Semiochemicals Used by Insect Parasitoids and Hyperparasitoids in Complex Chemical Environments and Their Application in Insect Pest Management

Abstract

Insect parasitoids are key biological agents within terrestrial ecosystems, offering a promising avenue for controlling insect pests. Hyperparasitoids are a group of insects that lay their eggs in or on the body of parasitoid hosts, which can greatly hamper the effectiveness of parasitoids. To optimize their reproductive success, adult parasitoids/hyperparasitoids must find sufficient food sources and mate partners (when they do not reproduce parthenogenetically) and locate suitable hosts for their offspring. To complete these tasks, parasitoids largely rely on their ability to detect relevant chemical cues (semiochemicals or infochemicals). In the last three decades, the identities of semiochemicals and their ethological significance have been widely characterized, and the possibility of using these chemical cues in insect pest management has received a lot of attention. Insects have evolved a highly sensitive and sophisticated chemosensory system adept at navigating complex and dynamic chemical environments. In this review, we first summarize the semiochemicals used by insect parasitoids, primarily including semiochemicals involved in food location, host foraging, and mate finding, while also addressing semiochemicals employed by hyperparasitoids. Next, we discuss recent progress in elucidating the chemosensory mechanisms underlying parasitoid responses to semiochemicals, with a focus on olfactory and gustatory pathways. Finally, we evaluate the potential applications of semiochemicals in pest management, highlighting the roles of parasitoids and hyperparasitoids. This paper aims to establish a theoretical framework for the effective employment of parasitoids in biological control of insect pests.

1. Introduction

The control of insect pests relies predominantly on the application of chemical pesticides. However, prolonged use of chemical pesticides has resulted in progressive increases in pest resistance, posing a serious threat to food safety and ecological security [1]. Therefore, there is an urgent need to develop efficient and environmentally sustainable pest management strategies. Using insects’ natural enemies, including parasitoids and predators, is a key strategy in biological pest control, offering a sustainable alternative to chemical pesticides [2]. Parasitoids are insects whose larvae develop in or on the bodies of other arthropods (mostly other insects), whereas the adults are free living. Most parasitoids are quite specialized and only attack a limited number of related host species, which minimizes the risk of harming non-target organisms of the ecosystem. This makes parasitoids key biocontrol agents within terrestrial ecosystems, offering a promising avenue for controlling agricultural and environmental pests [3]. For instance, in the Huang-Huai River Basin in China, the ichneumonid wasp Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae) parasitizes and kills up to 63.1% of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae [4]. Parasitoids have their own specialized natural enemies, i.e., hyperparasitoids. Hyperparasitoids are a group of insects in the fourth trophic level which lay their eggs in or on the body of other parasitoid hosts, which can greatly hamper the effectiveness of parasitoids [5].

Parasitoids and hyperparasitoids base foraging decisions on information from different trophic levels, and semiochemicals serve as critical cues [6,7,8,9]. In recent decades, numerous semiochemicals associated with the behavior of insect parasitoids/hyperparasitoids have been identified. For example, in the life cycle of the egg parasitoids (Hymenoptera) after emergence, they rely on semiochemicals to locate suitable mates and hosts, a process critical for successful mating and parasitism. In the egg parasitoid Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae), male-derived pheromones (4R,5R)-5-hydroxy-4-decanolide, (4R,5S)-5-hydroxy-4-decanolide, and 4-methylquinazoline induce mating behaviors of females [10,11,12]. Maize volatiles, such as (3E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT), and (E)-(1R,9S)-caryophyllene, induced by oviposition of the stemborer Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), could effectively attract the egg parasitoid Trichogramma bournieri Pintureau & Babault (Hymenoptera: Trichogrammatidae) [13]. Furthermore, some studies have verified that the use of semiochemicals can modify the behavior of parasitoids/hyperparasitoids, opening new perspectives in using semiochemicals to improve/curb the efficiency of parasitoids/hyperparasitoids [5,14,15,16,17,18]. Insects rely on chemosensory perception to locate food, hosts, and mate partners [18,19]. The understanding of semiochemicals used by parasitoids/hyperparasitoids in complex chemical environments and the chemosensory mechanism underlying semiochemical perception could facilitate the exploitation of parasitoids as bio-control agents. In this review, we systematically explore the semiochemicals used by parasitoid/hyperparasitoids wasps for food location, host foraging, and mate finding. We also summarize the chemosensory genes implicated in their detection and discuss the practical applications of these semiochemicals in insect pest management.

2. Semiochemicals for Food Location and Mate Finding

Although a minority of parasitoid species feed on their larval host to obtain protein, most adult parasitoids seek food sources that are enriched with carbohydrates, such as floral nectar and honeydews [20]. Plant volatiles commonly occurring in floral blends are the main triggers of food foraging behaviors in insect parasitoids [21]. One of such commonly occurring compounds is linalool [22]. This compound is an abundantly released component of floral volatiles. In the evergreen shrub Daphne mezereum L. (Myrtales: Thymelaeaceae), linalool accounted for 95% of total floral fragrance [23]. Linalool is known to be attractive to some parasitoid species [23,24]. For example, behavioral assays demonstrated that linalool is a semiochemical for food foraging of the ichneumonid wasp C. chlorideae [25].

