Trade-Off Between Fitness Gain and Cost Determines Profitability of a Peach Aphid Parasitoid

Temperature-dependent parasitism, development, and reproduction of Microplitis manilae (Hymenoptera: Braconidae) on Spodoptera litura (Lepidoptera: Noctuidae)

The braconid wasp Microplitis manilae (Hymenoptera: Braconidae) is a solitary, koinobiont endoparasitoid of young Spodoptera litura (Fabricius) larvae and one of its most crucial natural parasitic enemies. The establishment of a stable and sufficient population of M. manilae by rearing them in a laboratory is the cornerstone of field release management. However, when reared in a laboratory, M. manilae is affected by temperature. Here, we attempted to optimize the rearing temperature of M. manilae for parasitizing second-instar S. litura larvae by examining the effects of rearing at 5 temperatures: 16 °C, 20 °C, 24 °C, 28 °C, and 32 °C. Our results suggest that the development of the parasitoid progeny speeded up with shorter durations of the egg-larval, pupal, and preadult stages with the increase in temperature from 16°C to 32 °C. The parasitism rate and fecundity of M. manilae upon parasitizing second-instar S. litura larvae increased with rearing temperature from 16°C to 24 °C, with the peaks at 24 °C reaching 52.90% and 111.70 eggs/female, respectively, and then decreased at 28 °C to 32 °C. At 24 °C, the sex ratio (0.22) was the lowest, whereas the emergence rate (68.54%) did not differ significantly compared with those at other temperatures. Meanwhile, at 28 °C, the parasitism rate, fecundity, and emergence rate approached the levels at 24 °C, whereas the sex ratio was higher (0.29) than at 24 °C. These findings suggest 24 °C to 32 °C is the most appropriate temperature range for M. manilae mass rearing under laboratory conditions.

Similar host instar preferences by three sympatric parasitoids of Chielomenes sexmaculata (Coleoptera: Coccinellidae): potential host niche overlapping

Parasitoids employ diverse oviposition strategies to enhance offspring survival and maximise fitness gains from hosts. Ladybird parasitoids, significant natural enemies of ladybirds, have the potential to disrupt biocontrol efforts, yet their biology and ecology remain poorly understood. This study investigated the host-parasitoid interaction among three sympatric larval endoparasitoids of Cheilomenes sexmaculata (Coleoptera: Coccinellidae): Homalotylus hemipterinus (Hymenoptera: Encyrtidae), Nothoserphus mirabilis (Hymenoptera: Proctotrupidae) and Oomyzus scaposus (Hymenoptera: Eulophidae). Our objective was to understand host instar preferences from five perspectives related to host profitability, handling difficulty or parasitism decision-making, and to examine the occupation rates of each parasitoid in different host instars. Host profitability was determined by development time, adult offspring dry mass, sex ratio, brood size, parasitism success rate and host handling time. Host handling difficulty was evaluated through host defensive behaviour and handling time. Parasitism decision-making was evaluated through acceptance rate and preference score that considered the first reaction of female wasp to the host. Results showed that each parasitoid responded differently to the host from various perspectives. However, the first two suitable hosts of these parasitoids overlap on the third instar host, with first to third instar hosts being ideal for H. hemipterinus, and third to fourth instar hosts being ideal for N. mirabilis and O. scaposus. In the field, the occupation rate of each parasitoid in third instar host was influenced by the population of N. mirabilis, implying its superior competitiveness. This study reveals the host instar preferences of ladybird parasitoids and highlights the potential for interspecific competition.

Larval development of a parasitoid depends on host ecdysteroids

Parasitoids often exhibit high flexibility in their development depending on stages of their host at the parasitism, yet little is known about the mechanism underlying such flexibility. In the study, we evaluated the larval development time of the parasitoid Exorista sorbillans (Diptera: Tachinidae) on the lepidopteran model insect Bombyx mori (Lepidoptera: Bombycidae). We found that the development duration of E. sorbillans larvae parasitizing on the late-developmental silkworms was significantly shorter than that of the larvae parasitizing on the early-developmental hosts. Intriguingly, the 2nd-3rd instar molting of parasitoid always occurred when the ecdysteroid titers in the host were increased to higher levels. Furthermore, inhibiting the release of ecdysteroids to parasitic abdomen by thorax-abdomen ligation of the host only repressed the 2nd-instar growth and molting of E. sorbillans larvae, but had no effect on their pupation. Meanwhile, the ecdysone synthesis and 20-hydroxyecdysone (20 E) signaling in larval parasitoids were impeded after ligation treatment. Moreover, exogenous 20 E application could largely rescue the defect in 2nd instar growth and molting through stimulating ecdysone synthesis and signaling in E. sorbillans. Our results indicate that the parasitoid requires the host ecdysteroids to stimulate 20 E signaling and the subsequent 2nd-instar growth and molting. These findings will improve our understanding of the host utilization strategies of parasitoids, and contribute to the development of in vitro rearing procedures of tachinid parasitoids for biological control.

