Soft scale insect, repens scale (Pulvinaria rhododendri), on Rhododendron spp. in Northern Europe

NEWS - Repens scale (Pulvinaria rhododendri Kahrer & Hodgson, Hemiptera: Coccomorpha: Coccidae) was discovered in Rhododendron spp. (Ericaceae) in Denmark and Norway which appears taxonomically close to P. camelicola Signoret and P. floccifera Westwood with an uncertain geographic origin, but is likely to have been introduced to northern Europe via imported live plants.

P. rhododendri is distinguished from P. floccifera, P. camelicola and all other Pulvinaria species by a combination of characters-a medial band of large conical spinous setae extending from the anal plate anterior to the head and simple pores with strongly sclerotic margins present in a narrow medial band between the large conical spinous setae.

The new species also differs from P. floccifera by the presence of a type III tubular canal submarginally between the lateral stigmatic clefts (vs. absent). The species is named after the genus of the host plant from which it is most commonly collected, Rhododendron L. P. rhododendri is also found on Vaccinium myrtillus L. (Ericaceae) but only when Rhododendron spp. are present nearby.

In Denmark, the species has been found in gardens on the mainland at Hadsund Syd, Jutland, and at Tyfelse on the island of Zealand, 25 km southwest of Copenhagen. These gardens are probably about 150 km apart. In Norway, P. rhododendri has been collected from Arendal and Grimstad on the south coast, about 250 km north of Jutland, Denmark.

The new species is only known from a small area in northern Europe and was probably first found on Rhododendron spp. in Norway in 2018 causing severe sooty mold infestations. This plant was eradicated but the population appears to have persisted as specimens were collected at the same location in 2022.

It is likely that the first discovery also referred to P. rhododendri and since it is still present, it is clearly growing and overwintering under northern European conditions, having survived two winters in between.

To date, no geographical surveys have been carried out to determine the distribution of the species. This host plant could spread more widely as it is cultivated, is a popular plant and is often moved in the plant trade.

The new species is potentially important as a pest of urban plants, especially Rhododendron spp., based on the distribution mentioned above. In the field, P. rhododendri is somewhat similar to P. camelicola which may have been mistaken for non-entomologists in the past.

P. rhododendri, like other Pulvinaria species, produces abundant honeydew, so the growth of sooty mold is a major problem, forming a thick, unsightly layer covering the leaves. However, so far it does not appear to cause host mortality or other problems.

Original research

Andreas Kahrer & Chris J. Hodgson (2024). A new species of the soft scale insect genus Pulvinaria Targioni Tozzetti (Hemiptera: Coccomorpha: Coccidae) on Rhododendron spp. in Northern Europe. Zootaxa 5512 (2): 233-239, DOI:10.11646/zootaxa.5512.2.7

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