Mate-finding strategies in parasitoids are mainly based on the use of intraspecific sex pheromones [26,27,28]. Parasitoids reproduce in the following ways: the newly emerged females reproduce parthenogenetically, take risks of producing haploid male offspring, or they reproduce sexually, yielding both male and female offspring [29,30,31]. The latter strategy is more commonly executed. For example, virgin females of the parasitoid Cotesia vestalis (Halliday) (Hymenoptera: Braconidae) are not attracted by herbivore-induced plant volatiles (HIPVs) until they mate [32]. Virgin males of the aphid parasitoid Lysiphlebus testaceipes (Cresson) (Hymenoptera: Aphidiinae) are only attracted by conspecific females but not by aphid-induced plant volatiles [33].

Currently, however, sex pheromone communication in parasitoids is less studied. Females of Ascogaster quadridentata Wesmael (Hymenoptera: Braconidae), a braconid parasitoid of codling moth Cydia pomonella L. (Lepidoptera: Tortricidae), release (Z,Z)-9,12-octadecadienal to attract males [34]. As we mentioned before, in N. vitripennis, male-derived pheromones (4R,5R)-5-hydroxy-4-decanolide, (4R,5S)-5-hydroxy-4-decanolide, and 4-methylquinazoline induce mating behaviors of females [10,11,12,31]. (E)-bergamotene serves as a male-produced sex pheromone in Melittobia digitata Dahams (Hymenoptera: Eulophidae) [35]. In the females of Dibrachys cavus (Walker) (Hymenoptera: Pteromalidae), the cuticular hydrocarbon 3-methylalkane is suggested as a key component of the contact sex pheromones [36]. A recent work by Guo et al. found that virgin males of C. chlorideae were attracted to the female-derived sex pheromones tetradecanal and 2-heptadecanone [37].

3. Semiochemicals for Host Foraging

3.1. HIPVS

Host foraging is crucial for the reproductive success of parasitoids. They locate hosts through a progressive process where HIPVs act as instrumental information bases [9,24,38,39,40]. The composition of HIPVs has been elucidated in numerous plant species by gas chromatography–mass spectrometry (GC-MS). Compared to undamaged plants, those infested by herbivore insects released an increased quantity and variety of volatile compounds [41,42,43,44,45]. According to the published data, HIPVs mainly comprise terpenoids, green leaf volatiles (GLVs), aromatic compounds, and amino acid derivatives.

Terpenoids, produced from isoprene pyrophosphate, are regarded as the most abundant class of compounds emitted by herbivore-infested plants. Major terpenoid volatiles emitted from plants include monoterpenoids such as geraniol, (E)-β-ocimene, and citral and sesquiterpenoids such as farnesol and (E)-β-caryophyllene. Additionally, two terpenoids with irregular structures, i.e., DMNT and TMTT, frequently occur in HIPVs [46,47,48,49,50,51]. GLVs, encompassing a range of 6-carbon aldehydes, alcohols, and esters, are generated through the oxidative degradation of leaf lipids and typically exhibit a grassy scent. Several GLVs, such as (Z)-2-hexenal, (Z)-3-hexenol, (E)-2-hexenyl acetate, and (Z)-3-hexenyl acetate, are commonly detected in the HIPVs emitted by various plant species such as maize, soybean, and wheat [41,52,53,54]. Aromatic compounds, derived from phenylalanine metabolism, consist of aromatic rings and different side structures. Additionally, indole derivatives, such as methyl salicylate (MeSA) and indole, arise as byproducts in biosynthesis of the amino acid tryptophan. Amino acid derivatives, synthesized from amino acids, include a diverse assay of amines, oximes, and nitriles. These compounds are often less captured in HIPVs compared to the other three classes of compounds [52,55,56].

Numerous studies indicated that parasitoids use two different modes to detect HIPVs: the “species-specific odor” recognition mode and the “compound ratio” recognition mode [56,57,58,59]. In the “species-specific odor” mode, parasitoids recognize compounds unique to a single species or a closely related group. This is exemplified by herbivorous insects specialized on Brassicaceae, such as the braconid wasp Diaeretiella rapae (M’Intosh) (Hymenoptera: Braconidae), a specialist parasitoid of aphids on Brassicaceae plants (Brassicales). Studies have demonstrated that D. rapae exhibits chemotaxis responses to various isothiocyanate compounds [59,60]. Conversely, the “compound ratio” recognition mode, which predominates among parasitoids, involves the detection of widely occurring volatile compounds, with discrimination based on the specific compound ratios. Closterocerus ruforum (Krausse) (Hymenoptera: Eulophidae), a parasitoid of the common pine sawfly Diprion pini L. (Hymenoptera: Diprionidae), displays chemotaxis toward pine leaves oviposited by D. pini, which emit elevated levels of (E)-β-farnesene compared to uninfested controls. Further behavioral experiments revealed that single (E)-β-farnesene alone is insufficient to attract C. ruforum. Attraction occurs only when it is combined with five other terpenoids, i.e., β-phellandrene, (E)-β-ocimene, (Z)-β-ocimene, (E)-β-caryophyllene, and α-humulene, in ratios matching those emitted by oviposited pine leaves [61]. Similarly, the braconid C. vestalis is attracted to a blend of heptanal, α-pinene, sabinene, and (Z)-3-hexenyl acetate in proportions mirroring those released by Plutella xylostella L. (Lepidoptera: Plutellidae)-infested cabbages [62].

