Using insects to detect, monitor and predict the distribution of Xylella fastidiosa: a case study in Corsica

Abstract

We sampled ca 2500 specimens of Philaenus spumarius (Hemiptera: Aphrophoridae) throughout Corsica without a priori knowledge on the presence of symptoms on plants. We screened 448 specimens for the presence of Xylella fastidiosa (Xf) using qPCR and a custom nested PCR. qPCR appeared versatile and under-estimated the prevalence of Xf. Nested PCR showed that Xf was present in all populations. Molecular results were validated by prediction on the distribution of Xf made from tests conducted on plants, which shows the pertinence of using vectors in risk assessment studies. Xf was detected in tenerals and adults. Thus, P. spumarius could acquire Xf from its host plant, mostly Cistus monspeliensis in Corsica, which may act as reservoir for the next season. This contrasts with other observations and suggests that management strategies may have to be adapted on a case-by-case basis. At least two genetic entities and several variants of Xf not yet identified on plants were present in the insects, which suggests ancient introductions of Xf and a probable underestimation of the current diversity of the strains present in Corsica. Interestingly 6% of the specimens carried two subspecies of Xf. Studies are required to better characterize the strains present in Corsica and to determine how the disease was introduced, spread and why no sign of a potential epidemic was detected earlier. This study shows that, when sensitive enough methods are implemented, spittlebugs (and more specifically P. spumarius for which species distribution modelling shows it could be a good sentinel for Europe) can be used to predict and better assess the exact distribution of Xf. Furthermore, Xf multiply only in their foregut and does not become circulative, which facilitates its detection.

Update of the Scientific Opinion on the risks to plant health posed by Xylella fastidiosa in the EU territory

“…• The vector is wherever olive production is (e.g. Cruaud et al, 2018). • Adults appear in May, are present in orchards from May to September-October with a population peak in late-May or June.…”

A Guild-Based Protocol to Target Potential Natural Enemies of Philaenus spumarius (Hemiptera: Aphrophoridae), a Vector of Xylella fastidiosa (Xanthomonadaceae): A Case Study with Spiders in the Olive Grove

“…For instance, A. cucurbitina is a widespread species present at three different vertical strata, the tree canopy, bushes, and the herbaceous layer, whereas M. acalypha prefers the herbs near the ground [30]. The importance of A. cucurbitina relies on the fact that both the spider species and P. spumarius inhabit the olive tree canopy [42][43][44], thus there occurs spatial overlapping of the potential natural enemy and the pest. However, spittlebugs disperse in late spring to non-cultivated patches that act as natural reservoirs [45].…”

Pest survey card on Xylella fastidiosa

“…Although adult vectors could be collected from May onwards, surveillance of adult vectors in Apulia (Italy) should take place later during the summer period to increase the chances of finding infected insects ( Figure 3). In several warm and dry areas of the Mediterranean Basin, aestivation of adults has been observed (Cruaud et al, 2018) and therefore should be taken into consideration for the collection of the samples as during this period it is very difficult to capture the insects. The dates for adult collection also vary from year to year in the same location.…”