Rearing of parasitoid beetle Dastarcus kurosawai (Bothrideridae: Coleoptera) larvae using once-frozen Zophobas atratus (Coleoptera: Tenebrionidae) pupae

Polymorphism is a general phenomenon in the natural world evolving as a result of frequency-dependent selection. Such polymorphism has recently received increasing attention in parasitic cuckoo-host systems, in which egg polymorphism in hosts is regarded as a specific adaptation against cuckoo parasitism. Here, we provided a theoretical analysis to investigate the effect of host egg polymorphism on the rate of successful parasitism (SPR) by cuckoos to help better understand the role of egg polymorphism in cuckoo-host systems. Five key conclusions can be drawn from these calculations: (1) The contrast of egg phenotypes in hosts significantly reduces cuckoo fitness by reducing the SPR of cuckoos; (2) hosts can minimize the SPR of cuckoos by laying polymorphic eggs with equal proportion of the two morphs if cuckoos also evolve egg dimorphism as a counter-adaptation; (3) the escalation of egg polymorphism in hosts further reduces the SPR in cuckoos, although the efficiency of such an effect decreases with the number of host egg morphs; (4) cuckoos can maximize their SPR by laying a ratio of egg morphs equal to the ratio of host egg morphs; and (5) an increase in the number of cuckoo egg morph does not increase the total SPR of cuckoos for parasitizing hosts with polymorphic eggs. Egg polymorphism in hosts is regarded as a specific adaptation against cuckoo parasitism. Here, we provided a theoretical analysis to show that the disruptive evolution of egg phenotypes in hosts significantly reduces cuckoo fitness by reducing the rate of successful parasitism by cuckoos (SPR). Hosts with obvious egg polymorphism suffer low parasitism rates and they can minimize the SPR of cuckoos by laying polymorphic eggs with equal proportions of the two morphs if cuckoos also evolve egg dimorphism as a counteradaptation, whilst cuckoos can maximize their SPR by laying a ratio of egg morphs equal to the ratio of host egg morphs. Egg polymorphism in hosts is regarded as a specific adaptation against cuckoo parasitism. Here, we provided a theoretical analysis to show that the disruptive evolution of egg phenotypes in hosts significantly reduces cuckoo fitness by reducing the rate of successful parasitism (SPR) by cuckoos. Hosts with obvious egg polymorphism suffer low parasitism rates and they can minimize the SPR of cuckoos by laying polymorphic eggs with equal proportions of the two morphs if cuckoos also evolve egg dimorphism as a counteradaptation, whilst cuckoos can maximize their SPR by laying a ratio of egg morphs equal to the ratio of host egg morphs.

Narrow-spectrum insecticides are currently used to control populations of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), in eastern Canada. However, these could have nontarget impacts on other caterpillars – some of which may serve as alternative or alternate hosts to key parasitoids – that are also susceptible to control tactics. This study was conducted to determine how the insecticides, Bacillus thuringiensis variety kurstaki (Btk) and tebufenozide, used to control spruce budworm populations, impact caterpillar communities and associated parasitism rates. Post-treatment field sampling of caterpillars was conducted in 2018 and 2019 in New Brunswick, Canada, at sites treated with either Btk or tebufenozide and at control sites. Caterpillar species richness and abundance, community structure, and parasitism rates were assessed using molecular analyses for 659 collected caterpillars. We found that insecticide applications had no significant impact on abundance, species richness, or parasitism rate relative to the measurements made in the control sites. Nonetheless, a significantly higher caterpillar abundance and lower parasitism rate occurred in Btk-treated sites than in tebufenozide-treated sites. Overall, however, Btk and tebufenozide treatments did not negatively affect the non-budworm caterpillar community under the present conditions of low caterpillar densities, suggesting that parasitoids have alternative and alternate hosts after treatments that target the spruce budworm.

