You are reading a pest profile, a nursery tag, or a soil report and you hit it: Otiorhynchus sp. Or maybe Otiorhynchus spp. One p or two. They look almost identical, and most people treat them as interchangeable. They are not.
Both abbreviate the Latin word species. The single p abbreviates the singular: one species. The double p follows the Latin convention for pluralizing an abbreviation, the same way “p.” means one page and “pp.” means multiple pages.
sp. means one unidentified species within a genus. The writer knows the genus but has not identified (or chosen not to name) the exact species. A lab report that reads Phytophthora sp. is telling you: we confirmed it is Phytophthora, but we have not pinned down which one. That distinction matters because treatment can differ between species in the same genus.
spp. means multiple species within a genus, discussed collectively. A field guide that lists Malacosoma spp. as tent caterpillar hosts is referring to all tent caterpillar species at once: the western, the forest, and the eastern. It is a deliberate shorthand for “this applies to the whole group.”
Both end with a period because both are abbreviations. You will occasionally see “sp” or “spp” without the period. That is sloppy formatting, not a different meaning.
A few patterns you will encounter in practice: Rhododendron spp. on a pesticide label means the product is labeled for use on rhododendrons generally, not one species. Pseudomonas sp. in a diagnostic report means the lab identified the genus but the species-level identification is pending or inconclusive. Quercus spp. in a planting guide means the advice applies to oaks as a group.
One last detail: the genus is always italicized, the abbreviation is not. Acer sp., not Acer sp. You will see it done wrong constantly. Now you will notice.
Sources
- International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code, 2018)
- Bridson, G.D.R. & Smith, E.R. (1991). B-P-H/S: Botanico-Periodicum-Huntianum/Supplementum. Hunt Institute for Botanical Documentation.