3.2. Host-Associated Chemical Cues

Stimuli used in habitat locations (i.e., long-range) are generally plant volatiles or herbivore pheromones, while stimuli used in host locations (i.e., short-range) are nonvolatile herbivore products [2,3,63]. Parasitoids primarily use HIPVs in long-range foraging [64]. Upon arrival at the herbivore-infested plant, they employed nonvolatile chemical cues from the herbivore hosts [6].

Stimuli derived from the herbivore host are generally the most reliable sources of information. Unfortunately, as herbivore insects are small elements in complex environments, concentrations of the produced chemicals are always tiny. Herbivores are often conspicuous to their parasitoids that exploit kairomones in host locations. For example, parasitoids of the bark beetle I. pini were attracted to the aggregation pheromone 2-methyl-6-methylene-2,7-octadien-4-ol of host insects [65]. Sex pheromones from certain moth species similarly attract parasitoids [66]. For instance, sex pheromones of the stem borer Sesamia nonagrioides (Lefèbvre) (Lepidoptera: Noctuidae) attract the egg parasitoid Telenomus busseolae Gahan (Hymenoptera: Scelionidae) [67]. Egg parasitoids of the genus Trichogramma (Hymenoptera) use sex pheromones of the adult hosts, such as sex pheromones of Manduca sexta L. (Lepidoptera: Sphingidae) [68].

Parasitoids can also use semiochemicals directing the stage of the herbivore. For example, Opius lectus Gahan (Hymenoptera: Braconidae), an egg-larval parasitoid of the apple maggot flies Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), identifies the adult fly’s oviposition-deterring pheromone emitted from the fruit, while O. alloeus Mues, a parasitoid of late larval stages of R. pomonella, was not influenced by the pheromone [69]. Host larvae feces are also attractive to the parasitoids. Wind-tunnel assays showed that feces from the second-instar larvae of P. rapae L. strongly attract the parasitoid Cotesia rubecula (Marshall) (Hymenoptera: Braconidae) [70].

In addition to volatile chemicals, contact chemicals released by herbivore insects can also guide parasitoids to their hosts. Behavioral studies showed that the egg parasitoid T. basalis uses n-nonadecane, a cuticular hydrocarbon from the southern green stink bug N. viridula, to discriminate between female and male hosts [71,72]. Trissolcus brochymenae (Ashmead) (Hymenoptera: Scelionidae), an egg parasitoid of the brassicaceae pest Murgantia histrionica (Hahn), displayed an intense searching behavior in open arenas treated with contact semiochemicals from the host insects [73]. Polypeptides isolated from the host cocoon of Acrolepiopsis assectella (Zeller) (Lepidoptera: Acrolepiidae) was involved in host recognition behavior by the pupal parasitoid Diadromus pulchellus (Wsm) (Hymenoptera: Ichneumonidae) [74]. Probing–oviposition behavior of the parasitoid Eriborus terebrans Gravenhorst (Hymenoptera: Ichneumonidae) were readily stimulated in contact bioassays of the feces, oral secretion, and cuticles of Ostrinia nubilalis (Guenée) caterpillar [75], and sugars and amino acids in these compounds have been identified as semiochemicals in host location by the parasitoids [76,77]. Several other studies have found that volatiles originating from larval feces and sex pheromones from adult hosts can act together as semiochemicals, favoring host location of parasitoids [78,79,80].

Allomones are a type of allelochemical that elicit a behavioral or physiological response in the receiver that is beneficial to the sender [2,3]. These chemical signals are often used by organisms as a defense mechanism. Parasitoids could also be able to detect allomones. For example, defensive compounds such as (E)-2-decenal emitted from the metathoracic gland of Nezara viridula L. (Hemiptera: Pentatomidae) adults acts as a long-range kairomone orienting the egg parasitoid Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) females to their hosts [81].

4. Semiochemicals Used by Hyperparasitoids

Hyperparasitoids are insects in the fourth trophic level that parasitize the larvae/pupae of primary parasitoids (the third trophic level) (

Table 1. Semiochemical sources and effects on hyperparasitoid species.

