How to escape from insect egg parasitoids: a review of potential factors explaining parasitoid absence across the Insecta

Knockdown of vitellogenin receptor based on minute insect RNA interference methods affects the initial mature egg load in the pest natural enemy Trichogramma dendrolimi

Vitellogenin receptor (VgR) plays a crucial role in oogenesis by mediating endocytosis of vitellogenin and a portion of the yolk proteins in many insect species. However, the function of VgR in minute parasitoid wasps is largely unknown. Here, we applied Trichogramma dendrolimi, a minute egg parasitoid, as a study model to investigate the function of VgR in parasitoids. We developed RNA interference (RNAi) methods based on microinjection of prepupae in T. dendrolimi. RNAi employs nanomaterial branched amphipathic peptide capsules (BAPC) as a carrier for double-stranded RNA (dsRNA), significantly enhancing delivery efficiency. Also, artificial hosts without medium were used to culture the injected prepupae in vitro. Utilizing these methods, we found that ovarian growth was disrupted after knockdown of TdVgR, as manifested by the suppressed development of the ovariole and the inhibition of nurse cell internalization by oocytes. Also, the initial mature egg load in the ovary was significantly reduced. Notably, the parasitic capacity of the female adult with ovarian dysplasia was significantly decreased, possibly resulting from the low availability of mature eggs. Moreover, ovarian dysplasia in T. dendrolimi caused by VgR deficiency are conserved despite feeding on different hosts. The results confirmed a critical role of TdVgR in the reproductive ability of T. dendrolimi and provided a reference for gene functional studies in minute insects.

Differential effects of plant-beneficial fungi on the attraction of the egg parasitoid Trissolcus basalis in response to Nezara viridula egg deposition

There is increasing evidence that plant-associated microorganisms play important roles in defending plants against insect herbivores through both direct and indirect mechanisms. While previous research has shown that these microbes can modify the behaviour and performance of insect herbivores and their natural enemies, little is known about their effect on egg parasitoids which utilize oviposition-induced plant volatiles to locate their hosts. In this study, we investigated how root inoculation of sweet pepper (Capsicum annuum) with the plant-beneficial fungi Beauveria bassiana ARSEF 3097 or Trichoderma harzianum T22 influences the olfactory behaviour of the egg parasitoid Trissolcus basalis following egg deposition by its host Nezara viridula. Olfactometer assays showed that inoculation by T. harzianum significantly enhanced the attraction of the egg parasitoid, while B. bassiana had the opposite effect. However, no variation was observed in the chemical composition of plant volatiles. Additionally, fitness-related traits of the parasitoids (wasp body size) were not altered by any of the two fungi, suggesting that fungal inoculation did not indirectly affect host quality. Altogether, our results indicate that plant inoculation with T. harzianum T22 can be used to enhance attraction of egg parasitoids, which could be a promising strategy in manipulating early plant responses against pest species and improving sustainable crop protection. From a more fundamental point of view, our findings highlight the importance of taking into account the role of microorganisms when studying the intricate interactions between plants, herbivores and their associated egg parasitoids.

Morphology and ultrastructure of the colleterial glands in Myzinum flower wasps (Tiphiidae, Hymenoptera)

The strategies used by females to ensure the survival of their offspring are one of the key elements in insect reproductive biology. Ectoparasitoid hymenopterans, such as tiphiid wasps, use their ovipositor to deliver a single egg on the host body, where the larva develops. Here we describe the colleterial glands in females of two species of Myzinum and discuss the homology with other glands among Hymenoptera. The glands appear as two large opaque round sac-like structures that are connected to the oviduct. Histological sections revealed a compound gland, with a single layer of class-1 epithelial cells surrounded by a layer of class-3 gland cells, which is uncommon in Hymenoptera. Ultrastructure points to active protein synthesis, consistent with the role of colleterial glands. Despite the consensus that colleterial glands in Hymenoptera evolved with two distinct forms, the venom gland and Dufour's gland, the three glands were present in both Myzinum species. The few studies regarding the structure, function, and homology among the female accessory glands in Hymenoptera do not allow to make conclusive statements about the evolution of these glands. Therefore, the finding in Myzinum is probably the reflection of a lack of information, rather than an exception within Aculeata.

