Chemical ecology of egg parasitoids associated with true bugs

Parasitoids representing some 15 families of Hymenoptera develop in insect eggs; three of these families, Platygastridae (= Scelionidae), Mymaridae, and Encyrtidae, are associated with Heteroptera. Several species of heteropteran egg parasitoids are or may be important for biological pest control. Successful parasitism of insect herbivores by insect parasitoids arises through several phases of host searching, which lead female wasps to the vicinity of, or in contact with, their hosts. During the host location process, females encounter and explore a variety of stimuli, among which chemical cues (i.e., semiochemicals or infochemicals) play a pivotal role. Female parasitoids are under selection pressure to efficiently invest their limited time on the location and exploitation of host-derived stimuli. In general, the levels of reliability and detectability of a particular stimulus are inversely correlated. Female parasitic wasps adopt differing strategies to solve this dilemma. In this paper we focus on the various host selection strategies employed by heteropteran egg parasitoids and possible means whereby the chemically mediated behavior of these wasps may be exploited to enhance biological pest control

Chemical ecology of insect parasitoids: towards a new era

Over the course of evolutionary time, insect parasitoids have developed diverse strategies for using chemical compounds to communicate with various protagonists within their environment (i.e. conspecifi cs, their hosts, and the plants on which their hosts are living). Unravelling the evolutionary meaning of such chemical communication networks not only provides new insights into the ecology of these insects but also contributes to improving the use of parasitoids for the control of insect pests in biological control programmes. A book covering our current knowledge of the chemical ecology of insect parasitoids is therefore particularly timely and will appeal to a large number of potential readers worldwide, from university students to senior scientists. Internationally recognized specialists were invited to contribute chapters to this book, examining the main topics and exploring the most interesting issues in the fi eld of chemical ecology of insect parasitoids. The chapters are organized so as to present the most signifi cant knowledge and discoveries made over recent decades, and their potential uses in pest control

Egg parasitoid exploitation of plant volatiles induced by single or concurrent attack of a zoophytophagous predator and an invasive phytophagous pest

Zoophytophagous insect predators can induce physiological responses in plants by activating defence signalling pathways, but whether plants can respond to facultative phytophagy by recruiting natural enemies remains to be investigated. In Y-tube olfactometer bioassays, using a system including a Vicia faba plant, the zoophytophagous predator Podisus maculiventris and the egg parasitoid Telenomus podisi, we first demonstrated that T. podisi females are attracted by broad bean plants damaged by feeding activity of P. maculiventris and on which host egg masses had been laid, while they are not attracted by undamaged plants or plants damaged by feeding activity alone. In a second experiment, we evaluated the impact of the invasive phytophagous pest Halyomorpha halys on this plant volatile-mediated tritrophic communication. Results showed that the invasive herbivorous adults do not induce plants to recruit the native egg parasitoid, but they can disrupt the local infochemical network. In fact, T. podisi females are not attracted by volatiles emitted by plants damaged by H. halys feeding alone or combined with oviposition activity, nor are they attracted by plants concurrently infested by P. maculiventris and H. halys, indicating the specificity in the parasitoid response and the ability of the invasive herbivore in interrupting the semiochemical communication between plants and native egg parasitoids. To the best of our knowledge, this is the first study showing that zoophytophagous predator attacks induce indirect plant defences similarly to those defence strategies adopted by plants as a consequence of single or concurrent infestations of herbivorous insects

Chemical Ecology of Floral Resources in Conservation Biological Control

Conservation biological control aims to enhance populations of natural enemies of insect pests in crop habitats, typically by intentional provision of flowering plants as food resources. Ideally, these flowering plants should be inherently attractive to natural enemies to ensure that they are frequently visited. We review the chemical ecology of floral resources in a conservation biological control context, with a focus on insect parasitoids. We highlight the role of floral volatiles as semiochemicals that attract parasitoids to the food resources. The discovery that nectar-inhabiting microbes can be hidden players in mediating parasitoid responses to flowering plants has highlighted the complexity of the interactions between plants and parasitoids. Furthermore, because food webs in agroecosystems do not generally stop at the third trophic level, we also consider responses of hyperparasitoids to floral resources. We thus provide an overview of floral compounds as semiochemicals from a multitrophic perspective, and we focus on the remaining questions that need to be addressed to move the field forward

Female-released sex pheromones mediating courtship behavior in Lysiphlebus testaceipes males.

Ethological aspects and chemical communication at close-range between the sexes of Lysiphlebus testaceipes Cresson (Hymenoptera: Braconidae) have been investigated through behavioral bioassays and chemical analysis. The attractiveness toward males of whole-body extracts of females and males in hexane and acetone was evaluated, adopting male fanning behavior as a key behavioral component. Also, the activity of polar and nonpolar fraction of female-body extract in hexane obtained using solid-phase extraction technique was investigated. In order to identify cuticular compounds, male and female whole-body extracts with hexane and acetone were analyzed by gas chromatography-mass spectrometry. The results showed that males exhibit a behavior including 4 phases when exposed to virgin females: premount, mount, copulation, and post-copulation. A preliminary courtship of the male included wing fanning, an extension and vibration of the wings for 1 to 2 seconds. Also, some original aspects not described for other species were carried out. The average duration of the entire sequence of events was 138.80 ± 19.51 sec. Also, males displayed significantly more wing fanning behavior in response to female whole-body hexane extracts (70.83%) than female whole-body acetone extracts (33.3%). Furthermore, males did not respond to male-body extracts or to the control (pure hexane and acetone), suggesting that the sex pheromone is composed of cuticular hydrocarbons that are also involved in the male courtship behavior. When hexane extracts of whole females were fractionated on silica gel and exposed to males, more activity was recorded for the nonpolar fraction (50.0%) than the polar fraction (27.7%), but no significant statistical difference was found. Significant differences were detected comparing the control (not fractionated extract) with the polar fraction, but not with the nonpolar fraction. A homologous series of n-alkanes with chain lengths from C19 to C30 carbon atoms was identified and quantified in the solvent extracts of wasp males and females. Between male and female extracts, there was a statistically significant difference in the average quantity of some of these hydrocarbons, such as C27, C28, and C29

