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Abstract
The lack of understanding of complex food-web interactions has been a major gap in the history of biological control. In particular, a better understanding of the functioning of pest food-webs and how they vary between native and invaded geographical ranges is of prime interest for biological control research and associated integrated pest management. Technical limitations associated with the deciphering of complex food-webs can now be largely overcome by the use of high throughput DNA sequencing techniques such as Illumina MiSeq. We tested the efficiency of this next generation sequencing technology in a metabarcoding approach, to study aphid food-webs using the cabbage aphid as model. We compared the variations in structure and composition of aphid food-webs in the species' native range (United Kingdom, UK) and in an invaded range (New Zealand, NZ). We showed that Illumina MiSeq is a well suited technology to study complex aphid food-webs from aphid mummies. We found an unexpectedly high top down pressure in the NZ cabbage aphid food-web, which coupled to a large ratio of consumer species / prey species and a lack of potential inter-specific competition between primary parasitoids, could cause the NZ food-web to be more vulnerable than the UK one. This study also reports for the first time the occurrence of a new hyperparasitoid species in NZ, as well as new associations between hyperparasitoids parasitoids and the cabbage aphid in this country. We conclude that the complexity of aphid food-webs in agricultural systems could often be underestimated, particularly at higher trophic levels; and that the use of high throughput DNA sequencing tools, could largely help to overcome this impediment.
Highlights
According to the Enemy Release Hypothesis (ERH) (Elton 1958), phytophagous insects that migrate or are introduced into a new region may be released from predation and parasitism pressure
When combining reads from both countries, the detection rate for the third trophic level reached 99%, while only just over 37% of the samples analysed with Illumina MiSeq produced DNA sequences for the second trophic level (Fig. 2), which constitutes a highly significant difference (Mc Nemar test, X2 = 8.0497, df = 1, P < 0.01)
Our results revealed new associations as the hyperparasitoid Al. leunissii had never been reported before as attacking D. rapae or any other parasitoid within a B. brassicae host
Summary
According to the Enemy Release Hypothesis (ERH) (Elton 1958), phytophagous insects that migrate or are introduced into a new region may be released from predation and parasitism pressure This is because some or all of their natural enemies may not occur in the newly colonized region, either because it is outside of their distribution range or because they did not manage to migrate themselves (MacLeod et al 2010). If natural enemies colonized the new region as well, they may themselves be ‘freed’ from the restraints imposed by higher trophic levels after migration (e.g. Gómez-Marco et al 2015) This would mechanistically result in higher predation pressure on the pest species and is the principle on which classical biological control hinges
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