. Then, we evaluated relative expression of plant defense-related genes on inoculated plants induced by stink bug feeding. We found evidence that T. harzianum T22 affects tomato defense responses against N. viridula nymphs leading to reduction of growth rate. Our results also showed that T. harzianum T22 enhances plant direct defenses by an early increase of transcript levels of jasmonic acid marker genes. Yet this effect was time-dependent and only detected 8 h after herbivore induction. Taken together, our findings provide better understanding on the mechanisms underlying tomato induced resistance against herbivorous stink bugs

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

Mediterranean ants can increase nymph mortality in the stink bug Nezara viridula without interfering with its egg parasitoid Trissolcus basalis

Ants (Hymenoptera: Formicidae) play a relevant ecological role across terrestrial ecosystems. Recent studies suggest that the presence of ants in crops could lead to a decrease in the populations of insect pests, but how these actions can vary along the different trophic levels is not well known. The southern green stink bug, Nezara viridula (L.) (Hemiptera: Pentatomidae), is a cosmopolitan agricultural pest which is regularly found on horticultural agroecosystems closely associated with its main egg parasitoid, Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae). We conducted laboratory experiments to test whether two Mediterranean ant species, the generalist predators Crematogaster scutellaris (Olivier) and Tapinoma magnum Mayr, attack N. viridula eggs or nymphs, and whether they interfere with the parasitization activity of T. basalis. The experiment showed that both ant species significantly increased the mortality of N. viridula nymphs, whereas they do not attack their eggs and do not interfere with the egg parasitoids. Our results suggest that ants and egg parasitoids may have an integrable role in biocontrol strategies against this pest

Insect oviposition in herbaceous plants attracts egg parasitoids despite fungal phytopathogen infection

Egg parasitoids are important natural enemies of several insect pests. The ability to kill the pest before it can inflict damage to the plant makes egg parasitoids ideal candidates for biological control. Several studies have shown that egg parasitoids exploit oviposition-induced plant volatiles (OIPVs) to locate host eggs laid on plant organs. Yet such studies have often overlooked that, in nature, plants frequently suffer concurrent attack by insect herbivores and phytopathogens. These dual attacks can modify the emission of induced plant volatiles, which may potentially interfere with the host location abilities of egg parasitoids. We investigated this research question using the following study organisms: the broad bean Vicia faba, the plant pathogen Stemphylium sp., the southern green stink bug Nezara viridula and its associated egg parasitoid Trissolcus basalis. We showed that T. basalis is able to exploit OPIVs in order to locate N. viridula egg masses even when V. faba plants were previously infected by Stemphylium sp. Chemical analyses indicate that the egg parasitoid ability to exploit OIPVs persists despite significant alterations of the volatile blends emitted by plants suffering multiple biotic stresses. This study highlights the importance of incorporating the complexity of multiple biotic stresses when studying parasitoid foraging behavior, in order to comprehend how to enhance the effectiveness of natural enemies in crop protection