Prospects of herbivore egg-killing plant defenses for sustainable crop protection

Due to a growing demand of food production worldwide, new strategies are suggested to allow for sustainable production of food with minimal effects on natural resources. A promising alternative to the application of chemical pesticides is the implementation of crops resistant to insect pests. Plants produce compounds that are harmful to a wide range of attackers, including insect pests; thus, exploitation of their natural defense system can be the key for the development of pest-resistant crops. Interestingly, some plants possess a unique first line of defense that eliminates the enemy before it becomes destructive: egg-killing. Insect eggs can trigger (1) direct defenses, mostly including plant cell tissue growth or cell death that lead to eggs desiccating, being crushed or falling off the plant or (2) indirect defenses, plant chemical cues recruiting natural enemies that kill the egg or hatching larvae (parasitoids). The consequences of plant responses to eggs are that insect larvae do not hatch or that they are impeded in development, and damage to the plant is reduced. Here, we provide an overview on the ubiquity and evolutionary history of egg-killing traits within the plant kingdom including crops. Up to now, little is known on the mechanisms and on the genetic basis of egg-killing traits. Making use of egg-killing defense traits in crops is a promising new way to sustainably reduce losses of crop yield. We provide suggestions for new breeding strategies to grow egg-killing crops and improve biological control

First report of Melittobia australica Girault in Europe and new record of M. acasta (Walker) for Italy

Melittobia acasta and M. australica are newly recorded from Sicily, Italy, and the second species is reported in Europe for the first time. A short historical background about Melittobia parasitoid wasps, their hosts, and distribution, with emphasis in those two species is presented together with illustrations to facilitate their identification. Brief discussion about the presence and possible distribution of the species in Sicily is also included

Factors influencing brood sex ratio in the egg parasitoid Trissolcus basalis (Woll.) (Hymenoptera: Scelionidae)

This study examines the sex ratio response of the egg parasitoid Trissolcus basalis (Woll.) to both the presence of other conspecific females and to their traces when ovipositing on Nezara viridula (L.) egg masses. In both conditions T basalis females lay a higher sex ratio (proportion of males) qualitatively agreeing with the local mate competition (LMC) theory. The ability of T basalis to shift its sex ratio could be achieved by decreasing the number of laid eggs per wasp, which would increase the proportion of male eggs due to the sequence effect, i.e. "male-first strategy", and by direct oviposition of a higher proportion of male eggs. In ali likelihood the extemal marking pheromone left by previous females provides the stimulus for sex ratio adjustment to the following females. Key words: arrhenotoky, local mate competition, Nezara viridula, egg mass.I FATTORI CHE INFLUENZANO LA SEX RATIO DELLA PROGENIE DEL PARASSITOIDE OOFAGO TRISSOLCUS BASALIS (WOLL.) (HYMENOPTERA: SCELIONIDAE) In questo studio sono state esaminate le variazione nella sex ratio del parassitoide oofago Trissolcus basalis (Woll.) mentre ovidepone su una ovatura di Nezara viridula (L.) in risposta sia alla presenza di femmine appartenenti alla stessa specie che alle loro sostanze marcanti. In entrambi i casi le femmine del T. basalis hanno prodotto una sex ratio maggiore (espressa come proporzione dei maschi) in accordo con quanto previsto dalla teoria della competizione localizzata per l'accoppiamento, local mate competition (LMC). La capacità delle femmine del parassitoide di modificare la loro sex ratio è dovuta sia alla diminuzione del numero di uova parassitizzate per ciascuna femmina, con il conseguente aumento del numero di uova maschili (effetto sequenza); che ad un diretto aumento del numero di uova che daranno maschi. Con tutta probabilità il feromone di marcatura esterno deposto dalla prima femmina ovideponente fornisce lo stimolo alle femmine successive per modificare la loro sex ratio. Parole chiave: riproduzione arrenotoca; competizione localizzata per l'accoppiamento, Nezara viridula, ovatura.

Effect of host kairomones and oviposition experience on the arrestment behavior of an egg parasitoid

Chemical residues left by walking adults of the southern green stink bug, Nezara viridula (L.) (Heteroptera: Pentatomidae) induce arrestment behavior in the egg parasitoid Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) leading to prolonged periods of walking on contaminated areas and to systematic return to the stimulus after encountering the treatment borders. In this study, we quantified how the hierarchical value of residues from host adults and oviposition experience can influence the arrestment behavior of T. basalis females. Our results showed that: (1) female wasps perceived host residues at different hierarchical levels depending on the host gender, with a clear preference for the chemical residues deposited by host females rather then host males; (2) wasps' arrestment response to chemical residues of host females became weaker when wasps were not rewarded by an oviposition experience, and stronger following successful oviposition; (3) repeated encounters with host male chemical residues, followed or not by oviposition experience, did not cause wasps to change their innate arrestment response; (4) in the unrewarded condition, arrestment responses of wasps varied according to the time elapsed between successive visits to areas contaminated by host females: responses were weak with a short interval (less than 24 h) and stronger with a long interval (more than 72 h), suggesting that this unrewarded experience, i.e. encounter with female traces not followed by host egg location, fade within a few hours. The potential significance of these results to the host location behavior of T. basalis in the field is discussed

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