Semiochemical Sources	Hyperparasitoid Species *	Effects	Components	References
HIPVs
HIPVs emitted by cabbage plants damaged by parasitized caterpillars	Lysibia nana	Attraction	DMNT	[84]
HIPVs induced by parasitized caterpillars	Pteromalus semotus	Attraction	NA	[85]
Host-associated chemicals
Contact cues from the parasitized aphids	Dendrocerus carpenteri Asaphes suspensus Syrphophagus aphidivorus Alloxysta victrix	Attraction	NA	[82]
Body chemicals of the parasitized caterpillars	Baryscapus galactopus	Attraction	2,3-butanedione	[86]
Aphid honeydew (no difference between healthy and parasitized aphids)	Dendrocerus carpenteri Asaphes suspensus Syrphophagus aphidivorus Alloxysta victrix	Antennal drumming behaviors	NA	[87]
Cuticular hydrocarbons of the caterpillars	Alloxysta victrix	Attraction	NA	[88]
Intraspecific pheromones
Intraspecific females	Dendrocerus carpenteri	Mate finding	NA	[89]
Intraspecific (both males and females)	Alloxysta victrix	Mate finding	6-Methyl-5-hepten-2-one	[90]

* This table lists the findings cited in the article. NA: Components were not identified yet.

). Hyperparasitoid larvae develop at the expense of parasitoids, which can greatly hamper the efficacy of parasitoids in biological control [82]. For example, in cotton fields in northern China, the hyperparasitoids Syrphophagus taeniatus (Förster) and S. aphidivorus (Mayr) (Hymenoptera: Encyrtidae) have been observed to ferociously attack the aphid parasitoids Lysiphlebus testaceipes (Cresson) (Hymenoptera: Aphidiidae) and Lipolexis scutellaris Mackauer (Hymenoptera: Braconidae), which host the cotton aphid species Toxoptera citricida (Kirkaldy) and Aphis gossypii Glover (Homoptera: Aphididae) [83].

Over the past two decades, a few studies of the chemical cues exploited by hyperparasitoids in host locations have been documented [84,91,92,93]. Like their parasitoid hosts, hyperparasitoids use chemical information to locate their hosts (although other physical cues may be involved). HIPVs are found to influence behaviors of hyperparasitoids [85,94]. Chemical compositions of the HIPV blend change according to the parasitism status of the attacking herbivores, allowing hyperparasitoids to locate their host [84,86]. Lysibia nana (Gravenhorst) (Hymenoptera: Ichneumonidae), a specialized hyperparasitoid of the cocoons of genus Cotesia, prefers plant volatiles emitted by cabbage plants attacked by parasitized caterpillars than plant volatiles induced by unparasitized caterpillars [94]. Injection with polydnavirus and/or venom of the parasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) affects the composition of oral secretions of herbivores and thereby changes the profile of volatiles emitted by the host plant, making these volatiles reliable indicators of host presence to L. nana [95]. Pteromalus semotus (Walker) (Hymenoptera: Pteromalidae), another hyperparasitoid of Cotesia parasitoids (Hymenoptera), is attracted by plant volatiles induced by parasitized herbivores [86].

Semiochemicals involved in the foraging behavior in several aphid-associated hyperparasitoid species have been extensively studied. Some species were shown to respond to HIPVs, while others have responses to aphid-associated chemical cues [87,96,97]. A study on four species of aphid–hyperparasitoids demonstrated that contact cues of parasitized aphids are vital in parasitoid locations [88]. Aphid honeydews may also emit semiochemicals that could be exploited by aphid–hyperparasitoids. Aphid–hyperparasitoids from Encyrtidae, Alloxystidae, and Megaspilidae show antennal drumming responses to substrates smeared with aphid honeydews [87,96].

During foraging at a close range, hyperparasitoids may also use information from their parasitoid host or the host insect of the parasitoids (the second trophic level). For example, the hyperparasitoid Baryscapus galactopus (Ratzeburg) (Hymenoptera: Eulophidae) uses body chemicals of the herbivore insect to discriminate parasitized P. rapae from unparasitized ones [96,97]. Analysis of volatiles from caterpillar bodies found that 2,3-butanedione was present in higher concentrations in the headspace of parasitized caterpillars [88]. Hyperparasitoids may also respond to cuticular hydrocarbons of the herbivore host. Alloxysta victrix (Westwood) (Hymenoptera: Charipinae) females have a longer dwelling time on spots with extracts of parasitized aphids compared with those of unparasitized aphids [88].

The information available for sex communication in hyperparasitoids is limited. In the aphid hyperparasitoid Dendrocerus carpenteri (Curtis) (Hymenoptera: Ceraphronidae), males were attracted by a sex pheromone released by conspecific females [89]. According to Y-tube olfactometer experiments, 6-Methyl-5-hepten-2-one, produced by the aphid hyperparasitoid A. victrix, acts as an intraspecific sex pheromone in sex communication [90].