Contrasting consequences of different defence strategies in a natural multihost–parasite system

Six species of legume or cereal-feeding aphids, Acyrthosiphon pisum, Aphis craccivora, Megoura crassicauda, Rhopalosiphum maidis, Rhopalosiphum padi and Sitobion akebiae (Hemiptera: Aphididae), were tested as candidates for alternative hosts of Aphidius gifuensis (Hymenoptera: Braconidae), an indigenous natural enemy attacking vegetable pest aphids Aulacorthum solani and Myzus persicae (Hemiptera: Aphididae). A. pisum, R. maidis and S. akebiae were accepted by A. gifuensis. S. akebiae showed the most successful parasitism by A. gifuensis among the six aphid species tested, with a mummification rate of 71.7% and emergence rate of 96.7%. No parasitism was observed on R. padi, an alternative host available in the banker-plant system with an exotic parasitic wasp, Aphidius colemani (Hymenoptera: Braconidae). A. gifuensis females reared on S. akebiae had the same developmental period and body size as those reared on M. persicae, with no significant differences. They also demonstrated successful parasitic performance in A. solani and M. persicae. These results suggest that S. akebiae should be a promising alternative host for use in a banker-plant system with A. gifuensis.

The diamondback moth Plutella xylostella is a devastating pest of cruciferous crops known to develop high levels of insecticide resistance. Here, we examined the possibility of classical biological control against this pest in the USA via the introduction of a specialist larval parasitoid, Diadegma semiclausum . We compared life-history traits of three populations of D. semiclausum from France, Greece, and the Netherlands, as well as their performance on their local P. xylostella host population and two novel host populations from the USA (one resistant to insecticides and one susceptible). In addition, the performance of D. semiclausum was evaluated under semi-field conditions with four commercial varieties of cruciferous crops, and a competition experiment was conducted with Diadegma insulare , the main parasitoid of diamondback moth in North America. Results revealed that all parasitoid populations were able to successfully develop in the two USA host populations, but parasitism success was lower on insecticide-resistant hosts. Parasitism success and parasitism rates were higher on the original host populations than on the USA host populations. In semi-field cages, parasitism rate was similar on the four crop varieties and was positively correlated with ambient temperature. Finally, D. semiclausum parasitism rate was 6.7 times higher than that of D. insulare , and parasitism rates of the two parasitoids were additive when they were placed together. These findings provide baseline data to support the selection of a D. semiclausum population likely to succeed if introduced in the USA and improve control of diamondback moth alongside D. insulare .

The ectoparasitoid Dastarcus helophoroides Fairmaire (Coleoptera: Bothrideridae) is an important natural enemy insect, which is artificially mass-reared and released into woodland to control medium and large longhorn beetle species. This study examined the developmental duration (days) of larvae and adult fitness (including numbers of adults emerging per host and mean body size) by exposing a single substitute host, a pupa of Zophobas morio (Coleoptera: Tenebrionidae), to different densities of D. helophoroides larvae. We showed that there was no significant effect on the rate of successful parasitism and cocoon formation, but emergence success and measures of individual adult body size (length, width, and weight) declined with increasing larval density. Larval period and cocoon period increased with larval density, while total weight of adults emerging per host increased initially before reaching a plateau. Our results suggest that a pupa of Z. morio could be successfully parasitized by a single D. helophoroides larva, but multiple D. helophoroides larvae can share one host. Excessive larval density caused intraspecific competition among D. helophoroides larvae, manifesting in extended developmental duration of immature stage and reduced fitness of adults. Furthermore, the tradeoff between the numbers of adults and body size may stabilize the population dynamics with detectable mutual interference, particularly in competing for limited host resources. These findings suggest six larvae per host would achieve the highest adult fitness and would enhance mass-rearing techniques as part of IPM strategies for longhorn beetles.

In Malawi, during the study period eggs of the rice-borer Diopsis macrophthalma were mainly parasitized by Trichogramma kalkae and Trichogramma pinneyi and further by Trichogrammatoidea simmondsi and a Paracentrobia species. Data are presented on rates of parasitism, locations and sizes of parasitized eggs, fecundity, longevity, development time, number of exit holes, clutch size per host egg, sex-ratio, emergence rate,alternative hosts and super- and multiple parasitism. Relations are discussed between 1) development time and clutch size, 2) number of ♂♂ per host egg and exit holes, 3) oviposition rate and host egg size and 4) sex-ratio and clutch size. In the first three parasitoids mentioned a fertilization/oviposition system is found that favours the presence of one♂ and a certain number of ♀♀ per Diopsis egg. Evidence is put forward that a fixed sex-regulation system exists during oviposition. T. kalkae was the most important egg parasitoid of D. macrophthalma, followed by T. pinneyi. No other hosts were found for T. kalkae, whereas T. pinneyi had one alternative main host and some hosts of lesser importance. In competition for eggs of D. macrophthalma T. kalkae was in various ways superior to T. pinneyi. This latter species seems more adapted to smaller host eggs.