Biodiversity of hymenopteran parasitoids

Parasitoid wasps are the most successful group of insect parasitoids, comprising more than half the known diversity of Hymenoptera and probably most of the unknown diversity. This lifestyle has enabled them to be used as pest control agents conferring substantial economic benefits to global agriculture. Major lineages of parasitoid wasps include Ichneumonoidea, Ceraphronoidea, Proctotrupomorpha, and a number of aculeate families. The parasitoid lifestyle arose only once among basal Hymenoptera, in the common ancestor of the Orussidae and Apocrita some 200+ Ma ago. The ancestral parasitoid wasp was probably an idiobiont on wood-living beetle larvae. From this comparatively simple biology, Hymenoptera radiated into an incredible diversity of hosts and parasitoid lifestyles, including hyperparasitoidism, kleptoparasitoidism, egg parasitoidism, and polyembryony, in several instances co-opting viruses to subdue their hosts. Many lineages evolved beyond the parasitoid niche, becoming secondarily herbivorous or predatory nest provisioners and eventually giving rise to most instances of insect societies.

Proactive classical biological control of Lycorma delicatula (Hemiptera: Fulgoridae) in California (U.S.): Host range testing of Anastatus orientalis (Hymenoptera: Eupelmidae)

Lycorma delicatula (Hemiptera: Fulgoridae), the spotted lanternfly, native to China, invaded and established in the northeast U.S. in 2014. Since this time, populations have grown and spread rapidly, and invasion bridgeheads have been detected in mid-western states (i.e., Indiana in 2021). This invasive pest presents a significant threat to Californian agriculture. Therefore, a proactive classical biological control program using Anastatus orientalis (Hymenoptera: Eupelmidae), a L. delicatula egg parasitoid native to China, was initiated in anticipation of eventual establishment of L. delicatula in California. In support of this proactive approach, the potential host range of A. orientalis was investigated. Eggs of 34 insect species either native or non-native to the southwestern U.S. were assessed for suitability for parasitism and development of A. orientalis. Of the native species tested, 10, 13, and one were Hemiptera, Lepidoptera, and Mantodea, respectively. Of the non-native species, eight Hemiptera and two Lepidoptera were evaluated. Host range tests conducted in a quarantine facility, exposed individually mated A. orientalis females (Haplotype C) to non-target and target (i.e., L. delicatula) eggs in sequential no-choice and static choice experiments to determine suitability for parasitization and development. Additionally, the sex ratio, fertility, and size of offspring obtained from non-target and target eggs were evaluated. Results of host range testing indicated that A. orientalis is likely polyphagous and can successfully parasitize and develop in host species belonging to at least two different orders (i.e., Hemiptera, Lepidoptera) and seven families (Coreidae, Erebidae, Fulgoridae, Lasiocampidae, Pentatomidae, Saturniidae and Sphingidae). Prospects for use of A. orientalis as a classical biological control agent of L. delicatula in the southwestern U.S. are discussed.

Adaptive Plasticity of Insect Eggs in Response to Environmental Challenges

Insect eggs are exposed to a plethora of abiotic and biotic threats. Their survival depends on both an innate developmental program and genetically determined protective traits provided by the parents. In addition, there is increasing evidence that (a) parents adjust the egg phenotype to the actual needs, (b) eggs themselves respond to environmental challenges, and (c) egg-associated microbes actively shape the egg phenotype. This review focuses on the phenotypic plasticity of insect eggs and their capability to adjust themselves to their environment. We outline the ways in which the interaction between egg and environment is two-way, with the environment shaping the egg phenotype but also with insect eggs affecting their environment. Specifically, insect eggs affect plant defenses, host biology (in the case of parasitoid eggs), and insect oviposition behavior. We aim to emphasize that the insect egg, although it is a sessile life stage, actively responds to and interacts with its environment.

Evaluation of the Parasitism Capacity of a Thelytoky Egg Parasitoid on a Serious Rice Pest, Nilaparvata lugens (Stål)