5. Chemosensory Mechanisms of Parasitoids

5.1. Olfactory Mechanisms

While behavioral responses have been extensively studied, the olfactory mechanisms governing semiochemical perception in parasitoids remain poorly understood. However, genome and transcriptome analyses have characterized odorant-binding protein (OBP) repertoires in at least 11 parasitoid species, advancing our understanding of their roles in olfactory stimulus detection [98,99,100,101,102,103,104,105,106]. For example, seven MmedOBPs in Microplitis mediator Haliday (Hymenoptera: Braconidae) have been functionally characterized, revealing binding affinities to diverse odorants [107]. Notably, MmedOBP4 exhibits strong affinities to aromatic compounds, while MmedOBP4 and MmedOBP6 bind terpenoids, such as α-humulene, nerolidol, and limonene [107], suggesting their involvement in HIPV perception.

Odorant receptor (OR) genes have been increasingly identified in parasitoids by genome and/or transcriptome analyses [105,106,108,109,110]. For example, the braconid species M. mediator possess 169 OR genes [110], and the jewel wasp N. vitripennis has 301 OR genes [108]. Two hundred and eleven ORs were identified in the antennal transcriptome of C. chlorideae [111]. Although a link between the number of ORs and the complexity of chemical space is not immediate, the increased number of OR genes in parasitoid species may relate to the detection of a wide range of semiochemicals [8,25,111].

In the ichneumonid wasp C. chlorideae, seven ORs show female head-biased expression, likely linked to host-seeking behaviors [111]. Among these, CchlOR62 is exclusively and highly expressed in female antennae, and when expressed in Xenopus oocytes, it responds specifically to (Z)-jasmone, an attractant for female C. chlorideae [111]. RNAi combined with behavioral assays of Anastatus japonicus Ashmead (Hymenoptera: Eupelmidae), a parasitoid of Tessaratoma papillosa Drury (Hemiptera: Tessaratomidae), demonstrated that AjapOR35 detects the oviposition attractants (E)-α-farnesene and β-caryophyllene [112]. Recently, odorant receptor co-receptor (ORco) genes of four Drosophila (Diptera)-parasitoid species were cloned. ORco-deficient male parasitoids, but not females, lost their courtship abilities. Moreover, ORco-deficient female parasitoids showed impaired host-searching performance and decreased oviposition rates [113]. To date, only one study has characterized ORs involved in sex pheromone detection in parasitoids [37]. In C. chlorideae, expression of six male antennae-biased ORs in Drosophila T1 neurons revealed that CchlOR18 and CchlOR47 were tuned to female-derived sex pheromone components, tetradecanal and 2-heptadecanone, respectively. RNAi of these ORs individually or together abolished attraction of male parasitoids to these compounds [37].

SNMPs, homologs of the human CD36 fatty acid transport proteins, are critical for chemoreception, particularly pheromone detection in Drosophila and some lepidopteran species [114]. In M. mediator, two SNMP transcripts, MmedSNMP1 and MmedSNMP2, were identified, with MmedSNMP1 showing significantly higher transcript levels in antennal flagella in both sexes, as evidenced by reverse-transcription PCR and in situ hybridization [115]. IRs, close relatives of the ancient ionotropic glutamate receptors, detect acids and amines [116]. RNAseq of M. mediator antennae identified 17 IRs, most of which are enriched in adult antennae [117]. Despite these advances, most olfactory genes in parasitoids remain uncharacterized, hindering the understanding of the olfactory mechanisms in parasitoids.

5.2. Gustatory Mechanisms

Parasitoids mainly exploit volatile semiochemicals in the initial steps of the host location whereas contact semiochemicals become progressively more important when parasitoids are in close proximity to their host insects. Most of the studies on chemosensory mechanism of parasitoids have been focused on the detection of volatile chemicals (olfaction) [7]. In contrast, relatively few studies have focused on the role of contact chemoreception (gustation) [118]. In the last two decades, a few studies have been executed in this field, especially on egg parasitoids. Egg parasitoids have developed specialized strategies for successful foraging, based on the exploitation of chemical cues which are related to the host eggs [66,119]. In Trichogrammatidae and Platygastridae, the main host-related information exploited by female wasps are kairomones associated with the herbivore host, such as scales left on the leaves by females or chemicals left behind by herbivore hosts while walking on the plants; these associated compounds may be linked to gustatory perception in parasitoids [119,120,121,122]. Methanolic extracts obtained from male and female host epidermis elicited action potentials in taste neurons housed in antennal sensilla chaetica of the egg parasitoid T. brochymenae, indicating that these sensilla are involved in the perception of non-volatile host kairomones [73,123].

Gustatory receptors (GRs) are mainly expressed in taste organs and are associated with contact chemoreception [124]. Unfortunately, our knowledge about the GRs in parasitoids is still lacking. Research in this direction is currently limited to the identification of GR genes by transcriptomic and/or genomics analyses [108,125]. The jewel wasp N. vitripennis has 47 apparently intact GR genes in their genome. Two candidate sugar receptors, a DmGR43a (detect fructose in Drosophila melanogaster (Meigen) (Diptera: Drosophilidae)) orthologue, and several additional GR lineages have been identified, including significant gene subfamily expansions related to the 10 GRs found in the honey bee Apis mellifera L. (Hymenoptera: Apidae) [108].

