Host plant richness and abundance drive populations of a super-generalist xylem-feeding insect - Raw data

Due to their wide diet breadth, generalist herbivores are often less sensitive to bottom-up forces compared to specialists. Despite this, some generalist insects can be significantly affected by the diversity and structure of the plant communities. Here, we investigated the effect of habitat and plant richness on the abundance of the super-generalist meadow spittlebug, Philaenus spumarius (Hemiptera: Aphrophoridae). We sampled 93 sites representing four habitat types (olive groves, vineyards, meadows, and field margins) across eight heterogeneous agricultural landscapes in central Italy. Nymphs were sampled by quadrat sampling in April along with the herbaceous vegetation community, while adults were sampled using a sweep net in June, August, and October. Nymph abundance was positively associated with plant richness and vegetation cover at a small scale (2 m2), a scale consistent with the limited movement capability of nymphs, while it did not differ between habitat types. A positive correlation between adult and juvenile densities was also observed. However, this effect weakened towards the end of the growing season. The findings indicate that bottom-up effects associated with plant community composition can significantly influence the local abundance of a super-generalist xylem-feeding insect. The positive relationship between plant species richness and nymph abundance suggests that broad host use does not necessarily buffer P. spumarius from local vegetation effects. Furthermore, the marked weakening of the juvenile–adult abundance relationship is consistent with increased dispersal activity happening in the fall in our study area. Given that P. spumarius exhibits variation in host use and population dynamics across its geographic range, the strength of bottom-up effects in this species, as well as its dispersal patterns, are likely context-dependent and may vary across environmental and geographical settings.

Methods
Sampling Methods
Juveniles (Nymphs)
Sampling period: late April 2022.

Method: quadrat sampling along 20 m transects positioned diagonally across each site.

Replicates: five quadrats (2 m² each) per site.

All nymphs within each quadrat were counted.

Vegetation variables recorded per quadrat:

vegetation cover (%)
vegetation height (cm)
plant richness (counts of species, genera and families)

Adults
Sampling periods: late June, mid-August, mid-October 2022.

Method: sweep net sampling.

Sampling effort: 80 sweeps on ground vegetation + 180 sweeps on woody canopy vegetation of each plot.

Counts are provided separately by sampling position (ground vs canopy).

No pest control or vegetation management targeting P. spumarius was applied in sampled habitats.