Pseudoligosita yasumatsui and Anagrus nilaparvatae are both egg parasitoids of the brown planthopper, Nilaparvata lugens (Stål) (BPH). In this study, we obtained a stable strain of P. yasumatsui reproduced via thelytoky through indoor rearing and screening. We assessed the parasitism capacity of this strain on eggs of N. lugens by comparing the parasitism preference and circadian rhythm of this strain to that of A. nilaparvatae, which is proved as the dominant egg parasitoid species of BPH in rice fields. The findings indicated that both egg parasitoids could parasitize fertilized and unfertilized BPH eggs, however, with a significant preference for fertilized eggs. The daily parasitization volume of P. yasumatsui was slightly higher than that of A. nilaparvatae. Both egg parasitoids preferred parasitizing 1-3-day-old BPH eggs, but the parasitism amount of 5-6-day-old BPH eggs by P. yasumatsui is higher than that by A. nilaparvatae. The parasitism events of both species of egg parasitoid wasps occurred primarily from 7:00-15:00 and the parasitism amount at night accounted for less than 15% of the total amount. The results indicate that this strain of P. yasumatsui reproduced via thelytoky could be valuable for rice planthopper control.

Oviposition selection in spotted lanternfly: impact of habitat and substrate on egg mass size and hatchability

Oviposition strategies adopted by insects (e.g., habitat selection, substrate preference, egg size, clutch size, structure, arrangement, parental care) are critical to the survival and development of their eggs. The impact of habitat and oviposition substrate on spotted lanternfly egg mass size and hatchability was studied in Pennsylvania through laboratory observations and field monitoring in 2019 and 2021. Eggs were arranged in single layers of 1-13 columns (1-18 eggs/column) on surfaces of various types of oviposition substrates, with the longest column(s) in the middle of the egg mass. Egg mass size was positively correlated with column number, with a mean of 26.6-35.1 (0-105) eggs/egg mass for different samples. Significant differences in egg mass size were observed between study sites, with larger egg masses found at Wertz (44.8), Sam Lewis (40.6), Pinnacle (39.1), Marsh Creek (37.9), Susquehannock (34.5), and Memorial Lake (33.3) and smaller egg masses at Nolde Forest (25.0), Gordon (24.4), and Antietam (21.0). Significant differences were also detected between types of oviposition substrates with smaller egg masses found on American hornbeam (22.7). In general, more (31.6%-48.0%) eggs hatched in the field compared with the laboratory (10.0%). Egg hatch success was positively correlated with egg mass size, with the highest rates recorded on American beech, American hophornbeam, black birch, black cherry, black locust, hackberry, Norway maple, red maple, and sweet cherry at Wertz, Marsh Creek, Memorial Lake, and Pinnacle. Potential (positive or negative) impacts of tree-of-heaven density, initial infestation, treatment history, and incubation conditions are discussed.

Evolution of parasitoid host preference and performance in response to an invasive host acting as evolutionary trap

The invasion of a novel host species can create a mismatch in host choice and offspring survival (performance) when native parasitoids attempt to exploit the invasive host without being able to circumvent its resistance mechanisms. Invasive hosts can therefore act as evolutionary trap reducing parasitoids' fitness and this may eventually lead to their extinction. Yet, escape from the trap can occur when parasitoids evolve behavioral avoidance or a physiological strategy compatible with the trap host, resulting in either host‐range expansion or a complete host‐shift. We developed an individual based model to investigate which conditions promote parasitoids to evolve behavioral preference that matches their performance, including host‐trap avoidance, and which conditions lead to adaptations to the unsuitable hosts. The model was inspired by solitary endo‐parasitoids attacking larval host stages. One important aspect of these conditions was reduced host survival during incompatible interaction, where a failed parasitization attempt by a parasitoid resulted not only in death of her offspring but also in host killing. This non‐reproductive host mortality had a strong influence on the likelihood of establishment of novel host–parasitoid relationship, in some cases constraining adaptation to the trap host species. Moreover, our model revealed that host‐search efficiency and genetic variation in host‐preference play a key role in the likelihood that parasitoids will include the suboptimal host in their host range, or will evolve behavioral avoidance resulting in specialization and host‐range conservation, respectively. Hence, invasive species might change the evolutionary trajectory of native parasitoid species, which is important for predicting biocontrol ability of native parasitoids towards novel hosts.