6. Application of Semiochemicals in Pest Management

Application of semiochemicals regulating parasitoid behaviors in pest management has been mainly focused on HIPVs. Current evidence highlights three primary strategies for leveraging HIPVs in parasitoid-based strategies: (1) deploying devices containing attractive odorants to enhance the attractiveness of host plants to parasitoids; (2) using neighboring plants to mediate tritrophic interactions; and (3) cultivating new plant varieties engineered to emit elevated levels of key volatiles that manipulate insect behaviors [2,3,7,14,126,127].

Hanging devices dispensing attractive odorants has gained widespread attention over the past three decades due to its efficacy and operational simplicity. A typical example is MeSA [126,127]. Beyond its role as a plant defense hormone, MeSA is induced by certain sucking insects [128]. Commercially available MeSA dispensers can successfully attract beneficial insects, such as lacewings and other parasitoids, to strawberry fields [128]. Similarly, (Z)-jasmone, an HIPV emitted by plants damaged by caterpillars or aphids [41,43,48,111,128,129], enhances parasitoid attraction when applied to crops like tomatoes [130] or rice [131]. Our field studies showed that its application to tobacco plants greatly increases the parasitism rate of H. armigera by the ichneumonid C. chlorideae [57,111]. Additionally, DMNT has been proven to be an attractant for a variety of parasitoids targeting caterpillars on cotton, maize, pepper, and cowpea [41,43,132,133,134,135,136]. In field cage studies, DMNT shows a strong attractive effect for M. mediator [136]. As a GLV, (Z)-3-hexenyl acetate, produced by numerous plant species following mechanical or herbivore damages [39,137], attracts Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) and Cotesia plutellae Kurdjumov (Hymenoptera: Braconidae) [79] and increases parasitism of Lygus lineolaris Goeze (Hemiptera: Miridae) by Anaphes iole Girault (Hymenoptera: Mymaridae) in cotton fields [138].

The use of companion plants exemplifies the “push–pull” strategy [15], which simultaneously attracts parasitoids in the main crop and repels herbivore pests to the trap crop (Figure 1). This approach has been proven effective in various contexts [91,139,140,141]. In sub-Saharan African, wheat plants sprayed with low levels of (Z)-jasmone repel the grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae) while attracting its parasitoids A. ervi [14,142,143]. Field plots of wheat were sprayed with (Z)-jasmone at a rate equivalent to 50 g per hectare (in four consecutive seasons in mid-May and early June), and aphid counts were conducted at weekly intervals. The researchers found that aphid infestations were significantly reduced in (Z)-jasmone-treated plots when compared to the control field [141]. Furthermore, this could also be explained by the “prime” effect. It has been reported that (Z)-jasmone could prime inducible production of defense compounds in intact plants, including wheat, maize, soybean, tomato, and pepper [144,145,146,147,148,149,150]. For example, the soybean Glycine max L. (Fabales: Fabaceae), after being treated with (Z)-jasmone, emits a blend of volatiles that attracts the stink bug egg parasitoid Telenomus podisi Ashmead (Hymenoptera: Scelionidae) [146].

Cultivating plant varieties with enhanced volatile emission to attract parasitoids has been practically fulfilled with genetically modified plants. Typical evidence came from Arabidopsis thaliana (L.) (Brassicales: Brassicaceae) engineered to increase emissions of putative attractants, demonstrating enhanced parasitoid attraction in greenhouse assays [151]. Likewise, transgenic tobacco emitting elevated levels of the aphid alarm pheromone (E)-β-farnesene repels the green peach aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) while attracting its parasitoid D. rapae in behavioral bioassays [152,153]. Moreover, researchers have reported that supplemental feeding is useful to enhance the efficiency of parasitoids. For example, the parasitoid complex of the bark beetle Dendroctonus frontalis (Coleoptera: Scolytidae) in the southern United States consists of several species that may be important in regulating parasitism rate. Supplemental feedings of these parasitoids with an artificial diet have been shown to increase their longevity and fecundity [154]. This exemplifies the “feeding stimulants” component in the “pull” element.