Trichogramma, a globally recognized egg parasitoid, plays a pivotal role in the biological control of lepidopteran pests. However, its mass production relies heavily on the large eggs of the Chinese oak silkworm (Antheraea pernyi Guérin-Méneville, COS), which poses a limitation due to varying compatibilities among different Trichogramma species. This study investigates the potential of Eri silkworm (Samia ricini William Jones, ES) eggs as an alternative host by evaluating the performance of 6 Trichogramma species (T. chilonis, T. dendrolimi, T. japonicum, T. leucaniae, T. ostriniae, and T. pretiosum). The suitability of ES eggs for Trichogramma rearing was demonstrated through significant improvements in key fitness parameters. Notably, T. leucaniae and T. ostriniae exhibited higher parasitism rates on ES eggs (45% and 64%, respectively) compared to COS eggs (32% and 41%). Additionally, T. japonicum, which failed to parasitize COS eggs, achieved an 18% parasitism rate on ES eggs. Both T. chilonis and T. dendrolimi showed remarkable performance on ES eggs, with parasitism and emergence rates exceeding 80%. Furthermore, females reared on ES eggs demonstrated superior reproductive potential, including enhanced longevity and fecundity, compared to those reared on COS eggs. This study highlights ES eggs as a highly promising factitious host for the mass production of Trichogramma species. Their broad compatibility and ability to enhance parasitoid fitness underscore their potential to revolutionize biocontrol strategies against lepidopteran pests in agriculture and forestry.

The Screaming Cowbird (Molothrus rufoaxillaris) is the most specialized brood-parasitic cowbird, relying almost entirely on the Bay-winged Cowbird (Agelaioides badius) as host. Recently, Screaming Cowbirds have expanded their range to areas where Bay-winged Cowbirds are absent, and they are exploiting the Chopi Blackbird (Gnorimopsar chopi). Interactions between Screaming Cowbirds and Chopi Blackbirds are largely unexplored, as is the reproductive success of the parasite in this host. Screaming Cowbirds, Chopi Blackbirds, and Bay-winged Cowbirds coexist in northeastern Argentina, providing an ideal system to explore interactions between a specialist brood parasite and an alternative host and to compare the reproductive success of the parasite in its main host and in an alternative host. Screaming Cowbirds parasitized both hosts throughout their breeding seasons (Chopi Blackbirds, mid-October to mid-January; Bay-winged Cowbirds, mid-November to mid-March). Frequency of parasitism was lower in Chop...

Host range is a key element of a parasite's ecology and evolution and can vary greatly depending on spatial scale. Generalist parasites frequently show local population structure in relation to alternative sympatric hosts (i.e. host races) and may thus be specialists at local scales. Here, we investigated local population specialization of a common avian nest-based parasite, the hen flea Ceratophyllus gallinae (Schrank), exploiting two abundant host species that share the same breeding sites, the great tit Parus major (Linnaeus) and the collared flycatcher Ficedula albicollis (Temminck). We performed a cross-infestation experiment of fleas between the two host species in two distinct study areas during a single breeding season and recorded the reproductive success of both hosts and parasites. In the following year, hosts were monitored again to assess the long-term impact of cross-infestation. Our results partly support the local specialization hypothesis: in great tit nests, tit fleas caused higher damage to their hosts than flycatcher fleas, and in collared flycatcher nests, flycatcher fleas had a faster larval development rates than tit fleas. However, these results were significant in only one of the two studied areas, suggesting that the location and history of the host population can modulate the specialization process. Caution is therefore called for when interpreting single location studies. More generally, our results emphasize the need to explicitly account for host diversity in order to understand the population ecology and evolutionary trajectory of generalist parasites.

We investigated the effect of mutual interference on the attack efficiency and the rate of successful parasitism on the parasitoid Spalangia cameroni (Perkins) attacking pupae of the stable fly Stomoxys calcitrans (L.). Female parasitoids (2, 4, 8, 16, or 32) were exposed to 100 fly pupae during 24 h. The number of pupae that were attacked and the number successfully parasitized increased with the parasitoid density and reached a maximum of ∼70 and 50, respectively. Parasitoid-induced mortality (PIM) was about 20 pupae, irrespective of parasitoid density. The per capita rates of attack, successful parasitism and parasitoid-induced mortality declined monotonously with parasitoid density. Progeny sex ratio was female biased for all parasitoid densities, but declined significantly with increasing parasitoid density from ∼70% females at the lowest density to ∼60% at the highest. Mutual interference was incorporated into a functional response model to predict the attack rate and the rate of successful parasitism at different temperatures, host densities and parasitoid densities. The model explained 93.5% of the variation in the observed number of attacked pupae and 91.5% of the variation in the number of successfully parasitized pupae. The model predicts that increasing parasitoid densities will increase the percentage of killed hosts, but only up to a certain density. Above this density, a further increase in parasitoid abundance will actually lead to a decline in the percentage parasitism. These findings may have some implications for using S. cameroni in biological control against flies using inundative releases.