Revision of the World Species of Megaphragma Timberlake (Hymenoptera: Trichogrammatidae)

Megaphragma species are important models for basic organismal research, and many are potential biological control agents. We present the first extensive revision of species of the genus Megaphragma based on morphological and molecular data. Our revision includes all previously described species, 6 of which are synonymized, and 22 of which are described here as new. We also provide the first key to all species of the genus and reconstruct their phylogeny based on 28S and CO1 molecular markers. The following species are synonymized with M. longiciliatum Subba Rao: M. aligarhensis Yousuf and Shafee syn. nov.; M. amalphitanum Viggiani syn. nov.; M. decochaetum Lin syn. nov.; M. magniclava Yousuf and Shafee syn. nov.; M. shimalianum Hayat syn. nov.M. anomalifuniculi Yuan and Lou syn. nov. is synonymized with M. polychaetum Lin. The following species are described as new: M. antecessor Polaszek and Fusu sp. nov.; M. breviclavum Polaszek and Fusu sp. nov.; M. chienleei Polaszek and Fusu sp. nov.; M. cockerilli Polaszek and Fusu sp. nov.; M. digitatum Polaszek and Fusu sp. nov.; M. fanenitrakely Polaszek and Fusu sp. nov.; M. funiculatum Fusu, Polaszek, and Viggiani sp. nov.; M. giraulti Viggiani, Fusu, and Polaszek sp. nov.; M. hansoni Polaszek, Fusu, and Viggiani sp. nov.; M. kinuthiae Polaszek, Fusu, and Viggiani sp. nov.; M. liui Polaszek and Fusu sp. nov.; M. momookherjeeae Polaszek and Fusu sp. nov.; M. nowickii Polaszek, Fusu, and Viggiani sp. nov.; M. noyesi Polaszek and Fusu sp. nov.; M. pintoi Viggiani sp. nov.; M. polilovi Polaszek, Fusu, and Viggiani sp. nov.; M. rivelloi Viggiani sp. nov.; M. tamoi Polaszek, Fusu, and Viggiani sp. nov.; M. tridens Fusu, and Polaszek sp. nov.; M. uniclavum Polaszek and Fusu sp. nov.; M. vanlentereni Polaszek and Fusu sp. nov.; M. viggianii Fusu, Polaszek, and Polilov sp. nov.

Optimal Conditions for Diapause Survival of Aprostocetus fukutai, an Egg Parasitoid for Biological Control of Anoplophora chinensis

Simple Summary

Diapause is a critical state of an insect’s life cycle when it undergoes arrestment of growth and/or reproduction to survive adverse environmental conditions and/or food shortage. Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle, Anoplophora chinensis, a high-risk invasive woodboring pest. The parasitoid overwinters as diapausing mature larva in the host egg and emerges in early summer in synchrony with the egg-laying of A. chinensis. Here, we determined the optimal conditions for diapause survival of this parasitoid. We showed that the parasitoid had a low (36.7%) diapause survival rate inside host eggs laid on potted plants due to desiccation or tree wound defense response under semi-natural conditions. Under laboratory rearing conditions, when parasitized host eggs were extracted from wood, the parasitoid did not survive at low humidity (44% RH). Survival rate increased with humidity, reaching the highest at 100% RH. Survival rate also increased with increasing chilling period temperature from 2 to 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature, but the reproductive fitness of the parasitoid was not significantly affected by the temperature regimes. Overall, high humidity (100% RH) and mild temperatures (12.5 °C) are the most suitable survival conditions for the diapausing parasitoid and thus should be used in laboratory rearing.

Abstract

Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle Anoplophora chinensis, a high-risk invasive pest of hardwood trees. The parasitoid overwinters as diapausing mature larvae within the host egg and emerges in early summer in synchrony with the egg-laying peak of A. chinensis. This study investigated the parasitoid’s diapause survival in parasitized host eggs that either remained in potted trees under semi-natural conditions in southern France or were removed from the wood and held at four different humidities (44, 75, 85–93 and 100% RH) at 11 °C or four different temperature regimes (2, 5, 10 and 12.5 °C) at 100% RH in the laboratory. The temperature regimes reflect overwintering temperatures across the parasitoid’s geographical distribution in its native range. Results show that the parasitoid resumed its development to the adult stage at normal rearing conditions (22 °C, 100% RH, 14L:10D) after 6- or 7-months cold chilling at both the semi-natural and laboratory conditions. It had a low survival rate (36.7%) on potted plants due to desiccation or tree wound defense response. No parasitoids survived at 44% RH, but survival rate increased with humidity, reaching the highest (93.7%) at 100% RH. Survival rate also increased from 21.0% at 2 °C to 82.8% at 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature. There was no difference in the lifetime fecundity of emerged females from 2 and 12.5 °C. These results suggest that 100% RH and 12.5 °C are the most suitable diapause conditions for laboratory rearing of this parasitoid.