Several studies tested the application of host-associated volatiles to enhance the efficacy of parasitoids. Among these, host sex pheromones and aggregation pheromones have been applied either as tools for monitoring parasitoid populations or for enhancing parasitism rate in the field. For example, a blend of the synthetic sex pheromone of Aphis fabae Scopoli (Hemiptera: Aphididae), (4aS,7S,7aR)-(+)-nepetalactone and (1R,4aS,7S,7aR)-(-)-nepetalactol could effectively attract three parasitoid species, i.e., Praon abjectum Haliday, P. dorsal Haliday, and P. volucre Haliday (Hymenoptera: Braconidae), to water traps [155]. In the Mediterranean basin, the sex pheromone of citrus mealybug Planococcus citri (Risso) and vine mealybug P. ficus (Hemiptera: Pseudococcidae), (S)-(+)-lavandulyl senecioate, applied in the vineyards, citrus or fig orchards could significantly increase the mealybug parasitism rate by Anagyrus sp. (Hymenoptera) [156]. In the soybean fields of South Korea, traps baited with the aggregation pheromone from Riptortus pedestris Fabricius (Hemiptera: Alydidae) could effectively attract its egg parasitoids, Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) and Gryon japonicum Ashmead (Hymenoptera: Scelionidae) [157]. Also, recent research identified that the synthetic compound (2,4,4-trimethyl-2-cyclohexenyl) methyl butyrate could attract two parasitoids, Anagyrus sawadai Ishii (Hymenoptera: Encyrtidae) and Leptomastix dactylopii Howard (Hymenoptera: Encyrtidae), which could effectively control the mealybug Planococcus kraunhiae Kuwana (Hemiptera: Pseudococcidae) [158,159]. Relevant semiochemicals for parasitoids cited in this review were summarized in

Table 2. Parasitoid species and elicited behaviors by semiochemicals (or blends).

Semiochemical	Parasitoid *	Source	Effect	Reference
Pheromones
(4R,5R)-5-Hydroxy-4-decanolide, (4R,5S)-5-hydroxy-4-decanolide, and 4-methylquinazoline	Nasonia vitripennis	Intraspecific males	Mate finding	[10,11]
(Z,Z)-9,12-Octadecadienal	Ascogaster quadridentata	Intraspecific females	Mate finding	[34]
(E)-Bergamotene	Melittobia digitata	Intraspecific males	Mate finding	[35]
3-Methylalkane	Dibrachys cavus	Intraspecific females	Mate finding	[36]
Tetradecanal 2-heptadecanone	Campoletis chlorideae	Intraspecific female pheromone blend	Mate finding	[37]
HIPVs
(3E)-4,8-Dimethyl-1,3,7-nonatriene (DMNT)	Cotesia sesamiae	Single compound	Attraction	[13]
(E)-β-Ocimene Linalool Farnesene (Z)-Jasmone	Aphidius ervi	Single compound	Attraction	[24]
Linalool (Z)-Jasmone Farnesene (Z)-3-Hexenyl acetate	Campoletis chlorideae	Single compound	Attraction	[25,44,57,111]
3-Butenyl isothiocyanate 3-Isothiocyanatoprop-1-ene	Diaeretiella rapae	Single compound	Attraction	[30,59,60]
(Z)-3-Hexenyl acetate	Trichogramma chilonis Cotesia plutellae	Single compound	Attraction	[79,119]
(E)-α-Farnesene (+)-Aromadendrene (Z)-3-Hexenol	Anastatus japonicus	Single compound	Attraction	[112]
2-Ethyl-1-hexanol	Anastatus japonicus	Single compound	Repulsion	[112]
3,7-Dimethyl-1,3, 6-octatriene	Microplitis mediator	Single compound	Attraction	[136]
(Z)-3-Hexenal, (E)-2-hexenal, (Z)-3-hexenol, (Z)-3-hexenyl acetate, linalool, DMNT, indole, (E)-bergamotene, (E)-β-farnesene, (E)-nerolidol, and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT)	Cotesia marginiventris	Blend	Attraction	[46]
β-Phellandrene, (E)-β-ocimene, (Z)-β-ocimene, (E)-β-caryophyllene, and α-humulene	Closterocerus ruforum	Blend	Attraction	[61]
Heptanal, α-pinene, sabinene, and (Z)-3-hexenyl acetate	Cotesia vestalis	Blend	Attraction	[99]
(E)-2-Hexenyl (E)-2- hexenaoate, (E)-2-hexenyl (Z)-3-hexenoate, and tetradecyl isobutyrate	Ooencyrtus nezarae Gryon japonicum	Blend	Attraction	[157]
Host-associated chemicals
(Z)-11-Hexadecenyl acetate, (Z)-11-hexadecenol, (Z)-11-hexadecenal, and dodecyl acetate	Telenomus busseolae	Host sex pheromone blend	Attraction	[67]
Blend chemicals NA	Cotesia rubecula	Host larval feces	Attraction	[70]
n-Nonadecane	Trissolcus basalis	Host cuticles	Discrimination female hosts from male hosts	[71]
Polypeptides	Diadromus pulchellus	Host cocoon	Host searching behavior	[74]
Proteins, amino acids, triglycerides, and salts	Eriborus terebrans	Host larval feces and cuticles	Probing–oviposition behavior	[75]
Allyl isothiocyanate	Cotesia plutellae	Host larval feces	Attraction	[79]
(Z)-11-Hexadecenal, (Z)-11-hexadecenyl acetate, and (Z)-11-hexadecenol	Cotesia plutellae	Host sex pheromone blend	Attraction	[79]
Amino acids, sugars, and glucosinolates.	Trissolcus brochymenae	Host cuticles	Host searching behavior	[121,123]
(4aS,7S,7aR)-(+)-nepetalactone, (1R,4aS,7S,7aR)-(-)-nepetalactol	Praon abjectum P. dorsal P. volucre	Host sex pheromone blend	Attraction	[155]
(S)-(+)-lavandulyl senecioate	Anagyrus sp.	Host sex pheromone	Attraction	[156]

* This table lists the findings cited in the article. ^NA Components were not identified yet.

