Abstract
An accurate estimation of parasitism rates and diversity of parasitoids of crop insect pests is a prerequisite for exploring processes leading to efficient natural biocontrol. Traditional methods such as rearing have been often limited by taxonomic identification, insect mortality and intensive work, but the advent of high-throughput sequencing (HTS) techniques, such as DNA metabarcoding, is increasingly seen as a reliable and powerful alternative approach. Little has been done to explore the benefits of such an approach for estimating parasitism rates and parasitoid diversity in an agricultural context. In this study, we compared the composition of parasitoid species and parasitism rates between rearing and DNA metabarcoding of host eggs and larvae of the millet head miner, Heliocheilus albipunctella De Joannis (Lepidoptera, Noctuidae), collected from millet fields in Senegal. We first assessed the detection threshold for the main ten endoparasitoids, by sequencing PCR products obtained from artificial dilution gradients of the parasitoid DNAs in the host moth. We then assessed the potential of DNA metabarcoding for diagnosing parasitism rates in samples collected from the field. Under controlled conditions, our results showed that relatively small quantities of parasitoid DNA (0.07 ng) were successfully detected within an eight-fold larger quantity of host DNA. Parasitoid diversity and parasitism rate estimates were always higher for DNA metabarcoding than for host rearing. Furthermore, metabarcoding detected multi-parasitism, cryptic parasitoid species and differences in parasitism rates between two different sampling sites. Metabarcoding shows promise for gaining a clearer understanding of the importance and complexity of host-parasitoid interactions in agro-ecosystems, with a view to improving pest biocontrol strategies.
Highlights
Insect parasitoids are usually defined as species whose larvae develop by feeding in or on the body of an arthropod host, eventually killing it[1]
The relevance of this model is supported by the fact that assemblages of parasitoids associated with millet head miner (MHM) have been morphologically and genetically documented[41], and that significant larval mortality generally occurs during rearing processes (e.g. ~47% larval mortality in Sow et al.42), which is a major obstacle for the assessment of parasitism rates
A total of 8,045,424 reads, with 87.5% of reads belonging to MHM (1 genetic cluster), 6.1% of reads belonging to Masalia nubila (2 genetic haplotypes-it was the eggs of this pest that were collected on the millet heads, and it was impossible to distinguish between them and those of MHM; these sequences were removed from the dataset after filtering), 2.1% of reads belonging to parasitoids (4 genetic haplotypes) and 4.2% of artefactual reads(see Dataset S3 for further details on taxonomic abundances)
Summary
Insect parasitoids are usually defined as species whose larvae develop by feeding in or on the body of an arthropod host, eventually killing it[1]. DNA metabarcoding is efficiently used for the detection of host-parasite interactions, as illustrated in a wide range of organisms[20,21,22], including insects[23,24] This approach provides reliable, rapid and accessible identification of both immature and adult specimens, even for non-taxonomists, and without a priori knowledge on the specific composition of the food web[25]. Crop losses due to this pest are greatly mitigated by natural enemies, including predators and parasitoids[38,39,40,41], making it a relevant model for exploring the functional biodiversity involved in trophic webs and biological control The relevance of this model is supported by the fact that assemblages of parasitoids associated with MHM have been morphologically and genetically documented[41], and that significant larval mortality generally occurs during rearing processes The relevance of this model is supported by the fact that assemblages of parasitoids associated with MHM have been morphologically and genetically documented[41], and that significant larval mortality generally occurs during rearing processes (e.g. ~47% larval mortality in Sow et al.42), which is a major obstacle for the assessment of parasitism rates
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