Although I’m sure many of you are waiting as eagerly for the first flower buds, returning migrant birds, and other signs of spring as I am, I do still appreciate some of the nature observations that are just plain easier in the winter. Galls are certainly one of them. With no leaves on trees and shrubs, any galls remaining on woody stems are easily visible. Galls come in a variety of unique shapes and sizes; I’ve written about a number of galls before if you want to get a sense of their diversity, including the scrub oak gall, the wool sower gall, and the oak apple gall. Due to the unique structures that are formed from each insect-plant interaction, galls can provide definitive IDs for both the insect that initiated the gall, as well as, in many cases, the host plant that created the structure to cordon off the insect intruder. 

On a recent beach walk, I noticed numerous dark spiky structures on many of the beach rose (Rosa rugosa) stems. Each gall was roughly spherical and approximately the size of a golf ball. The shape, size, spines and host plant all point to the spiny rose stem gall wasp (Diplolepis spinosa). Although the name may be a mouthful, these wasps are quite diminutive, reaching only 1/8” to 1/4” long as adults. Although they utilize a number of different wild rose species across their North American range, their most likely host on Cape Cod is Rosa rugosa. The spiny rose stem galls are green when they are initially formed, but mature to brown, hard, woody structures covered in stiff spines. 

These galls are formed by the beach roses in response to the female wasp laying her eggs in a leaf bud before it opens in the spring. Once the eggs hatch and the larvae start to eat the rose’s stem tissue, the plant responds defensively by growing a thick layer of cells to enclose the larvae. By mid-August, the gall has reach full size and the larvae inside have consumed all available plant tissue within the structure. At this point they enter a pre-pupa stage; this is the life stage in which they’ll over winter. They won’t undergo their full pupal stage, and final stage of development, until temperatures warm to the mid-50s in the spring. Once the adults emerge, they will chew their way out of the gall to look for a mate. The adult life of the spiny rose stem gall wasp is brief, only 5 to 12 days, during which time they will mate and the females will lay eggs in new rose leaf buds, starting the cycle again. Given the small size and brief adult life span of these wasps, the spiky galls they produce on beach roses are one of our best indicators of their presence.