.

A push–pull strategy could also be applied for the management of hyperparasitoids. Specific semiochemicals may be applied to push hyperparasitoids away (or pull them into traps), thus keeping primary parasitoids away from the attacks of hyperparasitoids [92,93]. HIPVs are of potential interest for push–pull strategies for hyperparasitoid management acted out at longer distances. Cabbage plants emit HIPVs in reply to infestation by parasitized herbivores, which can attract hyperparasitoids [5,6,84]. Garden experiments showed that the hyperparasitoid L. nana preferred plants that were induced by C. glomerata-parasitized Pieris brassicae L. (Lepidoptera: Pieridae) caterpillars [84]. Semiochemicals that act over a relatively short range mainly refer to pheromones. Pheromones could be directly applied to the dispensers, for example, using containers with parasitized insects smeared with the pheromones. The function of 6-methyl-5-hepten-2-one as a species-specific sex pheromone in A. victrix has been demonstrated [90,160]. However, this compound induces dispersal effect in the aphid parasitoid Aphidius uzbekistanicus Luzhetzki (Hymenoptera: Braconidae) [161]. Research on semiochemical applications at the fourth trophic level is still in its early stages compared to the extensively studied primary parasitoids. Increased efforts are required to explore semiochemical application at the fourth trophic level.

7. Future Perspectives

Enormous efforts have been made in the identification of semiochemicals used by insect parasitoids and hyperparasitoids in complex chemical environments and their application in insect pest management [2,6,7,14,162,163,164,165]. However, there are still several questions needing to be tackled in the future. First, a common issue is the discrepancy between field results and laboratory test results. The main reason is that the use of semiochemicals to manipulate parasitoids’ behavior is accomplished in environments that are complex. Hence, biotic factors, e.g., the recipient organisms (including plants and other insects, etc.) and their associated physiological and/or behavioral changes should be investigated. Besides this, the consequences of using specific chemical compounds in the field at the population level should be researched, such as population dynamics in different times and spaces [162,163]. Abiotic factors such as temperature and humidity should also be carefully considered. For example, the parasitism of Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae) decreases by nearly 40% at temperatures exceeding 35 °C [166]. Second, application of our knowledge into commercial products for pest management needs to be accelerated. This involves the progress of synthetic biology in the production of semiochemicals. By engineering plant or yeast metabolic pathways, synthetic biology enables these organisms for producing specific semiochemicals [167]. Compared to industrial synthetic chemicals, this method could realize the purpose of consistent production and release of semiochemicals in the field. Also, it can overcome limitations of other natural induction approaches, such as herbivore damage synthetic compounds. Third, the chemosensory mechanisms underlying semiochemical perception in parasitoids/hyperparasitoids remain largely unknown. Chemosensory proteins, especially ORs and GRs tuned to behavior-relevant semiochemicals in parasitoids/hyperparasitoids, await functional characterization. Furthermore, sex communication underlying sex pheromone identification and corresponding chemosensory mechanisms in parasitoid/hyperparasitoids needs to be unveiled. Systematic functional characterization of chemosensory receptors is essential to fully elucidate how parasitoids exploit chemical cues of varying reliability and detectability in host locations. Also, elucidation of chemosensory receptors could pave the way for “reverse chemical ecology”. For example, in the parasitoid C. chlorideae, functional studies of female antennae-prominent CchlOR62 demonstrated that this receptor responds robustly to the HIPV component (Z)-jasmone. Behavioral assays confirmed that (Z)-jasmone strongly attracts female parasitoids and significantly enhances their parasitism efficiency in early-instar H. armigera larvae [111]. Fortunately, the technique of studying insect chemosensory receptors is being updated gradually [168]. For example, calcium imaging (or patch clamp) of HEK293/Sf9 cells and the Xenopus (Anura) oocyte expression system (coupled with two-electrode voltage clamp recording) served as simple yet effective techniques for in vitro gene expression and functional characterization and have been extensively applied in studies of insect chemosensory receptors [169,170]. An in vivo expression system for deorphanization of insects ORs., i.e., the “empty neuron system” of Drosophila, has also been successfully established [28,168]. Furthermore, the recently emerged CRISPR/Cas9 gene-editing technology, distinguished by its operational simplicity and stable efficacy, has been successfully utilized in numerous investigations validating insect chemosensory functions [171,172]. We believe that with the accumulation of our knowledge and the advancement of techniques, the aforementioned issues will be resolved effectively and progressively. This will concretely facilitate the development of a practical way to use parasitoids as a resource to curb the explode of insect pests within agricultural ecosystems.