Potential Aphid Population Regulation by Aphidius colemani (Hymenoptera: Aphidiidae) in Response to Host Density, Parasitoid Density, and Age

Aphidius colemani Viereck is an important parasitoid of green peach aphid [Myzus persicae (Sulzer)], a serious pest of greenhouse crops. Augmentation of natural enemies is the most used approach to biological control of greenhouse pests. Yet, the optimal release time, rate, and age of natural enemies are often unclear. Here we tested the influence of population density of both A. colemani and M. persicae, as well as parasitoid age, on parasitism and sex allocation. We aim is to provide information for cost-effective aphid biological control programs. We show that 1) parasitoid females increased parasitism and produced more female-biased offspring with an increase of aphid density, 2) a moderate increase of parasitoid release rate elevated parasitism and daughter production when aphid density was high, 3) parasitism rate declined with an increase of aphid density but the moderate increase of parasitoid release rate eased the decline rate, and 4) 3-d-old parasitoids parasitized more aphids than younger ones but parasitoid age had no effect on daughter production. These findings suggest that A. colemani females can adjust their ability to regulate the aphid population in response to aphid population dynamics. In augmentative programs, a moderate increase of parasitoid release when aphid density is high could achieve effective control and a release of 3-d-old parasitoids could lead to a better control outcome.

Host Preference and Fitness of Lysiphlebus testaceipes (Hymenoptera: Braconidae) in Different Instars of the Aphid Schizaphis graminum

Parasitoid fitness is strongly influenced by host quality for immature parasitoid development and by oviposition host choice patterns made by adult female parasitoids. This study aimed to determine immature host quality of Schizaphis graminum (Rondani) (Hemiptera: Aphididae) and Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae) host instars preference. To this end, the host quality of immature stages of S. graminum was assessed by rearing the parasitoid in all four instars of the aphid, placing each nymph stage of the aphid parasitized by L. testaceipes in separate Petri dishes with sorghum leaves over a 1% agar-water solution at 23 ± 1°C and a 12:12 h L:D photoperiod. The host-age preferences of the parasitoid between second and fourth instar nymphs were analyzed by choice (ten nymphs of each instar) and non-choice (ten nymphs of one instar) tests, observing parasitoid foraging in a 5-cm arena for 5 min under a stereoscopic microscope. Third and fourth instars were better hosts than first or second instars, with faster developmental times, resulting in larger wasps with bigger hind tibia size and more eggs in their ovarioles (i.e., higher initial egg load). Females preferred to oviposit in fourth instar aphids in both choice and non-choice tests. Selection by adult L. testaceipes females of older instars of S. graminum for oviposition maximizes parasitoid fitness as these instars are intrinsically more suitable for development of parasitoid offspring.

Impact of differences in nutritional quality of wingless and winged aphids on parasitoid fitness

Winged aphids are described as hosts of lesser quality for parasitoids because a part of their resources is used to produce wings and associated muscles during their development. Host lipid content is particularly important for parasitoid larvae as they lack lipogenesis and therefore rely entirely on the host for this resource. The goal of this study was to determine to what extent winged and wingless aphids differ from a nutritional point of view and whether these differences impact parasitoid fitness, notably the lipid content. We analysed the energetic budget (proteins, lipids and carbohydrates) of aphids of different ages (third instars, fourth instars and adults) according to the morph (winged or wingless). We also compared fitness indicators for parasitoids emerging from winged and wingless aphids (third and fourth instars). We found that in third instars, parasitoids are able to inhibit wing development whereas this is not the case in fourth instars. Both winged instars allow the production of heavier and fattier parasitoids. The presence of wings in aphids seems to have little effect on the fitness of emerging parasitoids and did not modify female choice for oviposition. Finally, we demonstrate that Aphidius colemani, used as a biological control agent, is able to parasitize wingless as well as winged Myzus persicae, at least in the juvenile stages. If the parasitism occurs in third instars, the parasitoid will prevent the aphid from flying, which could in turn reduce virus transmission.

Effective Biological Control Depends on Life History Strategies of Both Parasitoid and Its Host: Evidence from Aphidius colemani-Myzus persicae System

Mechanisms behind the success and failure of aphid biological control using parasitoids are largely unknown, probably because of the lack of knowledge of life history strategies of the insects involved. Here, we measured and compared life history strategies of Myzus persicae (Sulzer) (Hemiptera: Aphididae) and its parasitoid Aphidius colemani (Viereck) (Hymenoptera: Aphidiidae), providing essential information for evaluation of the potential of A. colemani to control M. persicae. Our results show that one A. colemani female parasitized ≈220 aphids within 1 wk regardless of the aphid age. Almost all aphids parasitized at <4th instar died before reaching adulthood, and those parasitized at ≥4th instar produced very few offspring, contributing little to population growth. Although having 21% lower intrinsic rate of increase and 33% longer life cycle than the aphid, the parasitoid possessed reproductive output and net population growth rate twice as high as the aphid, and reached maximum lifetime reproductive potential 1 wk earlier than the aphid. The life history strategies reported here imply that A. colemani is potentially a good biological control agent of M. persicae. On the basis of this study, we hypothesize that immediately after the onset of M. persicae, a release rate of ≈1:220 (female parasitoid:aphids) at a weekly interval during the first 3 wk could effectively control the pest. We suggest that the success of biological control of aphids using parasitoids largely depends on life history strategies of both insects involved and time of the season when they meet.