The parasitoid Asecodes mento (Walker, 1839) (Hymenoptera: Eulophidae) is the most important biocontrol agent of the strawberry leaf beetle Galerucella tenella (L.) (Coleoptera: Chrysomelidae) in northern Europe. Here, I investigated whether natural parasitism in organic strawberry plantations was affected by the presence of the alternative host plant meadowsweet (Filipendula ulmaria), and whether parasitism rates differed between plantations of different ages (6 to 79 years). I also investigated whether parasitoid brood size, body size and sex ratio differed between the two host plants in the field. Parasitism was very low (0%) in newly established plantations and increased to a plateau (∼40%) in fields where strawberries had been grown for approximately 20 years or longer. Such an extended colonization process is unacceptable for commercial growers. It would thus be desirable to find a method to catalyze parasitoid population buildup in young plantations. Parasitoid brood sizes were larger in beetles collected from meadowsweet, while body size and sex ratio did not differ between parasitoids collected from the two plants. These findings suggest that meadowsweet can export parasitoids to neighboring strawberry fields. Although this is a possibility, I did not find any significant differences in parasitism rates between isolated strawberry fields and fields adjacent to meadowsweet stands, indicating that effects of local vegetation are small on parasitism rates. Releasing parasitoids in newly established strawberry plantations may be a better strategy for quickly obtaining high parasitism than intercropping with meadowsweet.

Brood parasites rely entirely on the parental care of host species to raise the parasitic nestlings until independence. The reproductive success of avian brood parasites depends on finding host nests at a suitable stage (i.e. during egg laying) for parasitism and weakly defensive (i.e. non‐ejector) hosts. Finding appropriate nests for parasitism may, however, vary depending on ecological conditions, including parasite abundance in the area, which also varies from one year to another and therefore may influence coevolutionary relationships between brood parasites and their hosts. In this scenario, we explored: 1) the degree of laying synchronization between great spotted cuckoos Clamator glandarius and magpies Pica pica during two breeding seasons, which varied in the level of selection pressure due to brood parasitism (i.e. parasitism rate); 2) magpie responses to natural parasitism in the pre‐laying period and successfulness of parasitic eggs laid at this stage; and 3) magpie responses to experimental parasitism performed at different breeding stages. We found that, during the year of higher parasitism rate, there was an increase in the percentage of parasitic eggs laid before magpies started laying. However, the synchronization of laying was poor both years regardless of the differences in the parasitism rate. The ejection rate was significantly higher during the pre‐egg‐laying and the post‐hatching stages than during the laying stage, and hatching success of parasitic eggs laid during the pre‐egg‐laying stage was zero. Thus, non‐synchronized parasitic eggs are wasted and therefore poor synchronization should be penalized by natural selection. We discuss four different hypotheses explaining poor synchronization.

Ground cover management affects parasitism of Prays oleae (Bernard)

There are many different practices that contribute to conservation biological control, but little is known about their complementarity. We tested the effects of providing food and alternative hosts to parasitoids by intercropping a plum orchard with companion plants. Oats and vetch were intercropped into the orchard either as single-species (oats or vetch) or two-species (oats and vetch combined) intercrops within an inter-row. The trophic resources provided by these intercrops were assessed, along with the incidences of aphids and their parasitoids in plum trees. We found up to ten alternative host species provided by oats and vetch, and extrafloral nectar was available from the vetch and mixed strips. An effect of intercrop type and distance to plum trees was observed on aphid incidence during one sampling period. Parasitism rates in exclusion cages were affected by intercrop type, reaching almost 60% close to the mixed intercrop. However, no general tendency was observed upon whether oats, vetch or their mixture was associated with a lower incidence of aphids. We found no evidence that providing effective sources of food and alternative hosts for parasitoids increased aphid mortality in this study. The context-dependent efficiency of intercropping is discussed.