Progeny fitness determines the performance of the parasitoid Therophilus javanus, a prospective biocontrol agent against the legume pod borer

Djibril Aboubakar Souna,Anne-Nathalie Volkoff,Manuele Tamò,Ramasamy Srinivasan,Barry Robert Pittendrigh,Aimé Hippolyte Bokonon-Ganta,Mesmin Alizannon,Marc Ravallec

doi:10.1038/s41598-021-88644-3

Abstract

Therophilus javanus (Bhat & Gupta) is an exotic larval endoparasitoid newly imported from Asia into Africa as a classical biological control agent against the pod borer Maruca vitrata (Fabricius). The parasitoid preference for the five larval instars of M. vitrata and their influence on progeny sex ratio were assessed together with the impact of larval host age at the time of oviposition on development time, mother longevity and offspring production. In a choice situation, female parasitoids preferred to oviposit in the first three larval instars. The development of immature stages of the parasitoid was observed inside three-day-old hosts, whereby the first two larval instars of T. javanus completed their development as endoparasites and the third larval instar as ectoparasite. The development time was faster when first larval instars (two- and three-day-old) of the host caterpillars were parasitized compared to second larval instar (four-day-old). The highest proportion of daughters (0.51) was observed when females were provided with four-day-old hosts. The lowest intrinsic rate of increase (r) (0.21 ± 0.01), the lowest rate of increase (λ) (1.23 ± 0.01), and the lowest net reproductive rate (Ro) (35.93 ± 6.51) were recorded on four-day-old hosts. These results are discussed in the light of optimizing mass rearing and release strategies.

Highlights

Therophilus javanus (Bhat & Gupta) (Hymenoptera: Braconidae) is a solitary endoparasitoid attacking larval stages of the cowpea pod borer Maruca vitrata (Fabricius) (Lepidoptera: Crambidae)[1]
Host stage preference and the sex ratio—i.e. the ratio of males to females in the progeny — of the parasitoid were significantly affected by the larval instar of M. vitrata caterpillar (Table 1)
More males emerged from M. vitrata caterpillars parasitized at the first larval instar (χ2 = 5.7647, df = 1, p = 0.02)

Summary

Introduction

Therophilus javanus (Bhat & Gupta) (Hymenoptera: Braconidae) is a solitary endoparasitoid attacking larval stages of the cowpea pod borer Maruca vitrata (Fabricius) (Lepidoptera: Crambidae)[1]. Life traits estimation could provide additional and critical information on quantitative and ecological function of natural enemies[6], and population growth can be estimated by assessing fertility life table parameters[7] These parameters, when known for both a pest and its natural enemy, may help to plan the outcome of their interaction[8]. In tissue feeder parasitoids, feeding after egression can enhance adult fitness[10] For both tissue feeders and hemolymph feeders, the quality of the host parasitized by the mother parasitoid can affect progeny fitness, including immature survival, development, adult longevity, and fecundity[17,18]. The objectives of the present study were to determine the larval feeding behavior in T. javanus, investigate the impact of the host age on total development time, mother female longevity and offspring production, sex-ratio of the progeny, and estimate the life table parameters under laboratory conditions

Objectives

Methods

Results

Discussion

Conclusion